ICES Journal of Marine Science: Journal du Conseil

Fox, Clive, J;Albalat,, Amaya;Valentinsson,, Daniel;Nilsson, Hans, C;Armstrong,, Frank;Randall,, Peter;Catchpole,, Thomas

doi: 10.1093/icesjms/fsaa037pmid: N/A

Abstract When discarded from bottom trawl fisheries, survival of Nephrops norvegicus may be sufficiently high that this species can be exempted from the EU Landing Obligation. In three studies, Nephrops were sampled from trawlers in northern European waters, and the fate of individuals monitored for a minimum of 13 days in onshore tanks. Winter estimates of captive survival (means ± 95% confidence intervals), including immediate mortality during catch sorting, were 62 ± 2.8% for the West of Scotland, 57 ± 1.8% for the Farne Deeps (North Sea), and 67 ± 5.4% for the Skagerrak. The Farne Deeps fishery is not active in summer, but captive survival rates in summer in the other two areas were reduced to 47 ± 3.4% for West of Scotland and 40 ± 4.8% for the Skagerrak. Linear modelling of the West of Scotland and Skagerrak data suggested that higher survivals in winter were related to colder water or air temperatures, although temperatures during captive observation may also have had an impact. Net modifications in the Skagerrak study had an effect on survival, which was higher for Nephrops sampled from nets equipped with the more selective Swedish sorting grid compared to Seltra trawls. Introduction One of the main aims of the European Union’s reformed Common Fisheries Policy [Regulation (EU) No. 1380/2013] is to reduce unwanted catches through a phased landing obligation for regulated species, the obligation being fully implemented in 2019. While technical measures that allow unwanted animals to escape before being brought onto the vessel are encouraged (Catchpole et al., 2017), such measures rarely eliminate the unwanted components of the catch completely (Broadhurst et al., 2006). The landing obligation thus includes exemptions and flexibility tools including for “species for which scientific evidence demonstrates high survival rates, taking into account the characteristics of the gear, of the fishing practices and of the ecosystem”. Producing robust estimates of post-discard survival has thus become a focus for research because evidence from such studies influences whether exemptions will be granted (Morfin et al., 2017). Allowing continued discarding of organisms with demonstrated high survivability does make conservation sense since a high proportion should survive and contribute to the stock (Rihan et al., 2019). Nephrops norvegicus is a small decapod crustacean that has become increasingly important in many European fisheries from Norway to the Bay of Biscay (Ungfors et al., 2013). Because individual Nephrops are encased in a strong exoskeleton and lack gas-filled body cavities, it has been suggested that this species should be a suitable candidate for the high survivability exemption from the landing obligation. Most discard survival estimates have come from captive observation where Nephrops are sampled from fishing vessels and held in captivity, recording their survival over time. However, historical survival estimates from trawl fisheries have been rather variable. Published rates include 17–18% (Campos et al., 2015); 19 or 31% depending on area (Charuau et al., 1982); 42 or 75% depending on area, trawler type, and sea conditions (Edwards and Bennett, 1980); 51% (Méhault et al., 2016); and 56–70% (Guéguen and Charuau, 1975). Some of this variation may be due to differences in fishing gears, methods of handling, or environmental conditions, but the ICES Workshop on Methods for Estimating Discard Survival (WKMEDS) suggested that variable experimental approaches might also be an important factor (ICES, 2014). For example, some studies have monitored survival using cages or containers placed on the seabed (Guéguen and Charuau, 1975; Campos et al., 2015; Méhault et al., 2016), while other studies have monitored survival in aquaria. In some studies, monitoring times may also have been too short because mortality can be delayed (Wileman et al., 1999). As one of their outputs, WKMEDS produced methodological guidance with the aim of improving the robustness and reproducibility of results from discard survival experiments (ICES, 2014). For captive observations, recommendations included assessing initial animal condition using standardized criteria, monitoring for a sufficient time and incorporating control subjects to evaluate the effect of holding conditions. The main aim of the present study was to compare the survival of discarded Nephrops across three distinct northern European trawl fisheries. Although the research followed the WKMEDS recommendations, there were inevitably some methodological differences as the three studies were conducted by different research groups (Valentinsson and Nilsson, 2015; Armstrong et al., 2016; Fox and Albalat, 2018). Links between survival and biological (sex, damage, and vitality), environmental (sea and air temperature), and operational factors (haul duration, catch weights, and sorting times) were also examined in order to suggest changes in trawling practice that might increase the survival of discarded Nephrops. Methods Operational factors Study 1 was undertaken using the MFV Ocean Trust (PD787), a 24-m stern trawler operating out of Mallaig (Scottish west coast, ICES Division VIa, Figure 1). Fishing took place in winter and summer on commercial grounds that were reasonably close to Mallaig to allow experimental animals to be returned to the Scottish Association for Marine Science (SAMS) aquarium, which is ∼86 miles south by road, in reasonable time. Fishing gear comprised a commercial twin-rig Nephrops trawl with both nets fitted for half the hauls with 80-mm and half the hauls with 100-mm diamond-mesh cod-ends. A 200-mm square-mesh escape panel (SMP) was fitted in the top sheet of each net in accordance with local regulations, but the nets did not have any further selectivity modifications (Table 1). Figure 1. Open in new tabDownload slide Locations of experimental hauls. Figure 1. Open in new tabDownload slide Locations of experimental hauls. Table 1. Summary of study locations, season (W—winter; S—summer), fishing gears, number of hauls, dates, tow durations and catch weights (mean ± standard deviation), and the biological factors recorded on board the fishing vessels (Y—yes; N—no) Study . ICES Division . Season . Cod-end mesh (mm) . Gear mods . Number of hauls . Year . Dates . Tow duration (h) . Nephrops catch (kg) . Non- Nephrops catch (kg) . Biological factors recorded . CL . DL . DS . DM . DP . DV . 1 VIa W 80 SMP 6 2017 6 March–8 March 3.6 ± 0.3 189 ± 55 40 ± 11 Y Y Y Y Y Y 100 SMP 6 2017 15 February–17 February 3.9 ± 0.2 183 ± 81 58 ± 15 Y Y Y Y Y Y S 80 SMP 6 2016 19 August–17 September 3.6 ± 0.3 152 ± 43 95 ± 41 Y Y Y Y Y Y 100 SMP 6 2016 15 July–18 August 3.4 ± 0.5 309 ± 120 57 ± 31 Y Y Y Y Y Y 2 IVb W 80 NetGrid 12 2016 3 February–11 March 3.2 ± 0.4 121 ± 95 29 ± 12 N N N Y Y Y 3 IIIa W 70 SweGrid 3 2015 5 March–17 March 4.1 ± 0.8 29 ± 28 63 ± 39 N Y Y Y Y Y 90 Seltra 3 2015 05 March–17 March 4.1 ± 0.8 20 ± 26 212 ± 78 N Y Y Y Y Y S 70 SweGrid 3 2015 31 August–3 September 4.0 ± 0.1 28 ± 13 91 ± 12 N Y Y Y Y N 90 Seltra 3 2015 31 August–3 September 4.0 ± 0.1 23 ± 14 167 ± 167 N Y Y Y Y N Study . ICES Division . Season . Cod-end mesh (mm) . Gear mods . Number of hauls . Year . Dates . Tow duration (h) . Nephrops catch (kg) . Non- Nephrops catch (kg) . Biological factors recorded . CL . DL . DS . DM . DP . DV . 1 VIa W 80 SMP 6 2017 6 March–8 March 3.6 ± 0.3 189 ± 55 40 ± 11 Y Y Y Y Y Y 100 SMP 6 2017 15 February–17 February 3.9 ± 0.2 183 ± 81 58 ± 15 Y Y Y Y Y Y S 80 SMP 6 2016 19 August–17 September 3.6 ± 0.3 152 ± 43 95 ± 41 Y Y Y Y Y Y 100 SMP 6 2016 15 July–18 August 3.4 ± 0.5 309 ± 120 57 ± 31 Y Y Y Y Y Y 2 IVb W 80 NetGrid 12 2016 3 February–11 March 3.2 ± 0.4 121 ± 95 29 ± 12 N N N Y Y Y 3 IIIa W 70 SweGrid 3 2015 5 March–17 March 4.1 ± 0.8 29 ± 28 63 ± 39 N Y Y Y Y Y 90 Seltra 3 2015 05 March–17 March 4.1 ± 0.8 20 ± 26 212 ± 78 N Y Y Y Y Y S 70 SweGrid 3 2015 31 August–3 September 4.0 ± 0.1 28 ± 13 91 ± 12 N Y Y Y Y N 90 Seltra 3 2015 31 August–3 September 4.0 ± 0.1 23 ± 14 167 ± 167 N Y Y Y Y N CL, catch length profile; DL, discard length profile; DS, discard sex profile; DM, discard immediate mortality; DP, discard physical damage; DV, discard vitality. Open in new tab Table 1. Summary of study locations, season (W—winter; S—summer), fishing gears, number of hauls, dates, tow durations and catch weights (mean ± standard deviation), and the biological factors recorded on board the fishing vessels (Y—yes; N—no) Study . ICES Division . Season . Cod-end mesh (mm) . Gear mods . Number of hauls . Year . Dates . Tow duration (h) . Nephrops catch (kg) . Non- Nephrops catch (kg) . Biological factors recorded . CL . DL . DS . DM . DP . DV . 1 VIa W 80 SMP 6 2017 6 March–8 March 3.6 ± 0.3 189 ± 55 40 ± 11 Y Y Y Y Y Y 100 SMP 6 2017 15 February–17 February 3.9 ± 0.2 183 ± 81 58 ± 15 Y Y Y Y Y Y S 80 SMP 6 2016 19 August–17 September 3.6 ± 0.3 152 ± 43 95 ± 41 Y Y Y Y Y Y 100 SMP 6 2016 15 July–18 August 3.4 ± 0.5 309 ± 120 57 ± 31 Y Y Y Y Y Y 2 IVb W 80 NetGrid 12 2016 3 February–11 March 3.2 ± 0.4 121 ± 95 29 ± 12 N N N Y Y Y 3 IIIa W 70 SweGrid 3 2015 5 March–17 March 4.1 ± 0.8 29 ± 28 63 ± 39 N Y Y Y Y Y 90 Seltra 3 2015 05 March–17 March 4.1 ± 0.8 20 ± 26 212 ± 78 N Y Y Y Y Y S 70 SweGrid 3 2015 31 August–3 September 4.0 ± 0.1 28 ± 13 91 ± 12 N Y Y Y Y N 90 Seltra 3 2015 31 August–3 September 4.0 ± 0.1 23 ± 14 167 ± 167 N Y Y Y Y N Study . ICES Division . Season . Cod-end mesh (mm) . Gear mods . Number of hauls . Year . Dates . Tow duration (h) . Nephrops catch (kg) . Non- Nephrops catch (kg) . Biological factors recorded . CL . DL . DS . DM . DP . DV . 1 VIa W 80 SMP 6 2017 6 March–8 March 3.6 ± 0.3 189 ± 55 40 ± 11 Y Y Y Y Y Y 100 SMP 6 2017 15 February–17 February 3.9 ± 0.2 183 ± 81 58 ± 15 Y Y Y Y Y Y S 80 SMP 6 2016 19 August–17 September 3.6 ± 0.3 152 ± 43 95 ± 41 Y Y Y Y Y Y 100 SMP 6 2016 15 July–18 August 3.4 ± 0.5 309 ± 120 57 ± 31 Y Y Y Y Y Y 2 IVb W 80 NetGrid 12 2016 3 February–11 March 3.2 ± 0.4 121 ± 95 29 ± 12 N N N Y Y Y 3 IIIa W 70 SweGrid 3 2015 5 March–17 March 4.1 ± 0.8 29 ± 28 63 ± 39 N Y Y Y Y Y 90 Seltra 3 2015 05 March–17 March 4.1 ± 0.8 20 ± 26 212 ± 78 N Y Y Y Y Y S 70 SweGrid 3 2015 31 August–3 September 4.0 ± 0.1 28 ± 13 91 ± 12 N Y Y Y Y N 90 Seltra 3 2015 31 August–3 September 4.0 ± 0.1 23 ± 14 167 ± 167 N Y Y Y Y N CL, catch length profile; DL, discard length profile; DS, discard sex profile; DM, discard immediate mortality; DP, discard physical damage; DV, discard vitality. Open in new tab Study 2 was undertaken using the MFV Luc (SN36), an 18-m single-rig stern trawler operating out of North Shields (English northeast coast, ICES Division IVb). Experimental fishing took place at the southern edge of the Farne Deeps in winter only (Figure 1). The net had an 80-mm diamond-mesh cod-end and incorporated a NetGrid selectivity device (Table 1). The NetGrid consists of a four-panel box section with a fish escape hole inserted into a standard two-panel trawl with an inclined netting sheet (Armstrong et al., 2016). Study 3 was undertaken in winter and summer using two commercial, twin-rig stern trawlers, the Canopus (LL377; 12 m) and the Ternö (LL388; 14.9 m) fishing on commercial grounds in the eastern Skagerrak (Swedish southwest coast, ICES Division IIIa, Figure 1). Each vessel deployed a standard Swedish grid trawl (hereafter abbreviated as SweGrid) comprising 35-mm bar-spacing with a 70-mm square-mesh cod-end as described in Valentinsson and Ulmestrand (2008) and a Seltra trawl with 90 mm diamond-mesh cod-end and a 270-mm diamond-mesh escape window as described in Krag et al. (2016). Environmental factors Sea surface temperatures were measured at least once a day using a SonTek Castaway (SonTek, San Diego, CA, USA) CTD (study 1), an OxyGuard Handy Polaris 2 (study 2), and an SD204 (SAIV A/S, Bergen, Norway) CTD (study 3). Vertical water column profiles were only recorded in studies 1 and 3. However, thermal and salinity stratification is typically minimal in February–March at the trawl sites in study 2 (Janssen et al., 1999), so surface values for these parameters should have been close to those at the seabed. In all three studies, air temperatures were recorded in the catch sorting area of the fishing vessel for each haul using digital thermometers. Catch sorting, sampling, and biological factors In all three studies, the trawler crews were asked to follow their normal fishing and catch sorting practices. On all four of the fishing vessels, the catch is dropped into a flat-bottomed metal hopper, from where it is raked via a hatch to a sorting table. Drop height in study 1 was 1.5 m, it was <1 m in study 2, and it was 0.8–1 m in study 3. In study 1, we had to assume that effects on Nephrops (levels of physical damage, etc.) would be similar in both cod-ends as the catches were not kept separate but dropped sequentially into the hopper, following the normal fishing practice. In study 3, the catch from each net was kept separate by dividing the hopper using wooden boards. In study 1, the catch length profiles were based on measurements of at least 100 Nephrops taken unselectively from different parts of the catch. For studies 2 and 3, only those Nephrops selected for captive survival observation were measured on board. For these regions, the typical size ranges of Nephrops in the catches and discards were estimated using fisheries observer data collected between 2011 and 2017 for ICES Division IVb, functional unit 6, and from 2015 for ICES Division IIIa. In all four vessels, the normal practice is that discards are returned continuously to the sea throughout catch sorting via a chute at the end of the sorting table. For each haul in study 1 (summer), scientific staff sampled Nephrops being discarded from the start of catch sorting until a target of 100 live animals was reached. This was subsequently modified (winter season) to take a target of 100 Nephrops from the start, and an additional 50 towards the end of catch sorting. For study 2, ∼200 Nephrops were sampled randomly throughout catch sorting across the whole size range from each haul. For study 3, observers first estimated the amount of Nephrops likely to be discarded from the catches and then adjusted the rate of sampling to cover the catch sorting period. The number of dead Nephrops encountered during sampling was also recorded and used to estimate immediate mortality for each haul. In all three studies, individual carapace lengths of the sampled Nephrops were measured using digital callipers. Sex was recorded during studies 1 and 3 but not during study 2. In all three studies, each animal selected for captive observation was examined for signs of visible damage (Table 2) with care taken to examine both ventral and dorsal surfaces. The vitality of each animal was also assessed (excepting study 3, summer hauls). Nephrops sampled for captive observation were then placed into individual compartments in commercial tube-sets (Figure 2). Once sampling was completed, the tube-set boxes were closed using perforated lids and placed into insulated containers filled with seawater. Water in the on-board holding tanks was renewed periodically to ensure that conditions did not deteriorate during transport to the onshore holding facilities. Figure 2. Open in new tabDownload slide Tube-set box used to retain Nephrops in individual compartments for captive observation. Figure 2. Open in new tabDownload slide Tube-set box used to retain Nephrops in individual compartments for captive observation. Table 2. Codes for scoring Nephrops semi-quantitative assessments of vitality and damage Criterion . Description . Excellent Vigorous body movement; all limbs moving and tail extends horizontally, flexed or tail-flips Good All limbs moving but tail hangs limp, no tail-flips Poor Limited or no body movement but movement of maxillipeds Moribund Only slight movement of maxillipeds or limbs in response to gentle prodding Dead 0 No response/movement to physical stimuli No injury 1 Alive with no obvious visible injuries Chelae D1 Either claw missing or damaged D2 Both claws missing or damaged Rostrum DR Rostrum damaged Body PUN A puncture injury on thorax or tail Thorax THC A crush injury on the thorax THP A puncture injury on the thorax Tail TAC A crush injury on the tail TAP A puncture injury on the tail Eye EYE Damage to one or both eyes Leg LEG One or more walking legs missing or damaged Criterion . Description . Excellent Vigorous body movement; all limbs moving and tail extends horizontally, flexed or tail-flips Good All limbs moving but tail hangs limp, no tail-flips Poor Limited or no body movement but movement of maxillipeds Moribund Only slight movement of maxillipeds or limbs in response to gentle prodding Dead 0 No response/movement to physical stimuli No injury 1 Alive with no obvious visible injuries Chelae D1 Either claw missing or damaged D2 Both claws missing or damaged Rostrum DR Rostrum damaged Body PUN A puncture injury on thorax or tail Thorax THC A crush injury on the thorax THP A puncture injury on the thorax Tail TAC A crush injury on the tail TAP A puncture injury on the tail Eye EYE Damage to one or both eyes Leg LEG One or more walking legs missing or damaged Open in new tab Table 2. Codes for scoring Nephrops semi-quantitative assessments of vitality and damage Criterion . Description . Excellent Vigorous body movement; all limbs moving and tail extends horizontally, flexed or tail-flips Good All limbs moving but tail hangs limp, no tail-flips Poor Limited or no body movement but movement of maxillipeds Moribund Only slight movement of maxillipeds or limbs in response to gentle prodding Dead 0 No response/movement to physical stimuli No injury 1 Alive with no obvious visible injuries Chelae D1 Either claw missing or damaged D2 Both claws missing or damaged Rostrum DR Rostrum damaged Body PUN A puncture injury on thorax or tail Thorax THC A crush injury on the thorax THP A puncture injury on the thorax Tail TAC A crush injury on the tail TAP A puncture injury on the tail Eye EYE Damage to one or both eyes Leg LEG One or more walking legs missing or damaged Criterion . Description . Excellent Vigorous body movement; all limbs moving and tail extends horizontally, flexed or tail-flips Good All limbs moving but tail hangs limp, no tail-flips Poor Limited or no body movement but movement of maxillipeds Moribund Only slight movement of maxillipeds or limbs in response to gentle prodding Dead 0 No response/movement to physical stimuli No injury 1 Alive with no obvious visible injuries Chelae D1 Either claw missing or damaged D2 Both claws missing or damaged Rostrum DR Rostrum damaged Body PUN A puncture injury on thorax or tail Thorax THC A crush injury on the thorax THP A puncture injury on the thorax Tail TAC A crush injury on the tail TAP A puncture injury on the tail Eye EYE Damage to one or both eyes Leg LEG One or more walking legs missing or damaged Open in new tab Transport and onshore holding For study 1, the tube-set boxes were then transported by road from Mallaig to the SAMS aquarium. Cold blocks were added to the insulated containers and air supplied using a portable compressor during transportation. For study 2, once the trawler had returned to port, the tube-set boxes were placed directly into onshore holding tanks located at the quay. In study 3, boxes were moved directly from the fishing vessels into the Kristineberg Marine Research station aquarium. Oxygen levels, temperature, and ammonia were checked in the transport containers on arrival at the onshore holding facilities. Control animals In study 1, controls were recovered discard fraction Nephrops from the previous trip that showed no visual injuries. Between experiments, the control animals were held in a common tank containing pieces of plastic pipe to act as refuges and fed on finely chopped mussel (Mytilus edulis) every second day. Ten control animals were added to each box of test animals when the box was transferred to the onshore aquaria, except for the first hauls in each season when recovered Nephrops were not yet available. For study 2, control animals were sourced from a local creel fisher working in a different part of the Farne Deeps. These creel-caught controls were transferred to the quayside aquaria and held unfed for the 2 weeks before the first treatment monitoring. The next opportunity to collect control animals was for the third of three monitoring periods, during which the collection of the control and treatment animals was synchronized. For study 3, control Nephrops were caught using creels from an area where trawling is not allowed but with similar habitat, depth, and environmental conditions to the experimental trawl locations. The creels had a smaller than usual mesh size (20 mm) to catch smaller Nephrops of sizes comparable to those normally discarded by the trawlers. Control animals were added to the observation boxes on return to the quay, i.e. control animals were not held in captivity prior to the experiments. Observation tanks Observation tanks were supplied with running seawater at a sufficient rate for replacement at least every 2 h. Seawater for the SAMS aquarium is drawn from a sub-sand beach filter, and incoming water temperatures can become high in summer. Observation tanks in study 1 were therefore housed in a constant temperature room with additional chilling of the incoming water. For study 2, temperatures in the observation tanks followed those of the ambient pumped seawater because the observation tanks were located on the dockside. In study 3, seawater is drawn from a deep supply. The observation tanks were housed in a constant temperature room, but additional water chilling was not used. Observation tanks were also aerated in studies 1 and 3. In study 1, temperatures in the observation tanks were monitored every 10 min using Hobo TidbiT loggers (Onset Computer Corp., Bourne, MA) and salinity was checked daily using a Castaway CTD (SonTek). Dissolved oxygen (DO) was monitored daily using an YSI (Yellow Springs, OH, USA) Pro20 portable oxygen metre, but only during the winter studies due to equipment availability. Ammonia levels were checked daily using API saltwater test strips (Mars Fishcare, Chalfont, PA, USA). In study 2, temperature and DO in the onshore holding tanks were measured daily using a portable metre (OxyGuard Handy Polaris 2) but salinity was not monitored. In study 3, temperature, salinity and DO were measured daily using portable metres (WTW Multi 3510) and water samples collected and analysed for ammonia. Captive observations Nephrops were not fed during captive observation. In study 1, Nephrops survival was monitored every 2 days from 1 to 13 days post-sampling. For study 2, inspections occurred daily up to 15 days, plus an additional evaluation of remaining survivors at 21 days. For study 3, Nephrops were monitored daily up to 15 days post-sampling. In all cases, the boxes were lifted out of the observation tanks and the individual Nephrops checked in air. Exposure to air was usually sufficient to cause live individuals to move but any that showed no movement were gently stimulated with blunt forceps. If they still failed to react to physical stimuli, they were recorded as dead and removed from the box. Statistical analyses Nephrops sizes are reported as carapace lengths in mm. All statistical analyses were performed using R version 3.5.0 (R Core Team, 2018) with additional packages “boot”, “ordinal”, “survival”, and “wrs2”. Statistical test results were considered significant at the p-value <0.05 level. Statistical analyses—data collected on board the fishing vessels Exploratory analysis of sea and air temperatures, haul durations, catch weights, and catch sorting times by study was conducted using pairs plots and Kendall’s tau to screen for potential collinearity. Nephrops size data were visualized using length frequency histograms. Differences in immediate mortalities within each study were explored using boxplots and tested using a non-parametric two-way median test. Potential relationships between immediate mortality and available covariates (sea surface and air temperatures, haul duration, catch weights, catch sorting times, and gear modification in study 3) were explored using scatterplots and Kendall’s tau. Immediate mortalities were then modelled as the total count of alive vs dead Nephrops in each haul using quasi-binomial GLMs that were sequentially simplified by eliminating non-significant factors, starting with the full model (Crawley, 2014). The final model fits were assessed using Pearson residuals. Data for physical damage at the time of sampling were summarized and ranked to identify the most common injuries in each study. The mean rate of occurrence of the top five injury types in each study was computed. Non-symmetrical 95% confidence intervals for these means were estimated by boot-strapping as the percentage of occurrence for some injury types was close to zero. Exploratory analysis of potential relationships between the percentages of injured Nephrops and available covariates was conducted as described above for immediate mortality. Analysis of the vitality scores from study 2 showed an unexplained increase in the proportions in the “Excellent” category comparing hauls on the third and fourth February with later dates. To standardize the vitality scores as much as possible within and across the three studies, the “Excellent” category was combined with the “Good” category to create E/G and the “Poor” category combined with the “Moribund” category to create P/M. This was based on the argument that the criteria for separating high vitality from low vitality animals were likely to be more consistent than when assigning animals to the finer divisions (Table 2), under challenging field conditions. Because vitality might be related to the presence of physical injuries, chi-square tests were applied to the frequencies of animals with injury presence or absence by E/G or P/M categories. Data on physical injury and vitality were then combined by assigning individual Nephrops to one of the four categories: uninjured and E/G, uninjured and P/M, injured and E/G, and injured and P/M. Potential relationships between the percentage of Nephrops in each category and available environmental and operational covariates (sea surface and air temperatures, haul duration, catch weights, catch sorting time, and gear for study 3) were explored using scatterplots and Kendall’s tau and modelled using ordinal regression with a logit link for each study. Non-significant terms based on the Wald F-statistics were sequentially removed from the regression models and the proportional odds assumption of final models tested using the “nominal_test” in the R “ordinal” package. Statistical analyses—data from the captive observations Survival of control Nephrops was evaluated by study, and the effect of season for studies 1 and 3 tested using the Fisher’s exact test. The effect of biological factors (sex, presence of damage and vitality at time of sampling) on the survival of individual Nephrops in the captive observations was visualized using Kaplan–Meier survival curves with differences being tested using log-rank tests (Kleinbaum and Klein, 2012; Moore, 2016). Because the assumption of independence between each Nephrops within an observation box might be invalid, we first estimated mean survivals (plus standard errors and 95% confidence intervals) for each haul from the Kaplan–Meier estimator at the time of the final mortality event. These survival estimates were then used to generate group mean survivals by study, season, and gear. To account for the uncertainty in the underlying haul-based mean survival estimates, 95% confidence intervals were computed as twice the standard error incorporating propagation of error following formula (1), assuming each haul-based estimate to be independent within the group SEgp=∑iσi2n2,(1) where n is the total number of contributing estimates and σi2 are the variances of each contributing estimate in the group i. Potential relationships between mean survival in each study and available biological (percentages of Nephrops in each damage presence/absence, E/G or P/M group), environmental (sea surface and air temperatures), and operational (haul duration, catch weights, sorting time, and gear for study 3) covariates were explored using scatterplots and modelled using multiple linear regressions with sequential removal of non-significant terms (Crawley, 2014). While percentage data, such as survival, infringe the limits for Gaussian error distributions, this only becomes a serious issue for linear modelling if the response variable values lie close to 0 or 100. For a range of 30–70%, ordinary linear modelling can be reasonably applied (Long, 1997). Final model fits were assessed visually using Pearson residual plots. Results Environmental conditions during trawling Winter air temperatures in studies 1 (West of Scotland) and 2 (North Sea) were between 6.9 and 11.5°C but were colder in study 3 (Skagerrak). In summer, the air temperatures in both regions reached as high as 19°C. There was also a greater seasonal difference in sea surface temperatures comparing the West of Scotland with the Skagerrak. In study 1, there was little thermal stratification, even in summer, but this was apparent in study 3 (Table 3). In studies 1 and 3, near bottom salinities were ∼34 but surface waters in the Skagerrak were fresher with salinities 24–29. Salinity was not recorded in study 2. Table 3. Field and observation tank environmental conditions as ranges Study . Season . Field sampling environmental conditions . Captive observation tanks . Air temp. (°C) . Sea surface temp. (°C) . Bottom temp. (°C) . Sea surface sal. . Sea bottom sal. . CT set air temp. (°C) . Water temp. (°C) . Salinity . 1 W 6.9–11.5 7.9–8.4 8.0–8.5 34.5–34.7 34.5–34.7 5 6.7–8.2 30.0–34.0 S 13.8–19.0 13.3–14.7 12.3–14.3 34.0–34.9 34.0–34.9 5 5.7–13.0 31.0–33.0 2 W 8.0–10.0 6.5–7.6 – – – – 5.2–9.2 – 3 W 2.0–5.9 3.6–4.3 4.9–6.1 24.5–26.4 32.9–34.9 10 5.0–6.0 32.0–33.0 S 14.9–18.7 16.7–17.9 10.0–10.8 23.9–29.1 33.5–34.2 14 14.0–15.0 33.0–34.0 Study . Season . Field sampling environmental conditions . Captive observation tanks . Air temp. (°C) . Sea surface temp. (°C) . Bottom temp. (°C) . Sea surface sal. . Sea bottom sal. . CT set air temp. (°C) . Water temp. (°C) . Salinity . 1 W 6.9–11.5 7.9–8.4 8.0–8.5 34.5–34.7 34.5–34.7 5 6.7–8.2 30.0–34.0 S 13.8–19.0 13.3–14.7 12.3–14.3 34.0–34.9 34.0–34.9 5 5.7–13.0 31.0–33.0 2 W 8.0–10.0 6.5–7.6 – – – – 5.2–9.2 – 3 W 2.0–5.9 3.6–4.3 4.9–6.1 24.5–26.4 32.9–34.9 10 5.0–6.0 32.0–33.0 S 14.9–18.7 16.7–17.9 10.0–10.8 23.9–29.1 33.5–34.2 14 14.0–15.0 33.0–34.0 Season: W, winter; S, summer hauls; CT, constant temperature room. Open in new tab Table 3. Field and observation tank environmental conditions as ranges Study . Season . Field sampling environmental conditions . Captive observation tanks . Air temp. (°C) . Sea surface temp. (°C) . Bottom temp. (°C) . Sea surface sal. . Sea bottom sal. . CT set air temp. (°C) . Water temp. (°C) . Salinity . 1 W 6.9–11.5 7.9–8.4 8.0–8.5 34.5–34.7 34.5–34.7 5 6.7–8.2 30.0–34.0 S 13.8–19.0 13.3–14.7 12.3–14.3 34.0–34.9 34.0–34.9 5 5.7–13.0 31.0–33.0 2 W 8.0–10.0 6.5–7.6 – – – – 5.2–9.2 – 3 W 2.0–5.9 3.6–4.3 4.9–6.1 24.5–26.4 32.9–34.9 10 5.0–6.0 32.0–33.0 S 14.9–18.7 16.7–17.9 10.0–10.8 23.9–29.1 33.5–34.2 14 14.0–15.0 33.0–34.0 Study . Season . Field sampling environmental conditions . Captive observation tanks . Air temp. (°C) . Sea surface temp. (°C) . Bottom temp. (°C) . Sea surface sal. . Sea bottom sal. . CT set air temp. (°C) . Water temp. (°C) . Salinity . 1 W 6.9–11.5 7.9–8.4 8.0–8.5 34.5–34.7 34.5–34.7 5 6.7–8.2 30.0–34.0 S 13.8–19.0 13.3–14.7 12.3–14.3 34.0–34.9 34.0–34.9 5 5.7–13.0 31.0–33.0 2 W 8.0–10.0 6.5–7.6 – – – – 5.2–9.2 – 3 W 2.0–5.9 3.6–4.3 4.9–6.1 24.5–26.4 32.9–34.9 10 5.0–6.0 32.0–33.0 S 14.9–18.7 16.7–17.9 10.0–10.8 23.9–29.1 33.5–34.2 14 14.0–15.0 33.0–34.0 Season: W, winter; S, summer hauls; CT, constant temperature room. Open in new tab Catches and discarding practices Based on pairs plots (Supplementary Figures S1–S4), there were no obvious relationships between haul duration and catch weight in any of the three studies, but total catch sorting times were significantly related to the Nephrops catch weight in study 1 (Supplementary Figure S1) and to the total catch weight in study 2 (Supplementary Figure S2). For study 3, there did not seem to be any strong relationships between total sorting times and catch weights (Supplementary Figures S3 and S4). There was a noticeable difference in the relative weights of Nephrops vs. non-Nephrops in the catches, these being much lower in study 3, where Nephrops comprised as little as 20 kg per net haul (Table 1). In study 1, the non-Nephrops components of the catches were mainly spotted dogfish (Scyliorhinus canicula), rays (Rajidae), ling (Molva molva), mackerel (Scomber scombrus), various flatfish including dab (Limanda limanda), and juvenile gadoids such as cod (Gadus morhua), hake (Merluccius merluccius), and haddock (Melangrammus aeglefinus). In study 2, the non-Nephrops components of the catches were mostly small gadoids. In study 3, the majority of the catches was comprised flatfishes, gadoids, and other benthic invertebrates. Details of the individual hauls are given in Supplementary Table S1. In study 1, the size range of Nephrops caught was 15–66 mm with a dominant mode at 28 mm and the size range of discarded Nephrops was 16–36 mm (Figure 3a). The majority of discards (96%) in study 1 was larger than the Minimum Conservation Reference Size (MCRS) for this fishing area. In study 2, the size range of Nephrops in the catch was 20–55 mm with a dominant mode at 28 mm (Figure 3b). This size range also closely matches that recorded over 7 years by fisheries observers on English trawlers fishing in the Farne Deeps. Observer data for ICES Division IVb showed that similar sizes of Nephrops are typically discarded as in study 1 but, because the MCRS is larger in Division IVb, a smaller percentage (54%) of these discarded Nephrops was above the MCRS (Figure 3b). In study 3, fisheries observer data for 2015 showed that Nephrops in trawl catches from this area ranged from 20 to 69 mm. Discarded Nephrops in study 3 ranged from 20 to 58 mm with a minority (8%) being above MCRS (Figure 3c). Compared to the other areas, this reflects the larger MCRS in ICES Division IIIa at the time (Hornborg et al., 2017). Thus, in all three study areas, Nephrops were being discarded for reasons other than the animals being below the minimum legal size, this being a particularly prominent feature in studies 1 (ICES Division VIa) and 2 (ICES Division IVb). Figure 3. Open in new tabDownload slide Histograms of Nephrops length frequencies in the total catches (grey histograms) and discarded portions of the catches (open histograms) in the three study areas. Vertical dashed lines are the minimum landing size (MCRS) at time studies were completed. (a) Length frequencies in study 1 (ICES Division VIa). (b) Length frequencies 2011–2017 in ICES Division IVb. (c) Length frequencies for 2015 in ICES Division IIIa. Figure 3. Open in new tabDownload slide Histograms of Nephrops length frequencies in the total catches (grey histograms) and discarded portions of the catches (open histograms) in the three study areas. Vertical dashed lines are the minimum landing size (MCRS) at time studies were completed. (a) Length frequencies in study 1 (ICES Division VIa). (b) Length frequencies 2011–2017 in ICES Division IVb. (c) Length frequencies for 2015 in ICES Division IIIa. Immediate mortality In study 1 in winter, the mean immediate mortality [±95% lower confidence level (LCL), upper confidence level (UCL)] was 9.7% (7.8, 11.9), and in summer, it was 14.5% (11.9, 19.7). However, because of variability in the immediate mortalities, neither season nor cod-end mesh size was statistically significant (Figure 4; med2way test: season p = 0.13, cod-end p = 0.51). Plotting immediate mortality by haul against available covariates (Supplementary Figure S5) suggested that immediate mortality might be related to total catch weight, Nephrops catch weight, sorting time, and air temperature with a possible effect of sea surface temperature. However, sequential removal of least significant terms in the GLM resulted in the retention of sorting time alone (Table 4), although this factor was itself correlated with Nephrops catch weight (Supplementary Figure S1). In study 2, no immediate mortality was observed. In study 3 in winter, the mean immediate mortality (±95% LCL, UCL) was 1.6% (0, 3.2), but in summer, it increased to 14.6% (11.6, 17.8). The median immediate mortality was significantly related to season, but not to gear (Figure 4; med2way test: season p < 0.001, gear modification p = 0.08). Scatterplots for study 3 (Supplementary Figure S6) suggested that immediate mortality might be related to sea surface temperature and this term was retained in the final GLM (Table 4). Residual plots for the GLM models for studies 1 and 3 indicated reasonable fits. Figure 4. Open in new tabDownload slide Estimates of immediate mortality during catch sorting by study (1–3), season (W—winter, S—summer), and gear (cod-end mesh size and gear modification). Heavy vertical bar indicates median, box is the interquartile range, whiskers extend up to 1.5 times the interquartile range, and circle is an outlier beyond the whisker range. Figure 4. Open in new tabDownload slide Estimates of immediate mortality during catch sorting by study (1–3), season (W—winter, S—summer), and gear (cod-end mesh size and gear modification). Heavy vertical bar indicates median, box is the interquartile range, whiskers extend up to 1.5 times the interquartile range, and circle is an outlier beyond the whisker range. Table 4. GLM model results for immediate mortality modelled as counts of Nephrops alive vs. dead during catch sorting, family = quasi-binomial, link = logit . Estimate . SE . t-Value . p-Value . Study 1 intercept 2.67 0.24 11.2 <0.001 Study 1 sorting time 0.32 0.10 −3.2 0.005 Null deviance 86.3 df = 22 – – Residual deviance 59.0 df = 21 – – Dispersion 2.9 – – – Study 3 intercept 4.83 0.66 7.27 <0.001 Study 3 sea surface temperature 0.17 0.04 −4.42 0.001 Null deviance 67.1 df = 11 – – Residual deviance 15.6 df = 10 – – Dispersion 1.5 – – – . Estimate . SE . t-Value . p-Value . Study 1 intercept 2.67 0.24 11.2 <0.001 Study 1 sorting time 0.32 0.10 −3.2 0.005 Null deviance 86.3 df = 22 – – Residual deviance 59.0 df = 21 – – Dispersion 2.9 – – – Study 3 intercept 4.83 0.66 7.27 <0.001 Study 3 sea surface temperature 0.17 0.04 −4.42 0.001 Null deviance 67.1 df = 11 – – Residual deviance 15.6 df = 10 – – Dispersion 1.5 – – – Final models resulting from sequential removal of insignificant terms are shown for models where residual patterns were acceptable. Note that in study 2 no immediate mortality was observed. Open in new tab Table 4. GLM model results for immediate mortality modelled as counts of Nephrops alive vs. dead during catch sorting, family = quasi-binomial, link = logit . Estimate . SE . t-Value . p-Value . Study 1 intercept 2.67 0.24 11.2 <0.001 Study 1 sorting time 0.32 0.10 −3.2 0.005 Null deviance 86.3 df = 22 – – Residual deviance 59.0 df = 21 – – Dispersion 2.9 – – – Study 3 intercept 4.83 0.66 7.27 <0.001 Study 3 sea surface temperature 0.17 0.04 −4.42 0.001 Null deviance 67.1 df = 11 – – Residual deviance 15.6 df = 10 – – Dispersion 1.5 – – – . Estimate . SE . t-Value . p-Value . Study 1 intercept 2.67 0.24 11.2 <0.001 Study 1 sorting time 0.32 0.10 −3.2 0.005 Null deviance 86.3 df = 22 – – Residual deviance 59.0 df = 21 – – Dispersion 2.9 – – – Study 3 intercept 4.83 0.66 7.27 <0.001 Study 3 sea surface temperature 0.17 0.04 −4.42 0.001 Null deviance 67.1 df = 11 – – Residual deviance 15.6 df = 10 – – Dispersion 1.5 – – – Final models resulting from sequential removal of insignificant terms are shown for models where residual patterns were acceptable. Note that in study 2 no immediate mortality was observed. Open in new tab Injury and vitality during catch sorting The percentage of discarded Nephrops with at least one visible injury ranged between 23 and 67% of the animals examined from each haul. The most common injuries were loss or damage to one or both chelae, puncture, and crush wounds to the thorax or abdomen and damaged rostra (Table 5). Damage to one or more legs, the telson, or the eye occurred in <1% of the Nephrops examined. Scatterplots of the percentage of damaged Nephrops against available covariates failed to reveal significant relationships, except in study 1 with non-Nephrops catch weight and in study 3 with sea surface temperature (Supplementary Figures S7 and S8). For vitality, the percentage in the E/G category in each haul was related to sea surface temperature in studies 1 and 2 and to haul duration in the winter hauls of study 3 (Supplementary Figures S9 and S10). In all three studies, the presence of at least one physical injury tended to reduce the vitality score of individual Nephrops (study 1: Chisq = 107, df = 1, p < 0.001; study 2: Chisq = 228, df = 1, p < 0.001; study 3: Chisq = 13, df = 1, p < 0.001) justifying combining the presence of at least one physical injury with vitality. However, ordinal regression of Nephrops assigned to these combined injury plus vitality categories failed to identify any significant environmental or operational covariates. Table 5. Percentages of Nephrops showing at least one injury by type during catch sorting as mean (95% LCL—95% UCL from bootstrap in parentheses) Injury . Study 1 . Study 2 . Study 3 . Study 3 . SMP . NetGrid . Seltra . SweGrid . Damaged—at least one injury 40 (36–43) 32 (30–34) 45 (32–54) 37 (32–41) One chela missing/damaged 24 (22–26) 21 (19–23) 15 (8–25) 11 (6–15) Puncture wound 8 (6–10) 2 (4–5) 24 (9–41) 19 (9–32) Crush wound 5 (4–10) 6 (3–7) 4 (1–9) 5 (1–11) Damaged rostrum 3 (2–4) 4 (3–5) 4 (1–6) 3 (1–4) Two chelae missing/damaged 4 (3–5) 4 (3–5) 3 (1–7) 2 (1–3) Injury . Study 1 . Study 2 . Study 3 . Study 3 . SMP . NetGrid . Seltra . SweGrid . Damaged—at least one injury 40 (36–43) 32 (30–34) 45 (32–54) 37 (32–41) One chela missing/damaged 24 (22–26) 21 (19–23) 15 (8–25) 11 (6–15) Puncture wound 8 (6–10) 2 (4–5) 24 (9–41) 19 (9–32) Crush wound 5 (4–10) 6 (3–7) 4 (1–9) 5 (1–11) Damaged rostrum 3 (2–4) 4 (3–5) 4 (1–6) 3 (1–4) Two chelae missing/damaged 4 (3–5) 4 (3–5) 3 (1–7) 2 (1–3) Note that individual Nephrops may have had more than one injury type and that abdominal and cephalothorax injuries have been combined. Open in new tab Table 5. Percentages of Nephrops showing at least one injury by type during catch sorting as mean (95% LCL—95% UCL from bootstrap in parentheses) Injury . Study 1 . Study 2 . Study 3 . Study 3 . SMP . NetGrid . Seltra . SweGrid . Damaged—at least one injury 40 (36–43) 32 (30–34) 45 (32–54) 37 (32–41) One chela missing/damaged 24 (22–26) 21 (19–23) 15 (8–25) 11 (6–15) Puncture wound 8 (6–10) 2 (4–5) 24 (9–41) 19 (9–32) Crush wound 5 (4–10) 6 (3–7) 4 (1–9) 5 (1–11) Damaged rostrum 3 (2–4) 4 (3–5) 4 (1–6) 3 (1–4) Two chelae missing/damaged 4 (3–5) 4 (3–5) 3 (1–7) 2 (1–3) Injury . Study 1 . Study 2 . Study 3 . Study 3 . SMP . NetGrid . Seltra . SweGrid . Damaged—at least one injury 40 (36–43) 32 (30–34) 45 (32–54) 37 (32–41) One chela missing/damaged 24 (22–26) 21 (19–23) 15 (8–25) 11 (6–15) Puncture wound 8 (6–10) 2 (4–5) 24 (9–41) 19 (9–32) Crush wound 5 (4–10) 6 (3–7) 4 (1–9) 5 (1–11) Damaged rostrum 3 (2–4) 4 (3–5) 4 (1–6) 3 (1–4) Two chelae missing/damaged 4 (3–5) 4 (3–5) 3 (1–7) 2 (1–3) Note that individual Nephrops may have had more than one injury type and that abdominal and cephalothorax injuries have been combined. Open in new tab Conditions on board and during transport In study 1, the time elapsed between sampling and transfer of tube-boxes into the observation tanks varied from 3 to 9 h with the road transport normally taking ∼2 h. Oxygen levels on arrival at the aquarium were between 7.8 and 8.8 mg l−1. Ammonia levels were elevated but not >1 mg l−1. In study 2, Nephrops were held in on-board tanks on the fishing vessel for 2.5–5.5 h, oxygen saturation remained >90%, and the animals were then transferred directly to the quayside facility. In study 3, time elapsed between sampling aboard and the transfer of the boxes into the observation tanks varied from 2 to 4 h. Oxygen saturation was always >90%, and ammonia levels never exceeded 0.15 mg l−1. Conditions in the captive observation tanks In study 1, the mean water temperature in the observations tanks was 7.6°C in winter fluctuating by <1°C (Table 3). In summer, the mean water temperature was 9.4°C but with larger fluctuations when the chillers struggled to cope with high temperatures of the incoming seawater. However, temperatures did not exceed those measured at the trawling sites during summer (Table 3). Salinities in the observation tanks were slightly lower than equivalent bottom salinities at the trawling sites, reflecting the location of the SAMS aquarium seawater intake. DO was always above 8 mg l−1, and ammonia levels were usually undetectable but peaked at 1 mg l−1 on a single occasion when the water flow to one recovery tank became temporarily reduced. In study 2, water was drawn directly from the quayside and the tanks were not under temperature control. Nevertheless, as this study was only conducted in the winter, temperatures were generally close to the sea surface temperatures measured at the haul locations (Table 3). In study 3, observation tank temperatures only fluctuated by 1°C, averaging 5.5°C in winter and 14.5°C in summer. However, in summer, water temperatures were up to 5°C warmer than the bottom temperatures measured at the haul sites. Salinities were close to those measured at the haul sites (Table 3). Oxygen levels remained above 80% saturation throughout all experiments, and ammonia levels were barely detectable. Size and survival of control animals The size (mean ± standard deviation) of control Nephrops in study 1 was 25 ± 2.2 vs. 24 ± 2.4 mm in the test animals. In study 2, the relative sizes were 40 ± 3.8 and 32 ± 6.4 mm, respectively, and in study 3, the relative sizes were 38 ± 2.7 and 38 ± 4.6 mm, respectively. Survival for controls during the monitoring of captive Nephrops was 96% in study 1 (n = 170), 94% in study 2 (n = 214), and 97% in study 3 (n = 390). For studies 1 and 3, seasonal differences in control survival were not statistically significant (Fisher’s exact tests; p > 0.05). This was not tested for study 2, which took place only in winter. Factors affecting the survival of individual Nephrops Based on Kaplan–Meier curves and log-rank tests, sex was not a significant factor affecting individual survival in either study 1 or 3 (Supplementary Figure S11, log-rank tests: study 1, Chisq = 1.0, p = 0.3; study 3, Chisq = 1.9, p = 0.2). Sex was not recorded in study 2. The presence of physical injuries affected individual survival with puncture and crush injuries having the greatest negative impacts (Supplementary Figure S12). Kaplan–Meier curves (Figure 5) showed significant effects on individual survival for the Nephrops in the four presence of injury combined with vitality categories (log-rank tests df = 3: study 1, Chisq = 299, p < 0.001; study 2, Chisq = 610, p < 0.001; study 3, Chisq = 126, p < 0.001). In all three studies, survival of undamaged animals in excellent or good vitality was significantly higher than survival of injured Nephrops in a poor of moribund state at the time of sampling. Figure 5. Open in new tabDownload slide Kaplan–Meier survival curves relating the probability of survival of individual Nephrops in the observation tanks against the presence of physical damage combined with the vitality categories (E/G—“excellent” or “good” vs. P/M—“poor” or “moribund”). Figure 5. Open in new tabDownload slide Kaplan–Meier survival curves relating the probability of survival of individual Nephrops in the observation tanks against the presence of physical damage combined with the vitality categories (E/G—“excellent” or “good” vs. P/M—“poor” or “moribund”). Survival estimates Based on the overall survival curves (Supplementary Figure S13), ∼90% of the observed mortalities had occurred by 8 days and further mortalities had largely ceased by 10 days of observation. Final survival estimates by haul including immediate mortality are given in Supplementary Table S2, illustrated in Supplementary Figure S14, and presented grouped by study, season, and fishing gear in Table 6 and Supplementary Figure S15. Table 6. Final survival estimates from the tank-based observation experiments including immediate mortality Study . Season . Cod-end (mm) . Gear mod. . N . Final survival estimates (%) . Mean . Standard error . LCL . UCL . 1 W 80 SMP 6 60.7 2.8 57.3 64.0 W 100 SMP 6 64.6 2.9 61.2 68.0 W Both SMP 12 61.7 1.4 59.3 64.0 S 80 SMP 6 52.0 3.6 48.2 55.8 S 100 SMP 6 42.1 3.3 38.5 45.8 S Both SMP 12 47.1 1.7 44.4 49.7 Both Both SMP 24 55.3 0.7 53.6 57.0 2 W 80 NetGrid 12 57.2 0.9 55.3 59.2 3 W 70 SweGrid 3 75.2 3.1 69.1 81.4 W 90 Seltra 3 58.6 4.5 49.6 67.6 W Both Both 6 66.9 2.7 61.5 72.4 S 70 SweGrid 3 41.7 3.1 35.4 48.0 S 90 Seltra 3 37.7 3.5 30.7 44.7 S Both Both 6 39.7 2.4 35.0 44.4 Both Both Both 12 53.3 1.8 49.7 56.9 Study . Season . Cod-end (mm) . Gear mod. . N . Final survival estimates (%) . Mean . Standard error . LCL . UCL . 1 W 80 SMP 6 60.7 2.8 57.3 64.0 W 100 SMP 6 64.6 2.9 61.2 68.0 W Both SMP 12 61.7 1.4 59.3 64.0 S 80 SMP 6 52.0 3.6 48.2 55.8 S 100 SMP 6 42.1 3.3 38.5 45.8 S Both SMP 12 47.1 1.7 44.4 49.7 Both Both SMP 24 55.3 0.7 53.6 57.0 2 W 80 NetGrid 12 57.2 0.9 55.3 59.2 3 W 70 SweGrid 3 75.2 3.1 69.1 81.4 W 90 Seltra 3 58.6 4.5 49.6 67.6 W Both Both 6 66.9 2.7 61.5 72.4 S 70 SweGrid 3 41.7 3.1 35.4 48.0 S 90 Seltra 3 37.7 3.5 30.7 44.7 S Both Both 6 39.7 2.4 35.0 44.4 Both Both Both 12 53.3 1.8 49.7 56.9 Season as W—winter, S—summer. LCL and UCL are the 95% upper and lower confidence limits for the mean survival estimates averaged by gear, season, and study with error propagation from the individual haul-based survival estimates. Open in new tab Table 6. Final survival estimates from the tank-based observation experiments including immediate mortality Study . Season . Cod-end (mm) . Gear mod. . N . Final survival estimates (%) . Mean . Standard error . LCL . UCL . 1 W 80 SMP 6 60.7 2.8 57.3 64.0 W 100 SMP 6 64.6 2.9 61.2 68.0 W Both SMP 12 61.7 1.4 59.3 64.0 S 80 SMP 6 52.0 3.6 48.2 55.8 S 100 SMP 6 42.1 3.3 38.5 45.8 S Both SMP 12 47.1 1.7 44.4 49.7 Both Both SMP 24 55.3 0.7 53.6 57.0 2 W 80 NetGrid 12 57.2 0.9 55.3 59.2 3 W 70 SweGrid 3 75.2 3.1 69.1 81.4 W 90 Seltra 3 58.6 4.5 49.6 67.6 W Both Both 6 66.9 2.7 61.5 72.4 S 70 SweGrid 3 41.7 3.1 35.4 48.0 S 90 Seltra 3 37.7 3.5 30.7 44.7 S Both Both 6 39.7 2.4 35.0 44.4 Both Both Both 12 53.3 1.8 49.7 56.9 Study . Season . Cod-end (mm) . Gear mod. . N . Final survival estimates (%) . Mean . Standard error . LCL . UCL . 1 W 80 SMP 6 60.7 2.8 57.3 64.0 W 100 SMP 6 64.6 2.9 61.2 68.0 W Both SMP 12 61.7 1.4 59.3 64.0 S 80 SMP 6 52.0 3.6 48.2 55.8 S 100 SMP 6 42.1 3.3 38.5 45.8 S Both SMP 12 47.1 1.7 44.4 49.7 Both Both SMP 24 55.3 0.7 53.6 57.0 2 W 80 NetGrid 12 57.2 0.9 55.3 59.2 3 W 70 SweGrid 3 75.2 3.1 69.1 81.4 W 90 Seltra 3 58.6 4.5 49.6 67.6 W Both Both 6 66.9 2.7 61.5 72.4 S 70 SweGrid 3 41.7 3.1 35.4 48.0 S 90 Seltra 3 37.7 3.5 30.7 44.7 S Both Both 6 39.7 2.4 35.0 44.4 Both Both Both 12 53.3 1.8 49.7 56.9 Season as W—winter, S—summer. LCL and UCL are the 95% upper and lower confidence limits for the mean survival estimates averaged by gear, season, and study with error propagation from the individual haul-based survival estimates. Open in new tab Final survival and biological, environmental, and operational factors In study 1 (Scottish west coast), final survival estimates were significantly higher in winter than in summer (winter 62 ± 2.8% vs. summer 47 ± 3.4%; ANOVA: survival ∼ season, season F = 13.0, df = 1, P = 0.002). In this study, Nephrops were only sampled from the start of the catch sorting for the summer hauls and this could have resulted in some overestimation of survival. The approach was subsequently changed so that the entire catch sorting time was sampled for the winter hauls. In study 2 (Farne Deeps, North Sea), the mean final survival was 57 ± 1.8% but only assessed in winter. In study 3 (Skagerrak), final survival was again higher in winter than in summer (winter 67 ± 5.4% vs. summer 40 ± 4.8%). However, gear also had an effect with final mean survival being higher for Nephrops caught using trawls fitted with the Swedish grid (ANOVA: survival ∼ season × gear modification, season F = 67.5, df = 1, p < 0.001, gear modification F = 9.71, df = 1, p = 0.01, gear modification by season, p > 0.05). Scatterplots and Kendall’s tau suggested that the seasonal effect in study 1 might be linked to differences in air temperature, but catch sorting time was also significantly correlated with survival (Supplementary Figure S16). In the simplified multiple linear regression model of survival, sea surface temperature, and not air temperature, along with catch sorting time, were retained (Table 7). Seasonal effects were not tested for in study 2 as it was conducted in winter only. In study 2, final survival was just significantly correlated with the weight of non-Nephrops catch, but in a positive manner (Supplementary Figure S16). Multiple regression simplification failed to identify any significant predictors of final survival for study 2. However, it should be noted that although neither sea nor air temperature was selected as significant, the temperature range in this study was limited since all hauls were conducted in winter. For study 3, although there were apparent effects of sea surface and air temperature on final survival by gear, the correlations were not statistically significant (Table S17). Sea surface temperature was, however, retained in the simplified multiple linear regression models of final survival for study 3 (Table 7). Final survival results across all three studies appeared to be consistent with an overall temperature effect (Figure 6). However, because sea surface and air temperatures were correlated, it is not possible to say with any certainty which factor was having the stronger impact. In relation to biological factors, scatterplots and Kendall’s tau failed to identify any patterns of final survival for each haul with the proportions of Nephrops in the injury presence/absence combined with E/G or P/M vitality groups (Supplementary Figures S18 and S19). Figure 6. Open in new tabDownload slide Relationship between Nephrops final mean survival from each haul and recorded sea surface (left hand panel) and air temperatures (right hand panel) across all three studies. Solid lines, linear regressions; dotted lines, 95% CIs. Points labelled as study number plus season (W—winter, S—summer). The linear regressions are: survival = 74.0 − 1.9 × sea surface temperature (F = 26.0, df = 1.46, p ≤ 0.001, r2 = 0.36) and survival = 75.3 − 1.8 × air temperature (F = 22.2, df = 1.46, p < 0.001, r2 = 0.33). Figure 6. Open in new tabDownload slide Relationship between Nephrops final mean survival from each haul and recorded sea surface (left hand panel) and air temperatures (right hand panel) across all three studies. Solid lines, linear regressions; dotted lines, 95% CIs. Points labelled as study number plus season (W—winter, S—summer). The linear regressions are: survival = 74.0 − 1.9 × sea surface temperature (F = 26.0, df = 1.46, p ≤ 0.001, r2 = 0.36) and survival = 75.3 − 1.8 × air temperature (F = 22.2, df = 1.46, p < 0.001, r2 = 0.33). Table 7. Linear model results for mean final survival estimates including immediate mortality by haul against available operational covariates for each study . Estimate . Standard error . t-Value . p-Value . Study 1—SMP  Intercept 90.07 8.79 10.25 <0.001  Sea surface temperature −1.90 0.79 −2.42 0.025  Sorting time −6.52 2.60 −2.52 0.020  Residual standard error 10.52  Multiple R2 0.47  F-statistic 9.34 df = 2.21 0.001 Study 2—_NetGrid No significant operational covariates Study 3—Seltra  Intercept 64.83 1.41 45.98 <0.001  Sea surface temperature −1.56 0.11 −13.96 <0.001  Residual standard error 1.83  Multiple R2 0.98  F-statistic 194.9 df = 1.4 <0.001 Study 3—SweGrid  Intercept 84.91 6.51 13.05 <0.001  Sea surface temperature −2.47 0.52 −4.79 0.009  Residual standard error 8.46  Multiple R2 0.85  F-statistic 23.0 df = 1.4 0.009 . Estimate . Standard error . t-Value . p-Value . Study 1—SMP  Intercept 90.07 8.79 10.25 <0.001  Sea surface temperature −1.90 0.79 −2.42 0.025  Sorting time −6.52 2.60 −2.52 0.020  Residual standard error 10.52  Multiple R2 0.47  F-statistic 9.34 df = 2.21 0.001 Study 2—_NetGrid No significant operational covariates Study 3—Seltra  Intercept 64.83 1.41 45.98 <0.001  Sea surface temperature −1.56 0.11 −13.96 <0.001  Residual standard error 1.83  Multiple R2 0.98  F-statistic 194.9 df = 1.4 <0.001 Study 3—SweGrid  Intercept 84.91 6.51 13.05 <0.001  Sea surface temperature −2.47 0.52 −4.79 0.009  Residual standard error 8.46  Multiple R2 0.85  F-statistic 23.0 df = 1.4 0.009 Final models, resulting from the sequential removal of insignificant terms, are shown where residual patterns were acceptable. Open in new tab Table 7. Linear model results for mean final survival estimates including immediate mortality by haul against available operational covariates for each study . Estimate . Standard error . t-Value . p-Value . Study 1—SMP  Intercept 90.07 8.79 10.25 <0.001  Sea surface temperature −1.90 0.79 −2.42 0.025  Sorting time −6.52 2.60 −2.52 0.020  Residual standard error 10.52  Multiple R2 0.47  F-statistic 9.34 df = 2.21 0.001 Study 2—_NetGrid No significant operational covariates Study 3—Seltra  Intercept 64.83 1.41 45.98 <0.001  Sea surface temperature −1.56 0.11 −13.96 <0.001  Residual standard error 1.83  Multiple R2 0.98  F-statistic 194.9 df = 1.4 <0.001 Study 3—SweGrid  Intercept 84.91 6.51 13.05 <0.001  Sea surface temperature −2.47 0.52 −4.79 0.009  Residual standard error 8.46  Multiple R2 0.85  F-statistic 23.0 df = 1.4 0.009 . Estimate . Standard error . t-Value . p-Value . Study 1—SMP  Intercept 90.07 8.79 10.25 <0.001  Sea surface temperature −1.90 0.79 −2.42 0.025  Sorting time −6.52 2.60 −2.52 0.020  Residual standard error 10.52  Multiple R2 0.47  F-statistic 9.34 df = 2.21 0.001 Study 2—_NetGrid No significant operational covariates Study 3—Seltra  Intercept 64.83 1.41 45.98 <0.001  Sea surface temperature −1.56 0.11 −13.96 <0.001  Residual standard error 1.83  Multiple R2 0.98  F-statistic 194.9 df = 1.4 <0.001 Study 3—SweGrid  Intercept 84.91 6.51 13.05 <0.001  Sea surface temperature −2.47 0.52 −4.79 0.009  Residual standard error 8.46  Multiple R2 0.85  F-statistic 23.0 df = 1.4 0.009 Final models, resulting from the sequential removal of insignificant terms, are shown where residual patterns were acceptable. Open in new tab Discussion Factors affecting immediate Nephrops mortality Being caught in trawls results in a range of physiological and physical responses in Nephrops. Animals will exhibit vigorous tail flipping as they try to escape from the ground gear (Newland and Chapman, 1989), and such activity results in depletion of muscle ATP and increased levels of anaerobic metabolites (Albalat et al., 2009). Exposure of Nephrops to low salinity surface waters during net hauling may lead to further physiological stress, but this is only likely to be important in strongly salinity-stratified waters such as the Kattegat and Skagerrak (Harris and Ulmestrand, 2004). Although haloclines were present in our third study in the Skagerrak, the surface salinities were not as low as the salinity of 15 used in the laboratory experiments conducted by Harris and Ulmestrand (2004). Once on board fishing vessels, Nephrops are usually held in air during catch sorting resulting in multiple physiological and immunological changes associated with oxygen deprivation. The severity of these changes has been linked to temperature and the length of aerial exposure (Spicer et al., 1990; Albalat et al., 2009; Lund et al., 2009; Campos et al., 2015). In study 3, the temperature of the aerial exposure appeared to influence the level of immediate mortality while in study 1, immediate mortality was related to total catch sorting time. Factors affecting final survival Consistent with previous studies (Symonds and Simpson, 1971; Wileman et al., 1999; Campos et al., 2015; Albalat et al., 2016), we found clear links between the survival of individual Nephrops during captive observation and the presence of physical damage plus vitality at the time of sampling. Puncture and crush injuries in particular are known to lead to the loss of haemolymph in Nephrops often resulting in eventual circulatory collapse (Wileman et al., 1999). However, despite the clear link between physical damage plus vitality and survival at the individual level, only sea surface temperature consistently emerged as a significant predictor of final mean survival in studies 1 and 3. However, because sea surface and air temperatures were correlated, it was difficult to determine which factor was having more impact. Although several studies have highlighted the negative link between increased air temperatures and Nephrops survival (Spicer et al., 1990; Ridgway et al., 2006), being returned to warmer water in summer, either by being discarded at sea or when placed into observation tanks, might also reduce survival. Since metabolic costs are linked to temperature, elevated energetic costs might reduce an animal’s capacity for recovery during the summer months. In addition, bacterial and fungal growth rates are likely to be higher in summer, perhaps resulting in the poorer survival of injured Nephrops recovering in warmer water. Broadhurst et al. (2006) suggested that simple measures to keep catches cool, such as ensuring that hopper covers are closed after the nets have been emptied or installing chillers, might improve discard survival. However, such measures could be less beneficial in summer if lower survival rates are also due to animals recovering in warmer water. This could be tested in further captive observation trials if the water temperatures in the captive observation tanks were kept constant between seasons. Experimental design The conclusion that final survivals were linked to temperature must be treated with some caution because most of the hauls at the higher temperatures were from the third study. There is thus scope for inter-study effects to have contributed to the overall relationship. This problem could be overcome by randomly allocating hauls across the full range of covariates, but this is difficult to achieve in field-based studies where the activity, in this case trawling and its associated environmental conditions, is not under direct experimental control. Furthermore, temperatures in the observation tanks did not always coincide with those measured in the field. In particular, temperatures were colder in the observation tanks for study 1 summer hauls but warmer in study 3 summer captive observations. We are not aware of any discard recovery studies with Nephrops where the effects of different water temperatures during recovery have been investigated, but water temperatures in the observation tanks could have had some impact on the results. The considered opinion of WKMEDS (ICES, 2014) is that, to date, there are no satisfactory methods for adjusting discard survival estimates using control data. Therefore, it is currently recommended that the magnitude of the control mortality should be used as a measure of the validity of the observation method, where control mortalities close to zero suggest a more valid method for accurately estimating discard survival. In the present studies, mortality of control animals was <5% suggesting that the observational set-ups were not causing high levels of stress. It must be noted that control animals were added to the observation boxes when they reached the aquaria. Adding control animals to the observation boxes on board the trawlers was impractical because the control Nephrops would have had to be transported back to the haul locations, in some cases on the previous evening, and thus exposed to even more unrealistic stressors. However, any mortality in the test subjects resulting from being placed into insulated containers on board the trawlers and transported to the aquaria could not be identified with the approach used. Sourcing appropriate control animals for discard survival studies is also challenging (ICES, 2014; Campos et al., 2015; Méhault et al., 2016; Morfin et al., 2017; Mérillet et al., 2018). Although previous studies have also used recovered (Mérillet et al., 2018) or creel-caught Nephrops (Wileman et al., 1999), both approaches are open to challenge. The use of recovered animals might not represent the full health and robustness range of Nephrops caught in the trawls since recovered animals might be those more resilient to such stresses. However, this approach did ensure that control animals are of similar size to those being discarded. For creel-caught controls, their larger size compared to those being discarded may be an issue, but this potential problem was minimized in study 3 by using creels with a reduced mesh size. Studies 1 and 2 were based on single vessel while study 3 used two vessels. Any extrapolation of results must be made cautiously because of the variety of operations in the wider fishing fleet. Given the logistical challenges and costs of conducting discard survival experiments across multiple fishing vessels, relationships between survival during captive observation and vitality have been used to extrapolate captive observation findings to a larger number of vessels (Morfin et al., 2017). However, this approach relies on the assumption that survival depends only on vitality plus any environmental covariates identified as statistically affecting captive survival. In the present studies, mean survival by haul did not appear to be strongly linked to such factors, suggesting that a substantial part of the variability in survival is being driven by additional, un-measured factors. Limitations with captive observation survival estimates Several publications have pointed out that tank-based discard survival experiments are likely to overestimate true survival by ignoring predation mortality that may occur at the sea surface, in the water column, or when discarded animals reach the seabed (Symonds and Simpson, 1971; Raby et al., 2014, Morfin et al., 2017; Mérillet et al., 2018). Although seabirds probably do not take a large proportion of discarded Nephrops (Catchpole et al., 2006; Depestele et al., 2016), this predation risk can be minimized by releasing discards below the sea surface using a protective chute. Little work has been undertaken on the predation of discarded Nephrops during their descent through the water column, but Bergmann et al. (2002) suggested that discards would reach the seabed in a few minutes. As far as we are aware, there are no estimates of predation rates of live, discarded Nephrops once they reach the seabed, although the behaviour of small Nephrops released at depths of ∼100-m has been observed using a remotely operated vehicle (Fox and Albalat, 2018). It was reported that undamaged Nephrops, even after aerial exposure for up to 3 h, recovered rapidly and began exploring their environment and entering available burrows within 10 min. However, these observations were only made on a limited number of dives and the Nephrops could only be followed for a short time. The conclusions reached might not apply to grounds with higher abundances of predators, to damaged Nephrops, or to those previously exposed to prolonged elevated air temperatures. Furthermore, if Nephrops are discarded over unsuitable habitat, for example while steaming back to port, they will have no chance of finding suitable protection in burrows (Evans et al., 1994). The longer-term effects of discarding on Nephrops are also difficult to assess. Evans et al. (1994) demonstrated that animals lacking one chela were less successful in competing for food and shelter compared to uninjured Nephrops. In the present studies, this injury was seen in ∼20% of the discarded Nephrops and these animals may be at a competitive disadvantage when returned to the sea. Reducing the occurrence of such injuries should improve survival potential but is challenging as levels of physical damage are related to animal condition, gear type, haul duration, seabed condition, size of catches and composition, catch handling, and hopper design (Campos et al., 2015; Méhault et al., 2016). Oliver et al. (2017) suggested that trawls that are more selective will result in less physical damage to Nephrops in the net, thus potentially increasing discard survival. However, across all three studies, we were unable to establish a statistical link between the proportions of Nephrops with physical damage and final survival by haul, even though such injuries led to reduced survival at the individual level. Within study 3 (Skagerrak), there was an effect of gear with captive survival of discarded Nephrops from nets equipped with Swedish grids being higher. Swedish grid trawls are considered more selective than Seltra trawls (Madsen and Valentinsson, 2010). Unfortunately, vitality was only recorded on the winter hauls making it difficult to reach firmer conclusions regarding the interplay of gear selectivity, Nephrops condition, and subsequent survival. Comparison with other published studies Levels of immediate mortality recorded in studies 1 and 3 were quite similar to the 15.6% immediate mortality reported by Mérillet et al. (2018) when using a discard chute. In study 2, no immediate mortality was observed. This was unexpected and not explained by any obvious differences between the studies, such as tow lengths or catch weights (Table 1). There are a limited number of published Nephrops survival studies undertaken at different seasons, but Mérillet et al. (2018) reported higher survival in summer (57%) compared to spring (42%). This contrasts with findings of reduced final survival at higher temperatures in the present studies, and with other publications reporting a negative link between Nephrops survival and temperature (Méhault et al., 2016). The mean summer survival estimates in study 1 (47%) and study 3 (40%) were lower than a recent result of 64% reported by Oliver et al. (2017) off the west coast of Ireland using Seltra trawls and of 57% reported by Mérillet et al. (2018) for the Bay of Biscay using a modified discarding chute. This may reflect genuine differences between the fisheries because the experimental methodology across all these recent studies largely followed the WKMEDS guidelines. Conclusions and recommendations for future work Despite some operational differences between the three studies, the final survival estimates were reasonably consistent. In all three winter studies, over half the observed Nephrops survived a minimum of 13 days captive observation while, in the two studies conducted in summer, survival was between a third and a half. Although what constitutes “high survivability” is not defined in the Landing Obligation [Regulation (EU) No. 1380/2013], the results presented in the present study have been reviewed by the Scientific, Technical and Economic Committee for Fisheries and accepted by the European Commission as the basis for exemptions in the North Sea and west of Scotland. In the two studies conducted across seasons, final survival of discarded Nephrops was significantly higher in winter than in summer. Sea surface temperature was identified as affecting both immediate mortality and final survival in study 3 but only final survival in study 1. However, the effect of air temperature was only marginally weaker in the models making it hard to conclude which of these two correlated environmental factors might be driving the seasonal response. Altering fishing practices to keep catches cool during catch sorting may thus improve discard survival, particularly in summer. However, poorer captive observation survival in summer could also be related to animals recovering in warmer water, in which case cooling during catch sorting may have less positive effect. Further studies where water temperatures in the captive observation tanks are manipulated could be undertaken to test this. Our results also confirmed that physical damage to Nephrops significantly reduces their survival potential with puncture and crush injuries being most deleterious. Although we were unable to link statistically the overall levels of damage within hauls to resultant mean final survival, in study 3 better survival was observed from catches made with trawls equipped with a Swedish sorting grid compared to a Seltra trawl. Furthermore, in study 1, immediate mortality was lower in lighter hauls where the overall sorting times were also shorter. Improvements in gear selectivity may thus benefit survival of discarded Nephrops and recording levels of physical damage and vitality of Nephrops when future gear comparison studies are conducted could provide valuable additional data to test this hypothesis. Once on board, catch-handling practices that may lead to further damage should be avoided. There is potential that changes in hopper design, such as sloping floors allowing the catch to be pulled onto the sorting tables with the assistance of gravity (Albalat et al., 2016), or seawater hoppers (Broadhurst et al., 2006), might be beneficial in reducing damage and improving discard survival. Although we are not aware of any research into this in northern European Nephrops fisheries, the benefits of seawater hoppers for improving discard survival have received attention in Australian prawn fisheries (Ocean Watch Australia, 2004). However, it must be cautioned that placing catches into hoppers filled with low salinity seawater may cause additional stress, so that such measures may not be effective in improving survival in areas with reduced surface salinity, such as the Skagerrak. Similar considerations may also apply if un-chilled seawater hoppers are filled with warm surface seawater during summer months. Given that discard survival studies are expensive to conduct (Morfin et al., 2017), it is recognized that future studies need to be standardized as much as possible (ICES, 2014). Despite efforts at standardization, some differences were apparent between the three studies reported here. For example, physical water column parameters were not measured in a consistent manner, analysis of the vitality data raised some doubts about the consistency of on-board scoring, and temperatures in the observation tanks did not always reflect those in the field. Such problems need to be tackled through further training and inter-calibration between laboratories conducting discard survival studies. Acknowledgements The authors would like to extend their sincere thanks to the skippers and crews of the participating fishing vessels. The authors would also like to acknowledge the anonymous reviewers whose comments have helped shape and improve the manuscript. 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V., Harris R. R. 1999 . Roundfish and Nephrops survival after escape from commercial gear. Final report, EC Contract No: FAIR-CT95-0753. 240 pp. © International Council for the Exploration of the Sea 2020. All rights reserved. For permissions, please email: [email protected] This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)