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Climate change or mismanagement?

Climate change or mismanagement? Environ Biol Fish (2022) 105:1363–1380 https://doi.org/10.1007/s10641-021-01209-1 Rainer Froese  · Eva Papaioannou  · Marco Scotti Received: 5 July 2021 / Accepted: 23 December 2021 / Published online: 10 January 2022 © The Author(s) 2022 Abstract Climate change and deoxygenation are have a high probability of impaired reproductive suc- affecting fish stocks on a global scale, but disentan - cess. It is pointed out that allowed catches were regu- gling the impacts of these stressors from the effects of lated by management and adhered to by the fishers, overfishing is a challenge. This study was conducted meaning that unregulated fishing did not occur. Thus, to distinguish between climate change and misman- mismanagement (quotas that were too high and gears agement as possible causes for the drastic decline that selected too small sizes) and not climate change in spawning stock size and reproductive success in appears to be the primary cause of the bad status of cod (Gadus morhua) and herring (Clupea harengus) cod and herring in the Western Baltic Sea. in the Western Baltic Sea, when compared with the good or satisfactory status and reproductive success Keywords Climate change · Mismanagement · of the other commercial species in the area. Available Gear selectivity · Overfishing · Western Baltic Sea · data on water temperature, wind speed, and plank- Cod · Herring ton bloom during the spawning season did not reveal conclusive correlations between years with good and bad reproductive success of cod or herring. Notably, Introduction the other commercial species in the area have very similar life history traits suggesting similar resilience Climate change and deoxygenation are affecting the against stress caused by climate change or fishing. oceans in ways that transform the environment that The study concludes that severe, sustained overfishing commercial fish depend on for survival, growth, and plus inappropriate size selectivity of the main fishing reproduction (Brander 2010; Cheung et  al. 2013; gears have caused the decline in spawning stock bio- Poloczanska et  al. 2016). Climate change has been mass of cod and herring to levels that are known to suggested as a potential cause for declines and col- lapses of commercial fisheries (Möllmann et al. 2003; Rijnsdorp et al. 2009; Akimova et al. 2016), but such R. Froese (*) · E. Papaioannou · M. Scotti  declines and collapses have also occurred before cli- GEOMAR Helmholtz Centre for Ocean Research, mate change had a major impact on the marine envi- Düsternbrooker Weg 20, 24105 Kiel, Germany ronment (Jackson et  al. 2001; Hobday et  al. 2011), e-mail: [email protected] basically as the result of fishing pressure exceed - E. Papaioannou  ing maximum sustainable levels. Rather than being e-mail: [email protected] the primary cause of fish stock declines, climate M. Scotti  change has been suggested as a factor aggravating the e-mail: [email protected] Vol.: (0123456789) 1 3 1364 Environ Biol Fish (2022) 105:1363–1380 effects of overfishing (Pörtner and Peck 2010; Bodini options for rebuilding and sustainable future use of et  al. 2018; Free et  al. 2019; Möllmann et  al. 2021). cod (Gadus morhua) and herring (Clupea harengus). Some studies (Hare et al. 2016; Spencer et al., 2019) A novelty of the study lies in its use of data such argue that climate vulnerability assessments need to as length, weight, gonad weight, and stomach content be conducted as part of a future fisheries manage - collected by commercial fishers in Kiel Bight in the ment framework. However, disentangling impacts Western Baltic Sea (Froese et al. 2020). of climate change from impacts of inappropriate management is not straightforward as, for example, recruitment failure may be caused by unsuitable envi- Materials and methods ronmental conditions as well as too small spawning stock size. Water temperature, wind speed, and chlorophyll a The purpose of this study is to distinguish between concentration climate change and mismanagement (e.g., too high catches or gears with inappropriate selectivity) as The GEOMAR Helmholtz Centre for Ocean Research possible causes for recent collapses of two previously in Kiel, Germany, situated near the southern tip of large and productive stocks in the Western Baltic Kiel Bight, maintains records taken in 8-min intervals Sea. Such distinction is important because if climate of water temperature, wind speed, and a number of change is the main cause of decline, then there may other daily weather measurements. These measure- be little that managers can do other than adjusting ments are taken on the roof of the institute and at the fishing to reduced productivity. If, however, misman - Kiel lighthouse, which is situated near the center of agement is the main cause of the decline, then end- Kiel Bight at 54° 29.9′ N 10° 16.4′ E. Water tempera- ing mismanagement may lead to restoration of good ture is determined at 5-m depth at the pier in front of stock status and profitable fishing despite impacts of the institute and at 1-m depth at the lighthouse. For climate change (Gaines et al. 2018). the purpose of this study, daily median measurements The Baltic Sea has already experienced substantial at the institute and the lighthouse were used. changes in its hydrography as a result of recent warm- A time series of chlorophyll a concentration (µg/l) ing, with an estimated increase in sea-surface temper- was extracted from the Oceanographic Database of ature (SST) of up to 1  °C per decade for the period the International Council for the Exploration of the 1990–2008. Projections of future climate change sug- Sea (ICES) (https:// ocean. ices. dk/ HydCh em/ HydCh gest that this trend will continue in the future, with em. aspx). It includes all stations in the Western Bal- an estimated 2  °C increase in summer SST in the tic Sea delimited by longitude = [9°, 11°] E and lati- southern parts of the Baltic Sea. Such warming and tude = [54°, 55°] N, from sea surface to a maximum the continuing influx of nutrients from agriculture are depth of 44 m. expected to lead to increasing areas of hypoxia and anoxia (Hansson et  al. 2019; BACC (Baltic Earth Selected years Assessment of Climate Change) II Author Team, 2015). There can be no doubt that these changes will The years 1997, 2003, 2016, 2019, and 2020 were affect ecosystems and species by triggering complex selected for closer examination with regard to envi- nonlinear dynamics (Reusch et  al. 2018; Ito et  al. ronmental conditions. These years showed better or 2019), but the resilience of commercial species will, worse reproductive success than the average of years in addition, be affected by their history of exploitation when spawning stock biomass (SSB) was within and their current stock status (Free et al. 2019). safe biological limits, referring to a stock size below In order to disentangle the impacts of climate which recruitment is likely to be impaired (CFP change and fisheries, this study examines the status of 2013). To avoid confusion with number of recruits commercial species in the Western Baltic Sea. It com- being reported for ages zero or one, reproductive suc- pares their life history traits and related intrinsic resil- cess was reported for the year in which the spawning ience, their response to recent changes in environ- took place (WGBFAS 2021; HAWG 2021; Fig. 4). In mental conditions, their relative stock sizes, and their particular, the selected years represent good repro- history of exploitation. Finally, the study explores ductive success for cod in 1997 and 2003, for herring Vol:. (1234567890) 1 3 Environ Biol Fish (2022) 105:1363–1380 1365 in 1997 and 2003, and for plaice (Pleuronectes pla- mainly in the Kattegat (ICES 2021a). Data on life his- tessa) in 2016 and 2019. Below average reproductive tory traits (maximum length, length at which 50% of success was reported for cod in 2019 and 2020; for the females have reached maturity) were derived from herring in 2016, 2019, and 2020; and for plaice in records in DATRAS (2021), which were restricted to 2003. the Western Baltic Sea and years after 2000, and from commercial catches in Kiel Bight (Froese et al. 2020; Occurrence of Lasker-events this study). Fecundity ranges were derived from refer- ences compiled in FishBase (Froese and Pauly 2021). Lasker (1975, 1978) proposed that a period of calm Information on fishing pressure, stock size, age com - winds is essential for forming food-rich plankton position in the commercial catch, and recruitment patches that are crucial for the survival of first-feed - was derived from the stock assessments carried out ing fish larvae. Pauly ( 1989) suggested to name such by working groups of the International Council for periods “Lasker-events,” and for the purpose of this the Exploration of the Seas (ICES) (ICES 2021b; study we used continuous 4-day periods with median HAWG 2021; WGBFAS 2021). Timing of spawning wind speeds not exceeding 6 m/s as representative of was derived from the gonadosomatic index of females such events. For the selected years, the occurrence of caught by commercial fishers in Kiel Bight as part of Lasker-events was counted from March to May. The this study. The gonadosomatic index was calculated period was chosen because the planktonic larvae of as the weight of the ovaries divided by live body all considered species require high densities of zoo- weight. plankton as food during these months, and too strong winds are considered detrimental for optimal feeding Life history reference points conditions (Lasker 1975, 1978; Cushing and Hor- wood 1994). The largest length (L ) found for the Western Bal- max tic Sea either in DATRAS (2021) or in the data from Examined species and life history traits commercial fishers used in this study was taken as ref - erence point for maximum size and as a proxy for the The species considered in this study represent all asymptotic length parameter (L ) in formal growth inf commercially relevant species that reproduce in the analyses (Froese and Binohlan 2000). Growth in body Western Baltic Sea (Petereit et  al. 2014) (Table  1). weight has an inflection point at 30% of asymptotic Other commercial species are lemon sole (Microsto- body weight (W ), which corresponds to 2/3 L if inf inf mus kitt) and sprat (Sprattus sprattus), but they are of the exponent of the length–weight relationship is only minor commercial importance and do not repro- close to 3. L is the length at which cohort biomass opt duce in the area. The bulk of sprat distribution occurs reaches a maximum (Holt 1958), and it corresponds in the northeastern Baltic Sea such as the Gulf of to 2/3 L if the ratio between the average adult rate inf Finland (ICES 2020) while lemon sole can be found of natural mortality (M) and the somatic growth rate Table 1 Overview of life history traits of the commercial weight, t is the maximum reported age, and t is the age at max m fishes in the Western Baltic Sea, where L is the maximum which more than 50% of the females reach first maturity max observed length, W is the corresponding maximum body max Species Common L W t t Fecundity Eggs Larvae Main food max max max m name cm kg y y Millions Gadus morhua Cod 106 10.6 14 2–3 0.5–5 Pelagic Pelagic Benthic invertebrates; fish Scophthalmus maximus Turbot 58 4.2 22 3–4 5–15 Pelagic Pelagic Benthic invertebrates; fish Scophthalmus rhombus Brill 60 3.2 11 3–4 0.3–1.5 Pelagic Pelagic Benthic invertebrates; fish Pleuronectes platessa Plaice 57 1.5 25 2–4 0.05–0.5 Pelagic Pelagic Benthic invertebrates Platichthys flesus Flounder 52 1.4 26 2–4 0.4–2 Pelagic Pelagic Benthic invertebrates Limanda limanda Dab 47 1.1 12 2–3 0.05–0.15 Pelagic Pelagic Benthic invertebrates Clupea harengus Herring 36 0.26 17 3 0.013–0.065 Demersal Pelagic Zooplankton; benthic invertebrates Vol.: (0123456789) 1 3 1366 Environ Biol Fish (2022) 105:1363–1380 (K) is close to M/K = 1.5, as has been found in many content, gonad weight, and indication of spawning commercial species (Froese et al. 2016). For the pur- activity based on visual examination of gonads (Fro- pose of this study, it was assumed that 2/3 L is a ese et al. 2020). max reasonable proxy for L . Taking the allowed catch opt when cohort biomass is highest has the least nega- tive impact on the fished stock and results in a mean Results age and size similar to the natural one without fish - ing. Since it is not possible to catch fish only at L , a Environmental data opt similar size and age structure can also be obtained if fishing with sustainable rates starts at a smaller length Median daily water temperatures in January to May L = 0.56 L and includes all subsequent length in Kiel Bight for the selected years show a remark- c_opt inf and age classes (Froese et al. 2016). able dichotomy of interannual variability, with very different trajectories and differences in temperature of Data collected by fishers up to 5 °C in January to March, and very similar tra- jectories with only up to 2 °C interannual differences GEOMAR has a collaboration with several com- in April and May (Fig. 1). The hottest year on record mercial fishers operating in the southern Kiel Bight, was 2020, whereas 2021 was one of the coolest years south of a line from Damp to Fehmarn. The fishers after mid of February. operated with gill nets of 55- to 110-mm mesh size The high monthly and annual variability in tem- (knot to knot) which were set to up to 24  h in dif- perature in January to March makes it difficult to ferent locations, from shallow (e.g., 4  m) to deeper use temperature as a reliable environmental trigger areas (about 15  m). In addition, in 2021, one fisher for gonad development and spawning. For example, with a 14-m boat operated a small trawl with 120- if an increase from lower temperatures to 4  °C was mm stretched mesh size and a typical net opening of such a trigger, then that occurred in early February in 10–11-m width and 2-m height. The collected data 2016, in early March in 1997, in late March in 2003, were locality, depth, gear used, and date and time 2019, and 2021, and never in 2020. Reproductive suc- of deployment, and for every species, length, wet cess of cod, herring, and plaice in the selected years weight, stomach weight, indication of main stomach does not consistently favor or disfavor any one year. Fig. 1 Comparison of daily median water temperatures in outside of safe biological limits in a given year, then the spe- Kiel Bight for the years indicated in the legend. Reproductive cies name in the legend is enclosed in parentheses. Note that success in these years for cod, herring (her), and plaice (ple) the depicted data for plaice refer to the stock in ICES subdivi- is indicated as above + or below − the average of years when sions 21–23 [TmpWnd2.R] biomass was within safe biological limits. If the stock size was Vol:. (1234567890) 1 3 Environ Biol Fish (2022) 105:1363–1380 1367 However, reproductive success was below average and the lowest number of Lasker-events, plaice had when spawning stock biomass was outside of safe extraordinarily good recruitment (Fig.  4), whereas biological limits (SSB < B ) (CFP 2013) (Fig. 1). the bad recruitment of cod and herring may have pa The monthly distribution of median daily wind been caused by their spawning stock biomass being speed in March to May looked very similar across outside of safe biological limits. the selected years (Fig.  2), with the exception of Monthly density of phytoplankton as represented 2019, when median wind speed was highest. The by measurements of chlorophyll a (Chl) was simi- number of Lasker-events ranged from 3 in 2019 to lar across the selected years with available data 9 in 2003 and 2016. There was no apparent correla- (Fig.  3), with a peak in March and similar densi- tion between reproductive success of cod, herring, ties in May. , when densities in April and May were or plaice and the number of Lasker-events. In the lowest compared with the other years. Yet, 2019 year (2019) with the highest median wind speed Fig. 2 Boxplot of wind speeds in selected years in March to May in Kiel Bight, with indication of number of calm periods (Lasker-events, bold num- bers). Below the Lasker- event numbers, reproductive success of cod, herring, and plaice in the given year is indicated as above + or below − average. If stock size in the given year was outside of safe biologi- cal limits, the indicator is enclosed in parentheses. Note that the depicted data for plaice refer to the stock in ICES subdivisions 21–23 [TmpWnd.R] Fig. 3 Seasonal timing and density of phytoplankton blooms in the Western Baltic Sea. Phytoplankton dynamics are indicated by chlorophyll a (Chl) concen- tration Vol.: (0123456789) 1 3 1368 Environ Biol Fish (2022) 105:1363–1380 Fig. 4 Time series of rela- tive reproductive success, fishing pressure, and spawn - ing stock sizes for Western Baltic cod, herring, and plaice. Reproductive suc- cess is expressed relative to the average of years when the stock was within safe biological limits. Repro- ductive success in years outside of safe biological limits is indicated as dashed lines. Fishing pressure is expressed as annual fishing mortality relative to the maximum sustainable level (F/F ) and shown as dot- msy ted lines. Spawning stock size is shown relative to the one marking the border of safe biological limits was the year with extraordinary good reproductive the Western Baltic Sea have been overexploited at some success of plaice (Fig. 4). point during the past 30  years (1990–2020) (Table  2, Fig.  4) (HAWG 2021; WGBFAS 2021). However, Life history traits while the overexploitation of flatfish was less severe and ended in recent years (WGBFAS 2021) (Fig.  4), The commercially important fish species in the West - continuous overexploitation of cod and herring was 2–5 ern Baltic Sea consist of 5 flatfish species in addition times the maximum sustainable level since the 1990s to cod and herring. With the partial exception of her- (Fig. 4). ring, their life history traits are remarkably similar Recent (2020) stock status was above or around the with regard to habitat (demersal), maximum body level that can produce maximum sustainable yields size (1–10  kg), food (mostly benthic invertebrates, (MSY) (CFP 2013) for all commercial fish except cod some fish), longevity (11–26  years), age at matu - and herring, which were suffering from depleted stock rity (2–4  years), spawning season (spring), and high sizes and very low reproductive success (Table  2, fecundity with pelagic eggs and larvae. Herring dif- Fig.  4) (HAWG 2021; WGBFAS 2021). Herring has fers by being the smallest species (0.26  kg), feeding supported the highest catches in the area with nearly mostly on zooplankton, with demersal deposition of 200,000 tonnes in 1992, followed by cod with 49,000 eggs (Table 1). tonnes in 1996. In comparison, the sum of the high- est catches of flatfish is only about 15,000 tonnes. But Exploitation and recent stock status catches of herring in 2020 were only 22,000 tonnes and for cod 4,363 tonnes, i.e., about only 10% of pre- With the possible exceptions of dab (Limanda limanda) vious maximum catches, indicating the depleted sta- and flounder ( Platichthys flesus ), all commercial fish in tus of these stocks. Vol:. (1234567890) 1 3 Environ Biol Fish (2022) 105:1363–1380 1369 Vol.: (0123456789) 1 3 Table 2 Recent (2020) stock status, fishing pressure, commercial interest, and related comments for the commercial fishes in the Western Baltic Sea. The maximum catch is meant to give an impression of relative stock size and importance to the fishery Name Stock status Fishing pressure Commercial interest Comments Cod Truncated age/size structure; very low Severe overfishing in the past; ongoing Used to be the commercially most Only the 2016 year class supports the biomass; repeated recruitment failures overfishing. Max catch 49,000 tonnes important species in the fishery remaining fishery and reproduction; in 1996 gear selectivity is too small for this large species Turbot Below MSY level Above MSY level. Max catch 1,200 High market value but low abundance No MSY level assessment; gear selectiv- tonnes in 1996 ity is too small for this large species Brill Probably around MSY level Probably around MSY level. Max catch Good market value but low abundance No MSY level assessment; stock size 160 tonnes in 1995 indicator about stable since 2004; gear selectivity is too small for this large species Plaice Recently above MSY level Severe overfishing in the past; recently Currently the most important species in Recovery despite ongoing overfishing not near MSY level. Max catch 8,000 the fishery well understood tonnes in 2002 Flounder Apparently around MSY level Apparently around MSY level. Max Limited market in Germany No MSY level assessment; recent decline catch about 3,000 tonnes in 2001 in in catch and stock size suggests MSY the area level has been reached Dab Apparently above MSY level Apparently below MSY level; no No market in Germany, exported to the No MSY level assessment; stock size targeted fishing but common bycatch. Netherlands indicator and size structure suggest Max catch 3,100 tonnes in 1994 healthy stock size; gear selectivity optimal for this small species Herring Very low biomass; very low recruitment Severe overfishing. Max catch 194,000 Used to be the staple species Scientific advice is for zero catch since tonnes in 1992 2019 but fishing continues 1370 Environ Biol Fish (2022) 105:1363–1380 The age structure of cod in 2020 was strongly trun- provided by commercial fishers in Kiel Bight (Froese cated, with the stock consisting nearly exclusively of et  al. 2020 and this study). No comparison with the one year class of 4-year-old cod, instead of the seven reference points was possible for herring because they year classes being present in 1985–2020 (WGBFAS were not caught by the analyzed gears. 2021) (Fig.  5). Exploitation of this remaining year Mean weight in trawl catches of about 4  kg was class continued with a fishing pressure of F = 0.8 above W for cod; however, this is due to the absence opt compared to the maximum sustainable level of of smaller cod (Fig. 5), which would have been other- F = 0.26 applicable to a stock within safe biologi- wise retained by the gear designed to catch cod from msy cal limits (ICES 2021b). 35  cm and about 0.5  kg onward, and their presence The age composition of herring in commercial would have reduced the mean. Despite the distorted catches in 2020 in the Western Baltic Sea (HAWG size distribution, L and L were below L and mean c opt 2021) shows a severe lack of individuals in all age L in the 55- and 75-mm gill nets, indicating that c_opt classes when compared with the mean of 2000–2005, selectivity of all gears was too small for cod. indicating a severely reduced stock size and a lack of Selectivity of the 55-mm gill net was too small recruitment in recent years (Fig. 6). for turbot, plaice, and flounder, but adequate for dab. Selectivity of the 75-mm gill net was too small for Selectivity turbot but adequate for the other flatfish. Mean weight in the trawl catch was below W for opt Table 3 reports reference points for healthy size struc- plaice, flounder, and dab, and presumably also for the ture (L , W ), for optimum selectivity of gears larger turbot and brill for which insufficient data were opt opt (L , W ), and for maturation (L and L ) for available, indicating that the selectivity of the trawl c_opt c_opt m90 m50 the commercial fish species in the Western Baltic Sea. was too small for these species. For comparison with L and W , Table  3 presents Aggregated length frequencies for plaice, floun - opt opt the mean length and weight of the catch of common der, and dab in commercial gill net catches in 2020 gears, and for comparison with L the selectivity and 2021 in Kiel Bight show frequency peaks near c_opt (L ) of some of these gears. Mean length, mean weight, or above L (Fig.  7), indicating that these stocks c opt and mean length at first capture were derived from data were in reasonably good condition and not subject Fig. 5 Age composition of commercial catches of Western Baltic cod in mil- lions by age class in 2020 (dark gray) compared with mean catches in 1985–2000 (light gray) [WBS_2020. xlsx] Vol:. (1234567890) 1 3 Environ Biol Fish (2022) 105:1363–1380 1371 Fig. 6 Age composition of commercial catches of her- ring in the Western Baltic Sea, in millions at age. The light gray bars are the mean of 2000–2005 whereas the dark bars indicate the abun- dance in 2020 [WBS_2020. xlsx] Table 3 Reference values for length (L ) and weight (W ) 75-mm mesh size (knot to knot) and a bottom trawl with 120- opt opt for which increase in body weight is highest, length (L ) mm stretched mesh size, as observed in this study. Bold val- c_opt and weight (W ) when fishing should start, length at which ues indicate suitability of the gear for the respective species. c_opt 90% and 50% of the females reach maturity (L , L ). This Lengths are given in centimeters and weights (W , W ) in m90 m50 opt c_opt is compared with selectivity of gill nets with 55-mm and grams Name L W L W L L 55-mm net 75-mm net Trawl opt opt c_opt c_opt m90 m50 L L L L W mean c mean c mean Cod 71 3202 59 1984 36 27 46 40 58 40 4011 Turbot 38 1275 32 761 38 26 33 31 36 30 NA Brill 40 927 33 544 40 28 NA NA 43 36 NA Plaice 38 532 32 342 31 20 32 28 37 33 263 Flounder 35 471 29 283 33 21 32 28 36 31 285 Dab 31 339 26 184 23 17 29 27 33 30 197 Herring 22 77 18 46 16 14 NA NA NA NA NA to overfishing (Froese et al. 2016, 2018). The situ- Timing of spawning ation was different for the larger-sized turbot, for which the frequency peak occurred before L Gonadosomatic index values of female fish caught c_opt and several of the larger length classes were under- by commercial fishers in Kiel Bight from January to represented, indicating less satisfactory stock size May were available in 2020 and 2021 for cod and in and at least temporary overexploitation. 2021 for plaice and dab. Gonadosomatic index values Vol.: (0123456789) 1 3 1372 Environ Biol Fish (2022) 105:1363–1380 Fig. 7 Length frequencies for turbot, plaice, flounder, and dab in commercial catches in 2020 and 2021 in Kiel Bight, with gill nets ranging from 55- to 110-mm mesh size (knot to knot) [Kfish_27.R] above 0.07 were used as indication of active spawn- area (Kiel Bight) in January and February, when tem- ing (Froese et al. 2020). peratures in 1–5-m depth in recent years were 3–5 °C Cod spawning commenced in December (data higher than in, e.g., 1997 or 2003, but not in March before mid-February 2020 are missing), peaked in to May, when temperatures were very similar (Fig. 1). early March, and ended in early April in both 2020 Seasonal timing and density of phytoplankton blooms and 2021. Spawning of plaice commenced in Decem- show no major differences between years with good ber 2019 and ended at the end of February 2020. and bad reproductive success of cod and herring Spawning of dab started in January 2020 and ended (Fig.  3). The year 2019 had intermediate water tem- in early May (Fig. 8). peratures but strong wind, few Lasker-event, low chlorophyll a concentration in April and May, and below-average reproductive success of cod and her- Discussion ring; however, it supported extraordinary good repro- ductive success of plaice throughout the area (Fig. 4). The purpose of this study was to distinguish between All commercial species in the Western Bal- climate change and mismanagement as the possi- tic Sea  feed on benthic invertebrates (polychaete ble cause for the bad status of herring and cod in the worms, crabs, shrimps, bivalves, and small benthic Western Baltic Sea. The first approach to answer this fish) except for herring, which feeds on zooplank - question was to look at patterns of water temperature, ton and some benthic invertebrates. Large cod, tur- wind speed, and chlorophyll a density in selected bot, and brill also feed on larger fish (Table  1). All years when reproductive success of cod or herring species are highly fecund with pelagic eggs (except was above or below average. The second approach herring, which has demersal eggs) and planktonic was to compare life history traits and management of larvae (Table  1), and spawn in the area between Jan- herring and cod with those of the other commercial uary and May (Fig.  8). They mature between 2 and fishes in the area. 4  years of age, suggesting similar generation times and thus similar intrinsic rates of population increase Climate change and resilience (Pianka 2000). Based on a combination of life history traits, all species are classified in FishBase as having Influence of climate change on interannual variabil - medium resilience and potential to recover from low ity of water temperature may be visible in the study population sizes (Froese and Pauly 2021), and one Vol:. (1234567890) 1 3 Environ Biol Fish (2022) 105:1363–1380 1373 Fig. 8 Gonadosomatic index of females in Kiel Bight in 2020 for cod, plaice, and dab. Index values above 0.07 are interpreted as active spawn- ing. Data for cod in 2020 are incomplete as sampling started only in mid-Febru- ary [Kfish_27.R] Vol.: (0123456789) 1 3 1374 Environ Biol Fish (2022) 105:1363–1380 would expect them to have similar responses to envi- biomass was too small (as confirmed by Figs.  4 and ronmental changes in the Western Baltic Sea. 5) to supply all of its spawning area with sufficient Available length-frequency data for turbot, plaice, numbers of eggs during the whole duration of spawn- flounder, and dab in commercial catches in Kiel ing season. Bight (Fig. 7) show no gaps that would suggest recent Given that the other commercial stocks in the years with bad reproductive success. In contrast, the Western Baltic showing no signs of suffering from age composition of cod in commercial catches in the climate change are all flatfish, one could suspect that Western Baltic Sea in 2020 (WGBFAS 2021, Fig.  5) flatfish are somehow more resilient to climate change shows practically only one remaining year class (of than herring and cod. However, previous studies sug- 2016), and a comparison with mean age composi- gest that flatfish in the Baltic Sea have similar or even tion in 1985–2000 indicates that millions of cod are less resilience to climate change (MacKenzie et  al. missing in the other age classes, suggesting numerous 2007; HELCOM 2021). recruitment failures. This is confirmed by observed In summary, available time series of water tem- recruitment data, shown as reproductive success of perature, wind speed, and chlorophyll a concentra- the respective years in Fig.  4, with very low repro- tion show no conclusive connection with the repro- ductive success in 2007, 2015, and 2017, and accord- ductive failures of cod or the continuously declining ing to preliminary data also in 2018, 2019, and 2020 reproductive success in herring (Fig.  4). Instead, the (WGBFAS 2021). The period of low reproductive available data suggest that the environmental condi- success coincides with the periods from 1998 (ICES tions supported regular and even better-than-average 2018) or 2008 (WGBFAS 2021) onward, when reproductive success of the other spring-spawning spawning stock biomass was considered too low to commercial fish in the area. ensure successful reproduction. The age composition of herring in commercial catches in the Western Baltic Sea (Fig.  6) shows the Management presence of all age classes, suggesting no complete failure of reproduction in recent years. However, The second approach to disentangle impacts of cli- abundances especially at ages 1 to 4 are only 0.6–5% mate change or mismanagement as causes for the of the mean of 2000–2005 and lower than in subse- bad status of cod and herring in the Western Baltic quent year classes, highlighting hundreds of millions Sea was to compare exploitation and stock status of of missing herring and suggesting declining repro- herring and cod with those of the other commercial ductive success in recent years. This is confirmed fishes in the area, with the sub-population of cod in by observed recruitment data, shown as reproduc- the Öresund, and with three herring stocks in the tive success in the respective years in Fig.  4, with Central and Northeastern Baltic Sea. declining reproductive success since 2004 and lowest Since 1997, fishing pressure on cod was 3–5 times success on record in 2020. The period of low repro- the maximum value that the stock can support. Con- ductive success coincides with the period from 2003 sequently, spawning stock biomass shrunk by 64% onward, when spawning stock biomass is considered from 1997 to 2018 (WGBFAS 2021) (Fig. 4). Despite too low to ensure successful reproduction (HAWG the obvious biomass decline and the stock being out- 2021) (Fig. 4). side of safe biological limits (CFP 2013), severe over- It has been suggested (Froese et al. 2020) that the fishing never stopped, with F still 3 times the MSY unusually warm water temperature in January–March level in 2020 (ICES 2021b). Similarly, herring was 2020 (Fig. 1) has led cod to spawn too early such that subject to severe overfishing ( F/F > 1.5) from 1997 msy its zooplankton-feeding larvae missed the zooplank- to 2009 and 2016 to 2018, despite spawning stock ton bloom expected to occur in late March, April, and biomass being outside of safe biological limits and May. However, subsequent gonadosomatic index data declining by 60% from 1997 to 2020 (HAWG 2021) for female cod in Kiel Bight in 2020 (Fig. 8) confirm (Fig. 4). Since 2019, the International Council for the spawning activity until end of March, similar to the Exploration of the Sea (ICES) recommended closure regular spawning activity of plaice and dab (Fig.  8). of the herring fishery, but management allowed fish - Instead, it seems more likely that cod spawning stock ing to continue. Vol:. (1234567890) 1 3 Environ Biol Fish (2022) 105:1363–1380 1375 Similar to cod and herring, plaice was subject insufficient for brill, but given its similarity in size to very high levels of fishing pressure from 1999 and body shape to turbot, the turbot assessment to 2016, but other than in cod and herring, spawn- probably also applies to brill. ing stock biomass kept fluctuating around the bor - No length-composition data were available for her- der of safe biological limits (CFP 2013) and started ring, but the very high abundance of immature 1- and increasing in 2013, despite ongoing overfishing of 2-year-old herring in commercial catches shown in F/F = 1.5. The continued increase in biomass then Fig.  6 suggests that gear selectivity was inappropri- msy led to a decline in fishing pressure, below F in ate. For cod in the Western Baltic Sea, current com- msy 2020, ending overfishing of plaice for the first time mercial gear selectivity was difficult to establish in the time series. It is important to note that this because of the strongly truncated age structure, but recovery is not due to management reducing catches the minimum marketing size is 35  cm (WGBFAS to allow the stock to recover, because total catches 2021) whereas optimum length at first capture  is (landings + discards) increased from 3,594 tonnes in L = 59 cm (Table 3). Age composition in the com- c_opt 2013 to 4,470 tonnes in 2020 (+ 24%). The reasons mercial catch of 1985–2000 shows a high abundance for the recovery of the plaice stock despite ongoing of immature cod of 1–2  years of age. Together with overfishing and increasing catches are not well under - far too high fishing pressure, this inappropriate early stood and beyond the scope of this study. It may be onset and thus extended duration and impact of fish - a mixture of improved reproductive success since ing mortality is one of the reasons for the bad status 2009 (Fig.  4), reduced natural mortality of juveniles of the cod stock. due to absence of large cod before 2020 (Fig.  5), In summary, inappropriate selectivity of the main reduced competition for benthic food due to repeated commercial gears has contributed to the truncated age reproductive failures and thus absence of juvenile structure of cod (Fig.  5) and the distorted age struc- cod (Fig. 5), more appropriate selectivity of the main ture of herring, whereas the more appropriate gear gears (Table 3), and overfishing being less severe than selectivity for dab, plaice, and flounder has contrib - in cod and herring due to limited demand. But clearly, uted to their good size structure, stock status, and better than average plaice reproduction and increase apparent resilience against climate change. in stock size (Fig.  4) happened despite full exploita- It has been shown that the sub-population of cod tion and impact of climate change on the Western in the Öresund, which are part of the Western Baltic Baltic environment. Similarly, the available length cod stock (ICES 2021b), did not show a truncated age frequencies (Fig.  7) and stock size indicators (WGB- structure in 2010 (Svedäng and Hornborg 2017, their FAS 2021) of the other commercial fish in the area Fig. 3), presumably because trawling is banned in that (Table  3) do not show signs of reproductive failure, area since 1932, common gill nets have slightly larger depleted stock size, or severe overexploitation. mesh sizes (65  mm instead of 55  mm knot-to-knot), The importance of capturing fish at the right and the shipping traffic lanes act as de facto no-take size is not to be underestimated: it has been for- zones. However, fishing pressure was always high mally shown that it is theoretically impossible to and practically unlimited in the area, thus attracting collapse a stock if all individuals are allowed to fishers from neighboring areas when their  catches reproduce before capture (Myers and Mertz 1998). were low (Lindegren et al. 2013). For example, in the With regard to the appropriateness of management period 2017–2020, the mean proportion of Öresund of technical measures such as the size selectivity landings of the overall cod landings increased by 71% of the allowed gears, the peak in length frequen- compared to 2000–2016 (ICES 2021b, their Table 9). cies of plaice, flounder, and dab was slightly above In 2020, 89% of the Öresund landings consisted only L , suggesting that gear selectivity was appropri- of 4-year-old individuals, thus showing the same opt ate for these species (Table 3, Fig. 7). In turbot, the truncated age structure as the rest of the Western Bal- peak in frequencies was below L and L , sug- tic Sea (WGBFAS 2021, Table 2.32 at page 146) and opt c_opt gesting that gear selectivity was inappropriate (too suggesting that slightly larger mesh sizes, trawling small, too early) for this large species. This may be ban, and no-take zones are not sufficient to counteract one of the reasons for the less satisfactory stock sta- fishing pressure up to 5 times the maximum sustain - tus of turbot (WGBFAS 2021). Available data were able level (Lindegren et al. 2013, their Fig. 2b). Vol.: (0123456789) 1 3 1376 Environ Biol Fish (2022) 105:1363–1380 Fig. 9 Time series of relative reproductive suc- cess, fishing pressure, and spawning stock sizes for adjacent herring stocks in the Central Baltic, the Gulf of Riga, and the Gulf of Bothnia. Reproductive suc- cess is expressed relative to the average of years when the stock was within safe biological limits. Repro- ductive success in years outside of safe biological limits is indicated as dashed lines. Fishing pressure is expressed as annual fishing mortality relative to the maximum sustainable level (F/F ) and shown as dot- msy ted lines. Spawning stock size is shown relative to the one marking the border of safe biological limits There are three neighboring herring stocks to the and regular reproductive success (Figs.  4, 7, and 9), Western Baltic Sea, with different stock status and herring and cod (as well as herring in the Central Bal- reproductive success (Fig.  9). Gulf of Riga and Gulf tic) suffer from overexploitation, which has led to low of Bothnia herring show regular reproductive success population size outside of safe biological limits. Too fluctuating around the long-term mean with stock small spawning stock size reduced the probability to sizes within safe biological limits, apparently being spawn at the right place and time, which is a mov- resilient to effects of climate change so far. In con - ing, unpredictable target (Sinclair 1988). The small trast, reproductive success of Central Baltic herring spawning stock sizes were not caused by climate has been mostly below average, with spawning stock change but were the predicted and unavoidable result biomass fluctuating around the border of safe biologi - of fishing pressure far above maximum sustainable cal limits since 1995, caused by exploitation mostly levels, exacerbated by inappropriate gear selectivity. above the MSY level since 1990. This comparison Excessive fishing pressure continued even when the of herring stocks within the same large marine eco- stocks were already far outside of safe biological lim- system suggests that reproductive success was not a its with high probabilities of recruitment failure. function of climate change but rather a function of spawning stock biomass being inside or outside of safe biological limits (Figs. 4 and 9). Considerations for future management With the collapse of the high catches previously sup- Conclusion ported by large stocks of cod and herring, it will be tempting for managers to allow higher catches on The available data indicate that the commercial fish in the remaining species to fill the gap. However, fish - the Western Baltic Sea have similar resilience against ing pressure on these species is already at or near the pressures such as fisheries or climate change. But maximum sustainable level (Table  2, Fig.  7) and any while plaice, turbot, flounder, dab, and brill (as well increase will quickly lead to a decline in stock size as herring in the Gulf of Riga and the Gulf of Both- and catch. In any case, these stocks are too small to nia) show good or satisfactory relative population size replace the potential catches from healthy cod and Vol:. (1234567890) 1 3 Environ Biol Fish (2022) 105:1363–1380 1377 herring stocks (see maximum catches in Table  2). growing fast, reaching on average 4  kg for 5-year- Instead, the job at hand is to preserve high catches old cod in commercial trawl catches in Kiel Bight in (e.g., 90% of the MSY level) of the remaining flat - April 2021 (Table 3, Fig. 10). But Fig. 10 also shows fish stocks while getting serious about rebuilding the the problems of the only remaining year class being cod and herring stocks. For herring, the solution is targeted by the allowed catches, and the inappropriate straightforward and has been recommended by ICES setting of the minimum conservation reference size since 2019: stop all catch and bycatch, commercial (MCRS = 35 cm) and the corresponding selectivity of and recreational, in all of its area, until the stock has the gear, which would catch cod far smaller than the sufficiently recovered. This is straightforward because optimum size (L = 75  cm) or the optimum size at opt the gears used to catch herring are not used for any of first capture ( L = 59 cm). Also, cod are vulnerable c_opt the other species and there is no bycatch of herring to all the gears used to capture the other commercial in the main gears used for the other species. The age species. structure of herring is still complete and reproductive The required measures to conserve and rebuild success better than the recent average was observed the cod stock then are as follows: selectivity of the last in 2013, suggesting that herring reproduction is common gears has to be changed towards L for c_opt still functioning and is likely to improve once fish - cod. This will also benefit the size and age struc - ing stops and spawning stock biomass is allowed to ture of the larger flatfish (turbot, brill, plaice, and increase. For example, the estimated spawning stock flounder) and decrease the catch of unwanted small biomass in April 2020 was 58,434 tonnes while dab (Table 3, Fig. 7) and small or juvenile flatfish in catches in 2020 were 22,130 tonnes (HAWG 2021), general. For example, the selectivity of the allowed preventing a recovery of the stock. This remaining gill nets summarized in Table  3 shows that the fishing must end. 55-mm (knot to knot) gill nets are suboptimal for The situation is more complicated for cod, with cod and all flatfish but dab. The 75-mm gill nets are a severely truncated age structure (Fig.  5) and thus optimal for brill, plaice, flounder, and dab while still severely reduced genetic diversity and experience of suboptimal for turbot and cod, but a step in the right spawners. However, successful reproduction occurred direction. Mean body weight in the catch compared as recently as 2016, and the resulting year class was in with W suggests that selectivity of the 120  mm opt excellent condition (Froese et al. 2020) and has been (stretched mesh) trawls is inappropriate for cod and Fig. 10 Length frequency distribution of cod in com- mercial trawl catches with 120 mm stretched mesh size in Kiel Bight in April 2021. MCRS is the official mini- mum conservation reference size and probably close to the length L at which 50% of the individuals are retained by the gear. L c_opt is the length at first capture that would result in a mean length of L in the catch opt and L is the maximum max length observed in the area Vol.: (0123456789) 1 3 1378 Environ Biol Fish (2022) 105:1363–1380 Bundesamt für Naturschutz,  FKZ: 3521532201,  Rainer for flatfish (Table  3). Also, existing gears need to Froese,  FKZ: 3521532201,  Eva Papaioannou,  FKZ: be modified such that they continue to catch flat - 3521532201, Marco Scotti fish but reduce bycatch of cod. This can be achieved by strongly reducing the reach/height of the gears Declarations above the seafloor and by restrictions to trawling. Such measures need to remain in place until new Data and code availability All the data used here are pre- sented in the text and figures; data and code are in addition year classes have rebuilt the spawning stock to a available from https:// ocean rep. geomar. de/ 53154/. level that is large enough to ensure successful repro- The research reported herein did not involve human subjects duction. Subsequent exploitation should not exceed and/or live animals or cell lines. 90% of the maximum sustainable level. Fishers are not the culprits of the bad status of Consent The work described has not been published before cod and herring in the Western Baltic Sea, as their and is not under consideration for publication anywhere else. Its publication has been approved by all co-authors. RF collected catches corresponded, with small deviations in and analyzed the environmental and fisheries data; MS provided both directions, to the amounts set by management the plankton data and assessment; EP provided context for the (HAWG 2021; WGBFAS 2021). Fishers need com- environmental data; all authors reviewed and approved the final pensation for their losses caused by mismanage- draft. ment and incentives to actively avoid the capture of Conflict of interest The authors declare no competing inter- cod while fishing for flatfish, e.g., by avoiding times ests. and places where cod is likely to be present, and by modifying their gears such that cod are less likely to Open Access This article is licensed under a Creative Com- be caught or retained. mons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any The available data suggest that rebuilding of her- medium or format, as long as you give appropriate credit to the ring and cod with highly profitable future fisher - original author(s) and the source, provide a link to the Crea- ies with catches close to the maximum sustainable tive Commons licence, and indicate if changes were made. The level is still possible in the Western Baltic Sea, if images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated the prescriptions of the Common Fisheries Policy otherwise in a credit line to the material. If material is not of the EU (CFP 2013) are finally implemented as included in the article’s Creative Commons licence and your intended. Only such rebuilt stocks will be able to intended use is not permitted by statutory regulation or exceeds cope with the challenges posed by climate change. the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit Returning to the original question posed by this http:// creat iveco mmons. org/ licen ses/ by/4. 0/. study, the available data indicate that mismanage- ment and not climate change was responsible for the bad status of cod and herring. 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Climate change or mismanagement?

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Environ Biol Fish (2022) 105:1363–1380 https://doi.org/10.1007/s10641-021-01209-1 Rainer Froese  · Eva Papaioannou  · Marco Scotti Received: 5 July 2021 / Accepted: 23 December 2021 / Published online: 10 January 2022 © The Author(s) 2022 Abstract Climate change and deoxygenation are have a high probability of impaired reproductive suc- affecting fish stocks on a global scale, but disentan - cess. It is pointed out that allowed catches were regu- gling the impacts of these stressors from the effects of lated by management and adhered to by the fishers, overfishing is a challenge. This study was conducted meaning that unregulated fishing did not occur. Thus, to distinguish between climate change and misman- mismanagement (quotas that were too high and gears agement as possible causes for the drastic decline that selected too small sizes) and not climate change in spawning stock size and reproductive success in appears to be the primary cause of the bad status of cod (Gadus morhua) and herring (Clupea harengus) cod and herring in the Western Baltic Sea. in the Western Baltic Sea, when compared with the good or satisfactory status and reproductive success Keywords Climate change · Mismanagement · of the other commercial species in the area. Available Gear selectivity · Overfishing · Western Baltic Sea · data on water temperature, wind speed, and plank- Cod · Herring ton bloom during the spawning season did not reveal conclusive correlations between years with good and bad reproductive success of cod or herring. Notably, Introduction the other commercial species in the area have very similar life history traits suggesting similar resilience Climate change and deoxygenation are affecting the against stress caused by climate change or fishing. oceans in ways that transform the environment that The study concludes that severe, sustained overfishing commercial fish depend on for survival, growth, and plus inappropriate size selectivity of the main fishing reproduction (Brander 2010; Cheung et  al. 2013; gears have caused the decline in spawning stock bio- Poloczanska et  al. 2016). Climate change has been mass of cod and herring to levels that are known to suggested as a potential cause for declines and col- lapses of commercial fisheries (Möllmann et al. 2003; Rijnsdorp et al. 2009; Akimova et al. 2016), but such R. Froese (*) · E. Papaioannou · M. Scotti  declines and collapses have also occurred before cli- GEOMAR Helmholtz Centre for Ocean Research, mate change had a major impact on the marine envi- Düsternbrooker Weg 20, 24105 Kiel, Germany ronment (Jackson et  al. 2001; Hobday et  al. 2011), e-mail: [email protected] basically as the result of fishing pressure exceed - E. Papaioannou  ing maximum sustainable levels. Rather than being e-mail: [email protected] the primary cause of fish stock declines, climate M. Scotti  change has been suggested as a factor aggravating the e-mail: [email protected] Vol.: (0123456789) 1 3 1364 Environ Biol Fish (2022) 105:1363–1380 effects of overfishing (Pörtner and Peck 2010; Bodini options for rebuilding and sustainable future use of et  al. 2018; Free et  al. 2019; Möllmann et  al. 2021). cod (Gadus morhua) and herring (Clupea harengus). Some studies (Hare et al. 2016; Spencer et al., 2019) A novelty of the study lies in its use of data such argue that climate vulnerability assessments need to as length, weight, gonad weight, and stomach content be conducted as part of a future fisheries manage - collected by commercial fishers in Kiel Bight in the ment framework. However, disentangling impacts Western Baltic Sea (Froese et al. 2020). of climate change from impacts of inappropriate management is not straightforward as, for example, recruitment failure may be caused by unsuitable envi- Materials and methods ronmental conditions as well as too small spawning stock size. Water temperature, wind speed, and chlorophyll a The purpose of this study is to distinguish between concentration climate change and mismanagement (e.g., too high catches or gears with inappropriate selectivity) as The GEOMAR Helmholtz Centre for Ocean Research possible causes for recent collapses of two previously in Kiel, Germany, situated near the southern tip of large and productive stocks in the Western Baltic Kiel Bight, maintains records taken in 8-min intervals Sea. Such distinction is important because if climate of water temperature, wind speed, and a number of change is the main cause of decline, then there may other daily weather measurements. These measure- be little that managers can do other than adjusting ments are taken on the roof of the institute and at the fishing to reduced productivity. If, however, misman - Kiel lighthouse, which is situated near the center of agement is the main cause of the decline, then end- Kiel Bight at 54° 29.9′ N 10° 16.4′ E. Water tempera- ing mismanagement may lead to restoration of good ture is determined at 5-m depth at the pier in front of stock status and profitable fishing despite impacts of the institute and at 1-m depth at the lighthouse. For climate change (Gaines et al. 2018). the purpose of this study, daily median measurements The Baltic Sea has already experienced substantial at the institute and the lighthouse were used. changes in its hydrography as a result of recent warm- A time series of chlorophyll a concentration (µg/l) ing, with an estimated increase in sea-surface temper- was extracted from the Oceanographic Database of ature (SST) of up to 1  °C per decade for the period the International Council for the Exploration of the 1990–2008. Projections of future climate change sug- Sea (ICES) (https:// ocean. ices. dk/ HydCh em/ HydCh gest that this trend will continue in the future, with em. aspx). It includes all stations in the Western Bal- an estimated 2  °C increase in summer SST in the tic Sea delimited by longitude = [9°, 11°] E and lati- southern parts of the Baltic Sea. Such warming and tude = [54°, 55°] N, from sea surface to a maximum the continuing influx of nutrients from agriculture are depth of 44 m. expected to lead to increasing areas of hypoxia and anoxia (Hansson et  al. 2019; BACC (Baltic Earth Selected years Assessment of Climate Change) II Author Team, 2015). There can be no doubt that these changes will The years 1997, 2003, 2016, 2019, and 2020 were affect ecosystems and species by triggering complex selected for closer examination with regard to envi- nonlinear dynamics (Reusch et  al. 2018; Ito et  al. ronmental conditions. These years showed better or 2019), but the resilience of commercial species will, worse reproductive success than the average of years in addition, be affected by their history of exploitation when spawning stock biomass (SSB) was within and their current stock status (Free et al. 2019). safe biological limits, referring to a stock size below In order to disentangle the impacts of climate which recruitment is likely to be impaired (CFP change and fisheries, this study examines the status of 2013). To avoid confusion with number of recruits commercial species in the Western Baltic Sea. It com- being reported for ages zero or one, reproductive suc- pares their life history traits and related intrinsic resil- cess was reported for the year in which the spawning ience, their response to recent changes in environ- took place (WGBFAS 2021; HAWG 2021; Fig. 4). In mental conditions, their relative stock sizes, and their particular, the selected years represent good repro- history of exploitation. Finally, the study explores ductive success for cod in 1997 and 2003, for herring Vol:. (1234567890) 1 3 Environ Biol Fish (2022) 105:1363–1380 1365 in 1997 and 2003, and for plaice (Pleuronectes pla- mainly in the Kattegat (ICES 2021a). Data on life his- tessa) in 2016 and 2019. Below average reproductive tory traits (maximum length, length at which 50% of success was reported for cod in 2019 and 2020; for the females have reached maturity) were derived from herring in 2016, 2019, and 2020; and for plaice in records in DATRAS (2021), which were restricted to 2003. the Western Baltic Sea and years after 2000, and from commercial catches in Kiel Bight (Froese et al. 2020; Occurrence of Lasker-events this study). Fecundity ranges were derived from refer- ences compiled in FishBase (Froese and Pauly 2021). Lasker (1975, 1978) proposed that a period of calm Information on fishing pressure, stock size, age com - winds is essential for forming food-rich plankton position in the commercial catch, and recruitment patches that are crucial for the survival of first-feed - was derived from the stock assessments carried out ing fish larvae. Pauly ( 1989) suggested to name such by working groups of the International Council for periods “Lasker-events,” and for the purpose of this the Exploration of the Seas (ICES) (ICES 2021b; study we used continuous 4-day periods with median HAWG 2021; WGBFAS 2021). Timing of spawning wind speeds not exceeding 6 m/s as representative of was derived from the gonadosomatic index of females such events. For the selected years, the occurrence of caught by commercial fishers in Kiel Bight as part of Lasker-events was counted from March to May. The this study. The gonadosomatic index was calculated period was chosen because the planktonic larvae of as the weight of the ovaries divided by live body all considered species require high densities of zoo- weight. plankton as food during these months, and too strong winds are considered detrimental for optimal feeding Life history reference points conditions (Lasker 1975, 1978; Cushing and Hor- wood 1994). The largest length (L ) found for the Western Bal- max tic Sea either in DATRAS (2021) or in the data from Examined species and life history traits commercial fishers used in this study was taken as ref - erence point for maximum size and as a proxy for the The species considered in this study represent all asymptotic length parameter (L ) in formal growth inf commercially relevant species that reproduce in the analyses (Froese and Binohlan 2000). Growth in body Western Baltic Sea (Petereit et  al. 2014) (Table  1). weight has an inflection point at 30% of asymptotic Other commercial species are lemon sole (Microsto- body weight (W ), which corresponds to 2/3 L if inf inf mus kitt) and sprat (Sprattus sprattus), but they are of the exponent of the length–weight relationship is only minor commercial importance and do not repro- close to 3. L is the length at which cohort biomass opt duce in the area. The bulk of sprat distribution occurs reaches a maximum (Holt 1958), and it corresponds in the northeastern Baltic Sea such as the Gulf of to 2/3 L if the ratio between the average adult rate inf Finland (ICES 2020) while lemon sole can be found of natural mortality (M) and the somatic growth rate Table 1 Overview of life history traits of the commercial weight, t is the maximum reported age, and t is the age at max m fishes in the Western Baltic Sea, where L is the maximum which more than 50% of the females reach first maturity max observed length, W is the corresponding maximum body max Species Common L W t t Fecundity Eggs Larvae Main food max max max m name cm kg y y Millions Gadus morhua Cod 106 10.6 14 2–3 0.5–5 Pelagic Pelagic Benthic invertebrates; fish Scophthalmus maximus Turbot 58 4.2 22 3–4 5–15 Pelagic Pelagic Benthic invertebrates; fish Scophthalmus rhombus Brill 60 3.2 11 3–4 0.3–1.5 Pelagic Pelagic Benthic invertebrates; fish Pleuronectes platessa Plaice 57 1.5 25 2–4 0.05–0.5 Pelagic Pelagic Benthic invertebrates Platichthys flesus Flounder 52 1.4 26 2–4 0.4–2 Pelagic Pelagic Benthic invertebrates Limanda limanda Dab 47 1.1 12 2–3 0.05–0.15 Pelagic Pelagic Benthic invertebrates Clupea harengus Herring 36 0.26 17 3 0.013–0.065 Demersal Pelagic Zooplankton; benthic invertebrates Vol.: (0123456789) 1 3 1366 Environ Biol Fish (2022) 105:1363–1380 (K) is close to M/K = 1.5, as has been found in many content, gonad weight, and indication of spawning commercial species (Froese et al. 2016). For the pur- activity based on visual examination of gonads (Fro- pose of this study, it was assumed that 2/3 L is a ese et al. 2020). max reasonable proxy for L . Taking the allowed catch opt when cohort biomass is highest has the least nega- tive impact on the fished stock and results in a mean Results age and size similar to the natural one without fish - ing. Since it is not possible to catch fish only at L , a Environmental data opt similar size and age structure can also be obtained if fishing with sustainable rates starts at a smaller length Median daily water temperatures in January to May L = 0.56 L and includes all subsequent length in Kiel Bight for the selected years show a remark- c_opt inf and age classes (Froese et al. 2016). able dichotomy of interannual variability, with very different trajectories and differences in temperature of Data collected by fishers up to 5 °C in January to March, and very similar tra- jectories with only up to 2 °C interannual differences GEOMAR has a collaboration with several com- in April and May (Fig. 1). The hottest year on record mercial fishers operating in the southern Kiel Bight, was 2020, whereas 2021 was one of the coolest years south of a line from Damp to Fehmarn. The fishers after mid of February. operated with gill nets of 55- to 110-mm mesh size The high monthly and annual variability in tem- (knot to knot) which were set to up to 24  h in dif- perature in January to March makes it difficult to ferent locations, from shallow (e.g., 4  m) to deeper use temperature as a reliable environmental trigger areas (about 15  m). In addition, in 2021, one fisher for gonad development and spawning. For example, with a 14-m boat operated a small trawl with 120- if an increase from lower temperatures to 4  °C was mm stretched mesh size and a typical net opening of such a trigger, then that occurred in early February in 10–11-m width and 2-m height. The collected data 2016, in early March in 1997, in late March in 2003, were locality, depth, gear used, and date and time 2019, and 2021, and never in 2020. Reproductive suc- of deployment, and for every species, length, wet cess of cod, herring, and plaice in the selected years weight, stomach weight, indication of main stomach does not consistently favor or disfavor any one year. Fig. 1 Comparison of daily median water temperatures in outside of safe biological limits in a given year, then the spe- Kiel Bight for the years indicated in the legend. Reproductive cies name in the legend is enclosed in parentheses. Note that success in these years for cod, herring (her), and plaice (ple) the depicted data for plaice refer to the stock in ICES subdivi- is indicated as above + or below − the average of years when sions 21–23 [TmpWnd2.R] biomass was within safe biological limits. If the stock size was Vol:. (1234567890) 1 3 Environ Biol Fish (2022) 105:1363–1380 1367 However, reproductive success was below average and the lowest number of Lasker-events, plaice had when spawning stock biomass was outside of safe extraordinarily good recruitment (Fig.  4), whereas biological limits (SSB < B ) (CFP 2013) (Fig. 1). the bad recruitment of cod and herring may have pa The monthly distribution of median daily wind been caused by their spawning stock biomass being speed in March to May looked very similar across outside of safe biological limits. the selected years (Fig.  2), with the exception of Monthly density of phytoplankton as represented 2019, when median wind speed was highest. The by measurements of chlorophyll a (Chl) was simi- number of Lasker-events ranged from 3 in 2019 to lar across the selected years with available data 9 in 2003 and 2016. There was no apparent correla- (Fig.  3), with a peak in March and similar densi- tion between reproductive success of cod, herring, ties in May. , when densities in April and May were or plaice and the number of Lasker-events. In the lowest compared with the other years. Yet, 2019 year (2019) with the highest median wind speed Fig. 2 Boxplot of wind speeds in selected years in March to May in Kiel Bight, with indication of number of calm periods (Lasker-events, bold num- bers). Below the Lasker- event numbers, reproductive success of cod, herring, and plaice in the given year is indicated as above + or below − average. If stock size in the given year was outside of safe biologi- cal limits, the indicator is enclosed in parentheses. Note that the depicted data for plaice refer to the stock in ICES subdivisions 21–23 [TmpWnd.R] Fig. 3 Seasonal timing and density of phytoplankton blooms in the Western Baltic Sea. Phytoplankton dynamics are indicated by chlorophyll a (Chl) concen- tration Vol.: (0123456789) 1 3 1368 Environ Biol Fish (2022) 105:1363–1380 Fig. 4 Time series of rela- tive reproductive success, fishing pressure, and spawn - ing stock sizes for Western Baltic cod, herring, and plaice. Reproductive suc- cess is expressed relative to the average of years when the stock was within safe biological limits. Repro- ductive success in years outside of safe biological limits is indicated as dashed lines. Fishing pressure is expressed as annual fishing mortality relative to the maximum sustainable level (F/F ) and shown as dot- msy ted lines. Spawning stock size is shown relative to the one marking the border of safe biological limits was the year with extraordinary good reproductive the Western Baltic Sea have been overexploited at some success of plaice (Fig. 4). point during the past 30  years (1990–2020) (Table  2, Fig.  4) (HAWG 2021; WGBFAS 2021). However, Life history traits while the overexploitation of flatfish was less severe and ended in recent years (WGBFAS 2021) (Fig.  4), The commercially important fish species in the West - continuous overexploitation of cod and herring was 2–5 ern Baltic Sea consist of 5 flatfish species in addition times the maximum sustainable level since the 1990s to cod and herring. With the partial exception of her- (Fig. 4). ring, their life history traits are remarkably similar Recent (2020) stock status was above or around the with regard to habitat (demersal), maximum body level that can produce maximum sustainable yields size (1–10  kg), food (mostly benthic invertebrates, (MSY) (CFP 2013) for all commercial fish except cod some fish), longevity (11–26  years), age at matu - and herring, which were suffering from depleted stock rity (2–4  years), spawning season (spring), and high sizes and very low reproductive success (Table  2, fecundity with pelagic eggs and larvae. Herring dif- Fig.  4) (HAWG 2021; WGBFAS 2021). Herring has fers by being the smallest species (0.26  kg), feeding supported the highest catches in the area with nearly mostly on zooplankton, with demersal deposition of 200,000 tonnes in 1992, followed by cod with 49,000 eggs (Table 1). tonnes in 1996. In comparison, the sum of the high- est catches of flatfish is only about 15,000 tonnes. But Exploitation and recent stock status catches of herring in 2020 were only 22,000 tonnes and for cod 4,363 tonnes, i.e., about only 10% of pre- With the possible exceptions of dab (Limanda limanda) vious maximum catches, indicating the depleted sta- and flounder ( Platichthys flesus ), all commercial fish in tus of these stocks. Vol:. (1234567890) 1 3 Environ Biol Fish (2022) 105:1363–1380 1369 Vol.: (0123456789) 1 3 Table 2 Recent (2020) stock status, fishing pressure, commercial interest, and related comments for the commercial fishes in the Western Baltic Sea. The maximum catch is meant to give an impression of relative stock size and importance to the fishery Name Stock status Fishing pressure Commercial interest Comments Cod Truncated age/size structure; very low Severe overfishing in the past; ongoing Used to be the commercially most Only the 2016 year class supports the biomass; repeated recruitment failures overfishing. Max catch 49,000 tonnes important species in the fishery remaining fishery and reproduction; in 1996 gear selectivity is too small for this large species Turbot Below MSY level Above MSY level. Max catch 1,200 High market value but low abundance No MSY level assessment; gear selectiv- tonnes in 1996 ity is too small for this large species Brill Probably around MSY level Probably around MSY level. Max catch Good market value but low abundance No MSY level assessment; stock size 160 tonnes in 1995 indicator about stable since 2004; gear selectivity is too small for this large species Plaice Recently above MSY level Severe overfishing in the past; recently Currently the most important species in Recovery despite ongoing overfishing not near MSY level. Max catch 8,000 the fishery well understood tonnes in 2002 Flounder Apparently around MSY level Apparently around MSY level. Max Limited market in Germany No MSY level assessment; recent decline catch about 3,000 tonnes in 2001 in in catch and stock size suggests MSY the area level has been reached Dab Apparently above MSY level Apparently below MSY level; no No market in Germany, exported to the No MSY level assessment; stock size targeted fishing but common bycatch. Netherlands indicator and size structure suggest Max catch 3,100 tonnes in 1994 healthy stock size; gear selectivity optimal for this small species Herring Very low biomass; very low recruitment Severe overfishing. Max catch 194,000 Used to be the staple species Scientific advice is for zero catch since tonnes in 1992 2019 but fishing continues 1370 Environ Biol Fish (2022) 105:1363–1380 The age structure of cod in 2020 was strongly trun- provided by commercial fishers in Kiel Bight (Froese cated, with the stock consisting nearly exclusively of et  al. 2020 and this study). No comparison with the one year class of 4-year-old cod, instead of the seven reference points was possible for herring because they year classes being present in 1985–2020 (WGBFAS were not caught by the analyzed gears. 2021) (Fig.  5). Exploitation of this remaining year Mean weight in trawl catches of about 4  kg was class continued with a fishing pressure of F = 0.8 above W for cod; however, this is due to the absence opt compared to the maximum sustainable level of of smaller cod (Fig. 5), which would have been other- F = 0.26 applicable to a stock within safe biologi- wise retained by the gear designed to catch cod from msy cal limits (ICES 2021b). 35  cm and about 0.5  kg onward, and their presence The age composition of herring in commercial would have reduced the mean. Despite the distorted catches in 2020 in the Western Baltic Sea (HAWG size distribution, L and L were below L and mean c opt 2021) shows a severe lack of individuals in all age L in the 55- and 75-mm gill nets, indicating that c_opt classes when compared with the mean of 2000–2005, selectivity of all gears was too small for cod. indicating a severely reduced stock size and a lack of Selectivity of the 55-mm gill net was too small recruitment in recent years (Fig. 6). for turbot, plaice, and flounder, but adequate for dab. Selectivity of the 75-mm gill net was too small for Selectivity turbot but adequate for the other flatfish. Mean weight in the trawl catch was below W for opt Table 3 reports reference points for healthy size struc- plaice, flounder, and dab, and presumably also for the ture (L , W ), for optimum selectivity of gears larger turbot and brill for which insufficient data were opt opt (L , W ), and for maturation (L and L ) for available, indicating that the selectivity of the trawl c_opt c_opt m90 m50 the commercial fish species in the Western Baltic Sea. was too small for these species. For comparison with L and W , Table  3 presents Aggregated length frequencies for plaice, floun - opt opt the mean length and weight of the catch of common der, and dab in commercial gill net catches in 2020 gears, and for comparison with L the selectivity and 2021 in Kiel Bight show frequency peaks near c_opt (L ) of some of these gears. Mean length, mean weight, or above L (Fig.  7), indicating that these stocks c opt and mean length at first capture were derived from data were in reasonably good condition and not subject Fig. 5 Age composition of commercial catches of Western Baltic cod in mil- lions by age class in 2020 (dark gray) compared with mean catches in 1985–2000 (light gray) [WBS_2020. xlsx] Vol:. (1234567890) 1 3 Environ Biol Fish (2022) 105:1363–1380 1371 Fig. 6 Age composition of commercial catches of her- ring in the Western Baltic Sea, in millions at age. The light gray bars are the mean of 2000–2005 whereas the dark bars indicate the abun- dance in 2020 [WBS_2020. xlsx] Table 3 Reference values for length (L ) and weight (W ) 75-mm mesh size (knot to knot) and a bottom trawl with 120- opt opt for which increase in body weight is highest, length (L ) mm stretched mesh size, as observed in this study. Bold val- c_opt and weight (W ) when fishing should start, length at which ues indicate suitability of the gear for the respective species. c_opt 90% and 50% of the females reach maturity (L , L ). This Lengths are given in centimeters and weights (W , W ) in m90 m50 opt c_opt is compared with selectivity of gill nets with 55-mm and grams Name L W L W L L 55-mm net 75-mm net Trawl opt opt c_opt c_opt m90 m50 L L L L W mean c mean c mean Cod 71 3202 59 1984 36 27 46 40 58 40 4011 Turbot 38 1275 32 761 38 26 33 31 36 30 NA Brill 40 927 33 544 40 28 NA NA 43 36 NA Plaice 38 532 32 342 31 20 32 28 37 33 263 Flounder 35 471 29 283 33 21 32 28 36 31 285 Dab 31 339 26 184 23 17 29 27 33 30 197 Herring 22 77 18 46 16 14 NA NA NA NA NA to overfishing (Froese et al. 2016, 2018). The situ- Timing of spawning ation was different for the larger-sized turbot, for which the frequency peak occurred before L Gonadosomatic index values of female fish caught c_opt and several of the larger length classes were under- by commercial fishers in Kiel Bight from January to represented, indicating less satisfactory stock size May were available in 2020 and 2021 for cod and in and at least temporary overexploitation. 2021 for plaice and dab. Gonadosomatic index values Vol.: (0123456789) 1 3 1372 Environ Biol Fish (2022) 105:1363–1380 Fig. 7 Length frequencies for turbot, plaice, flounder, and dab in commercial catches in 2020 and 2021 in Kiel Bight, with gill nets ranging from 55- to 110-mm mesh size (knot to knot) [Kfish_27.R] above 0.07 were used as indication of active spawn- area (Kiel Bight) in January and February, when tem- ing (Froese et al. 2020). peratures in 1–5-m depth in recent years were 3–5 °C Cod spawning commenced in December (data higher than in, e.g., 1997 or 2003, but not in March before mid-February 2020 are missing), peaked in to May, when temperatures were very similar (Fig. 1). early March, and ended in early April in both 2020 Seasonal timing and density of phytoplankton blooms and 2021. Spawning of plaice commenced in Decem- show no major differences between years with good ber 2019 and ended at the end of February 2020. and bad reproductive success of cod and herring Spawning of dab started in January 2020 and ended (Fig.  3). The year 2019 had intermediate water tem- in early May (Fig. 8). peratures but strong wind, few Lasker-event, low chlorophyll a concentration in April and May, and below-average reproductive success of cod and her- Discussion ring; however, it supported extraordinary good repro- ductive success of plaice throughout the area (Fig. 4). The purpose of this study was to distinguish between All commercial species in the Western Bal- climate change and mismanagement as the possi- tic Sea  feed on benthic invertebrates (polychaete ble cause for the bad status of herring and cod in the worms, crabs, shrimps, bivalves, and small benthic Western Baltic Sea. The first approach to answer this fish) except for herring, which feeds on zooplank - question was to look at patterns of water temperature, ton and some benthic invertebrates. Large cod, tur- wind speed, and chlorophyll a density in selected bot, and brill also feed on larger fish (Table  1). All years when reproductive success of cod or herring species are highly fecund with pelagic eggs (except was above or below average. The second approach herring, which has demersal eggs) and planktonic was to compare life history traits and management of larvae (Table  1), and spawn in the area between Jan- herring and cod with those of the other commercial uary and May (Fig.  8). They mature between 2 and fishes in the area. 4  years of age, suggesting similar generation times and thus similar intrinsic rates of population increase Climate change and resilience (Pianka 2000). Based on a combination of life history traits, all species are classified in FishBase as having Influence of climate change on interannual variabil - medium resilience and potential to recover from low ity of water temperature may be visible in the study population sizes (Froese and Pauly 2021), and one Vol:. (1234567890) 1 3 Environ Biol Fish (2022) 105:1363–1380 1373 Fig. 8 Gonadosomatic index of females in Kiel Bight in 2020 for cod, plaice, and dab. Index values above 0.07 are interpreted as active spawn- ing. Data for cod in 2020 are incomplete as sampling started only in mid-Febru- ary [Kfish_27.R] Vol.: (0123456789) 1 3 1374 Environ Biol Fish (2022) 105:1363–1380 would expect them to have similar responses to envi- biomass was too small (as confirmed by Figs.  4 and ronmental changes in the Western Baltic Sea. 5) to supply all of its spawning area with sufficient Available length-frequency data for turbot, plaice, numbers of eggs during the whole duration of spawn- flounder, and dab in commercial catches in Kiel ing season. Bight (Fig. 7) show no gaps that would suggest recent Given that the other commercial stocks in the years with bad reproductive success. In contrast, the Western Baltic showing no signs of suffering from age composition of cod in commercial catches in the climate change are all flatfish, one could suspect that Western Baltic Sea in 2020 (WGBFAS 2021, Fig.  5) flatfish are somehow more resilient to climate change shows practically only one remaining year class (of than herring and cod. However, previous studies sug- 2016), and a comparison with mean age composi- gest that flatfish in the Baltic Sea have similar or even tion in 1985–2000 indicates that millions of cod are less resilience to climate change (MacKenzie et  al. missing in the other age classes, suggesting numerous 2007; HELCOM 2021). recruitment failures. This is confirmed by observed In summary, available time series of water tem- recruitment data, shown as reproductive success of perature, wind speed, and chlorophyll a concentra- the respective years in Fig.  4, with very low repro- tion show no conclusive connection with the repro- ductive success in 2007, 2015, and 2017, and accord- ductive failures of cod or the continuously declining ing to preliminary data also in 2018, 2019, and 2020 reproductive success in herring (Fig.  4). Instead, the (WGBFAS 2021). The period of low reproductive available data suggest that the environmental condi- success coincides with the periods from 1998 (ICES tions supported regular and even better-than-average 2018) or 2008 (WGBFAS 2021) onward, when reproductive success of the other spring-spawning spawning stock biomass was considered too low to commercial fish in the area. ensure successful reproduction. The age composition of herring in commercial catches in the Western Baltic Sea (Fig.  6) shows the Management presence of all age classes, suggesting no complete failure of reproduction in recent years. However, The second approach to disentangle impacts of cli- abundances especially at ages 1 to 4 are only 0.6–5% mate change or mismanagement as causes for the of the mean of 2000–2005 and lower than in subse- bad status of cod and herring in the Western Baltic quent year classes, highlighting hundreds of millions Sea was to compare exploitation and stock status of of missing herring and suggesting declining repro- herring and cod with those of the other commercial ductive success in recent years. This is confirmed fishes in the area, with the sub-population of cod in by observed recruitment data, shown as reproduc- the Öresund, and with three herring stocks in the tive success in the respective years in Fig.  4, with Central and Northeastern Baltic Sea. declining reproductive success since 2004 and lowest Since 1997, fishing pressure on cod was 3–5 times success on record in 2020. The period of low repro- the maximum value that the stock can support. Con- ductive success coincides with the period from 2003 sequently, spawning stock biomass shrunk by 64% onward, when spawning stock biomass is considered from 1997 to 2018 (WGBFAS 2021) (Fig. 4). Despite too low to ensure successful reproduction (HAWG the obvious biomass decline and the stock being out- 2021) (Fig. 4). side of safe biological limits (CFP 2013), severe over- It has been suggested (Froese et al. 2020) that the fishing never stopped, with F still 3 times the MSY unusually warm water temperature in January–March level in 2020 (ICES 2021b). Similarly, herring was 2020 (Fig. 1) has led cod to spawn too early such that subject to severe overfishing ( F/F > 1.5) from 1997 msy its zooplankton-feeding larvae missed the zooplank- to 2009 and 2016 to 2018, despite spawning stock ton bloom expected to occur in late March, April, and biomass being outside of safe biological limits and May. However, subsequent gonadosomatic index data declining by 60% from 1997 to 2020 (HAWG 2021) for female cod in Kiel Bight in 2020 (Fig. 8) confirm (Fig. 4). Since 2019, the International Council for the spawning activity until end of March, similar to the Exploration of the Sea (ICES) recommended closure regular spawning activity of plaice and dab (Fig.  8). of the herring fishery, but management allowed fish - Instead, it seems more likely that cod spawning stock ing to continue. Vol:. (1234567890) 1 3 Environ Biol Fish (2022) 105:1363–1380 1375 Similar to cod and herring, plaice was subject insufficient for brill, but given its similarity in size to very high levels of fishing pressure from 1999 and body shape to turbot, the turbot assessment to 2016, but other than in cod and herring, spawn- probably also applies to brill. ing stock biomass kept fluctuating around the bor - No length-composition data were available for her- der of safe biological limits (CFP 2013) and started ring, but the very high abundance of immature 1- and increasing in 2013, despite ongoing overfishing of 2-year-old herring in commercial catches shown in F/F = 1.5. The continued increase in biomass then Fig.  6 suggests that gear selectivity was inappropri- msy led to a decline in fishing pressure, below F in ate. For cod in the Western Baltic Sea, current com- msy 2020, ending overfishing of plaice for the first time mercial gear selectivity was difficult to establish in the time series. It is important to note that this because of the strongly truncated age structure, but recovery is not due to management reducing catches the minimum marketing size is 35  cm (WGBFAS to allow the stock to recover, because total catches 2021) whereas optimum length at first capture  is (landings + discards) increased from 3,594 tonnes in L = 59 cm (Table 3). Age composition in the com- c_opt 2013 to 4,470 tonnes in 2020 (+ 24%). The reasons mercial catch of 1985–2000 shows a high abundance for the recovery of the plaice stock despite ongoing of immature cod of 1–2  years of age. Together with overfishing and increasing catches are not well under - far too high fishing pressure, this inappropriate early stood and beyond the scope of this study. It may be onset and thus extended duration and impact of fish - a mixture of improved reproductive success since ing mortality is one of the reasons for the bad status 2009 (Fig.  4), reduced natural mortality of juveniles of the cod stock. due to absence of large cod before 2020 (Fig.  5), In summary, inappropriate selectivity of the main reduced competition for benthic food due to repeated commercial gears has contributed to the truncated age reproductive failures and thus absence of juvenile structure of cod (Fig.  5) and the distorted age struc- cod (Fig. 5), more appropriate selectivity of the main ture of herring, whereas the more appropriate gear gears (Table 3), and overfishing being less severe than selectivity for dab, plaice, and flounder has contrib - in cod and herring due to limited demand. But clearly, uted to their good size structure, stock status, and better than average plaice reproduction and increase apparent resilience against climate change. in stock size (Fig.  4) happened despite full exploita- It has been shown that the sub-population of cod tion and impact of climate change on the Western in the Öresund, which are part of the Western Baltic Baltic environment. Similarly, the available length cod stock (ICES 2021b), did not show a truncated age frequencies (Fig.  7) and stock size indicators (WGB- structure in 2010 (Svedäng and Hornborg 2017, their FAS 2021) of the other commercial fish in the area Fig. 3), presumably because trawling is banned in that (Table  3) do not show signs of reproductive failure, area since 1932, common gill nets have slightly larger depleted stock size, or severe overexploitation. mesh sizes (65  mm instead of 55  mm knot-to-knot), The importance of capturing fish at the right and the shipping traffic lanes act as de facto no-take size is not to be underestimated: it has been for- zones. However, fishing pressure was always high mally shown that it is theoretically impossible to and practically unlimited in the area, thus attracting collapse a stock if all individuals are allowed to fishers from neighboring areas when their  catches reproduce before capture (Myers and Mertz 1998). were low (Lindegren et al. 2013). For example, in the With regard to the appropriateness of management period 2017–2020, the mean proportion of Öresund of technical measures such as the size selectivity landings of the overall cod landings increased by 71% of the allowed gears, the peak in length frequen- compared to 2000–2016 (ICES 2021b, their Table 9). cies of plaice, flounder, and dab was slightly above In 2020, 89% of the Öresund landings consisted only L , suggesting that gear selectivity was appropri- of 4-year-old individuals, thus showing the same opt ate for these species (Table 3, Fig. 7). In turbot, the truncated age structure as the rest of the Western Bal- peak in frequencies was below L and L , sug- tic Sea (WGBFAS 2021, Table 2.32 at page 146) and opt c_opt gesting that gear selectivity was inappropriate (too suggesting that slightly larger mesh sizes, trawling small, too early) for this large species. This may be ban, and no-take zones are not sufficient to counteract one of the reasons for the less satisfactory stock sta- fishing pressure up to 5 times the maximum sustain - tus of turbot (WGBFAS 2021). Available data were able level (Lindegren et al. 2013, their Fig. 2b). Vol.: (0123456789) 1 3 1376 Environ Biol Fish (2022) 105:1363–1380 Fig. 9 Time series of relative reproductive suc- cess, fishing pressure, and spawning stock sizes for adjacent herring stocks in the Central Baltic, the Gulf of Riga, and the Gulf of Bothnia. Reproductive suc- cess is expressed relative to the average of years when the stock was within safe biological limits. Repro- ductive success in years outside of safe biological limits is indicated as dashed lines. Fishing pressure is expressed as annual fishing mortality relative to the maximum sustainable level (F/F ) and shown as dot- msy ted lines. Spawning stock size is shown relative to the one marking the border of safe biological limits There are three neighboring herring stocks to the and regular reproductive success (Figs.  4, 7, and 9), Western Baltic Sea, with different stock status and herring and cod (as well as herring in the Central Bal- reproductive success (Fig.  9). Gulf of Riga and Gulf tic) suffer from overexploitation, which has led to low of Bothnia herring show regular reproductive success population size outside of safe biological limits. Too fluctuating around the long-term mean with stock small spawning stock size reduced the probability to sizes within safe biological limits, apparently being spawn at the right place and time, which is a mov- resilient to effects of climate change so far. In con - ing, unpredictable target (Sinclair 1988). The small trast, reproductive success of Central Baltic herring spawning stock sizes were not caused by climate has been mostly below average, with spawning stock change but were the predicted and unavoidable result biomass fluctuating around the border of safe biologi - of fishing pressure far above maximum sustainable cal limits since 1995, caused by exploitation mostly levels, exacerbated by inappropriate gear selectivity. above the MSY level since 1990. This comparison Excessive fishing pressure continued even when the of herring stocks within the same large marine eco- stocks were already far outside of safe biological lim- system suggests that reproductive success was not a its with high probabilities of recruitment failure. function of climate change but rather a function of spawning stock biomass being inside or outside of safe biological limits (Figs. 4 and 9). Considerations for future management With the collapse of the high catches previously sup- Conclusion ported by large stocks of cod and herring, it will be tempting for managers to allow higher catches on The available data indicate that the commercial fish in the remaining species to fill the gap. However, fish - the Western Baltic Sea have similar resilience against ing pressure on these species is already at or near the pressures such as fisheries or climate change. But maximum sustainable level (Table  2, Fig.  7) and any while plaice, turbot, flounder, dab, and brill (as well increase will quickly lead to a decline in stock size as herring in the Gulf of Riga and the Gulf of Both- and catch. In any case, these stocks are too small to nia) show good or satisfactory relative population size replace the potential catches from healthy cod and Vol:. (1234567890) 1 3 Environ Biol Fish (2022) 105:1363–1380 1377 herring stocks (see maximum catches in Table  2). growing fast, reaching on average 4  kg for 5-year- Instead, the job at hand is to preserve high catches old cod in commercial trawl catches in Kiel Bight in (e.g., 90% of the MSY level) of the remaining flat - April 2021 (Table 3, Fig. 10). But Fig. 10 also shows fish stocks while getting serious about rebuilding the the problems of the only remaining year class being cod and herring stocks. For herring, the solution is targeted by the allowed catches, and the inappropriate straightforward and has been recommended by ICES setting of the minimum conservation reference size since 2019: stop all catch and bycatch, commercial (MCRS = 35 cm) and the corresponding selectivity of and recreational, in all of its area, until the stock has the gear, which would catch cod far smaller than the sufficiently recovered. This is straightforward because optimum size (L = 75  cm) or the optimum size at opt the gears used to catch herring are not used for any of first capture ( L = 59 cm). Also, cod are vulnerable c_opt the other species and there is no bycatch of herring to all the gears used to capture the other commercial in the main gears used for the other species. The age species. structure of herring is still complete and reproductive The required measures to conserve and rebuild success better than the recent average was observed the cod stock then are as follows: selectivity of the last in 2013, suggesting that herring reproduction is common gears has to be changed towards L for c_opt still functioning and is likely to improve once fish - cod. This will also benefit the size and age struc - ing stops and spawning stock biomass is allowed to ture of the larger flatfish (turbot, brill, plaice, and increase. For example, the estimated spawning stock flounder) and decrease the catch of unwanted small biomass in April 2020 was 58,434 tonnes while dab (Table 3, Fig. 7) and small or juvenile flatfish in catches in 2020 were 22,130 tonnes (HAWG 2021), general. For example, the selectivity of the allowed preventing a recovery of the stock. This remaining gill nets summarized in Table  3 shows that the fishing must end. 55-mm (knot to knot) gill nets are suboptimal for The situation is more complicated for cod, with cod and all flatfish but dab. The 75-mm gill nets are a severely truncated age structure (Fig.  5) and thus optimal for brill, plaice, flounder, and dab while still severely reduced genetic diversity and experience of suboptimal for turbot and cod, but a step in the right spawners. However, successful reproduction occurred direction. Mean body weight in the catch compared as recently as 2016, and the resulting year class was in with W suggests that selectivity of the 120  mm opt excellent condition (Froese et al. 2020) and has been (stretched mesh) trawls is inappropriate for cod and Fig. 10 Length frequency distribution of cod in com- mercial trawl catches with 120 mm stretched mesh size in Kiel Bight in April 2021. MCRS is the official mini- mum conservation reference size and probably close to the length L at which 50% of the individuals are retained by the gear. L c_opt is the length at first capture that would result in a mean length of L in the catch opt and L is the maximum max length observed in the area Vol.: (0123456789) 1 3 1378 Environ Biol Fish (2022) 105:1363–1380 Bundesamt für Naturschutz,  FKZ: 3521532201,  Rainer for flatfish (Table  3). Also, existing gears need to Froese,  FKZ: 3521532201,  Eva Papaioannou,  FKZ: be modified such that they continue to catch flat - 3521532201, Marco Scotti fish but reduce bycatch of cod. This can be achieved by strongly reducing the reach/height of the gears Declarations above the seafloor and by restrictions to trawling. Such measures need to remain in place until new Data and code availability All the data used here are pre- sented in the text and figures; data and code are in addition year classes have rebuilt the spawning stock to a available from https:// ocean rep. geomar. de/ 53154/. level that is large enough to ensure successful repro- The research reported herein did not involve human subjects duction. Subsequent exploitation should not exceed and/or live animals or cell lines. 90% of the maximum sustainable level. Fishers are not the culprits of the bad status of Consent The work described has not been published before cod and herring in the Western Baltic Sea, as their and is not under consideration for publication anywhere else. Its publication has been approved by all co-authors. RF collected catches corresponded, with small deviations in and analyzed the environmental and fisheries data; MS provided both directions, to the amounts set by management the plankton data and assessment; EP provided context for the (HAWG 2021; WGBFAS 2021). Fishers need com- environmental data; all authors reviewed and approved the final pensation for their losses caused by mismanage- draft. ment and incentives to actively avoid the capture of Conflict of interest The authors declare no competing inter- cod while fishing for flatfish, e.g., by avoiding times ests. and places where cod is likely to be present, and by modifying their gears such that cod are less likely to Open Access This article is licensed under a Creative Com- be caught or retained. mons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any The available data suggest that rebuilding of her- medium or format, as long as you give appropriate credit to the ring and cod with highly profitable future fisher - original author(s) and the source, provide a link to the Crea- ies with catches close to the maximum sustainable tive Commons licence, and indicate if changes were made. The level is still possible in the Western Baltic Sea, if images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated the prescriptions of the Common Fisheries Policy otherwise in a credit line to the material. If material is not of the EU (CFP 2013) are finally implemented as included in the article’s Creative Commons licence and your intended. Only such rebuilt stocks will be able to intended use is not permitted by statutory regulation or exceeds cope with the challenges posed by climate change. the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit Returning to the original question posed by this http:// creat iveco mmons. org/ licen ses/ by/4. 0/. study, the available data indicate that mismanage- ment and not climate change was responsible for the bad status of cod and herring. 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Journal

Environmental Biology of Fishes – Springer Journals

Published: Oct 1, 2022

Keywords: Climate change; Mismanagement; Gear selectivity; Overfishing; Western Baltic Sea; Cod; Herring

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