ICES Journal of Marine Science: Journal du Conseil

Drinkwater, Kenneth F.; Loeng, Harald; Megrey, Bernard A.; Bailey, Nick; Cook, Robin M.

doi: 10.1016/j.icesjms.2005.08.008pmid: N/A

Alheit, J.; Möllmann, C.; Dutz, J.; Kornilovs, G.; Loewe, P.; Mohrholz, V.; Wasmund, N.

doi: 10.1016/j.icesjms.2005.04.024pmid: N/A

The index of the North Atlantic Oscillation, the dominant mode of climatic variability in the North Atlantic region, changed in the late 1980s (1987–1989) from a negative to a positive phase. This led to regime shifts in the ecology of the North Sea (NS) and the central Baltic Sea (CBS), which involved all trophic levels in the pelagial of these two neighbouring continental shelf seas. Increasing air and sea surface temperatures, which affected critical physical and biological processes, were the main direct and indirect driving forces. After 1987, phytoplankton biomass in both systems increased and the growing season was extended. The composition of phyto- and zooplankton communities in both seas changed conspicuously, e.g. dinoflagellate abundance increased and diatom abundance decreased in the CBS. Key copepod species that are essential in fish diets experienced pronounced changes in biomass. Abundance of Calanus finmarchicus (NS) and Pseudocalanus sp. (CBS) fell to low levels, whereas C. helgolandicus (NS) and Temora longicornis and Acartia spp. (CBS) were persistently abundant. These changes in biomass of different copepod species had dramatic consequences on biomass, fisheries, and landings of key fish species: North Sea cod declined, cod in the CBS remained at low levels, and CBS sprat reached unprecedented high biomass levels resulting in high yields. The synchronous regime shifts in NS and CBS resulted in profound changes in both marine ecosystems. However, the reaction of fish populations to the bottom-up mechanisms caused by the same climatic shift was very different for the three fish stocks.

Greve, Wulf; Prinage, Sabine; Zidowitz, Heike; Nast, Jutta; Reiners, Frank

doi: 10.1016/j.icesjms.2005.03.011pmid: N/A

The timing of fish larvae abundance was investigated at Helgoland Roads in the southern North Sea for the years 1990, 1993–1996, 1998, and 1999. From the 44 species identified, 27 were sufficiently abundant in more than 3 years and had well defined seasonal changes to calculate their phenology. The passage of the 15%, 50%, and 85% annual cumulative abundance thresholds was used to define the “start of season”, “middle of season”, and “end of season”, respectively. The timing of a significant number of these events was correlated with the mean annual winter sea surface temperatures (SSTs) in weeks 1–10 with higher temperatures leading to earlier appearance. The timing of the end of season was also negatively correlated with the SSTs. Sole (Solea solea) is presented as an example of the general functional relationship.

Hansen, Bogi; Eliasen, Sólvá K.; Gaard, Eilif; Larsen, Karin M.H.

doi: 10.1016/j.icesjms.2005.04.014pmid: N/A

Previous investigations have shown that new primary production on the Faroe Shelf during the spring bloom varies considerably from one year to another, both with regard to timing and intensity. It has also been found that variations in new primary production are transmitted up the food chain to top predators such as cod and haddock. An observed inverse relationship between new primary production and zooplankton biomass, especially Calanus finmarchicus, might be due to grazing, but could also reflect a dependence of both new primary production and C. finmarchicus import on the horizontal exchange between waters on the shelf and off it. Here, we investigate this question, using observations and an idealized numerical model. We find that the large variability in new primary production is most likely due to the direct effect of variable horizontal exchange rates on phytoplankton reproduction (horizontal Sverdrup mechanism), rather than grazing. Enhanced horizontal exchange flushes phytoplankton from the shallow areas and limits primary production. Horizontal exchange rate seems most sensitive to horizontal density differences between on-shelf and off-shelf waters, which is governed by atmosphere–ocean heat exchange and precipitation. For the primary observational period from 1990 to 2003, a close relationship was found between air temperature in January–April and new primary production. Cold winters produced large density differences, small horizontal exchange rates, and intensive new primary production. Other, less direct, evidence indicates, however, that this relationship may not have been valid in a period before 1990.

Heath, Michael R.

doi: 10.1016/j.icesjms.2005.04.010pmid: N/A

Hydrographic, plankton, benthos, fisheries landings, and fish diet data from shelf sea areas in the Northeast Atlantic have been combined into an analysis of the foodweb structure and secondary production requirements of regional fisheries. Fish landings from the Baltic and North Sea are shown to be taken from a lower trophic level and are shown to be overall more planktivorous than those from shelf edge regions. The secondary production required per unit of landed fish from the North Sea was approximately half that for landings from the southwest approaches to the UK, referred to as the Celtic Seas, where zooplankton production accounted for only a small fraction of the secondary production demands of the fisheries. In the North Sea, variability in zooplankton production seems to have exerted a bottom-up effect on fish production, which in turn has exerted a top-down effect on the benthos. Conversely, Celtic Seas benthos production has been a bottom-up driver of fish production, which seems to have been independent of variability in plankton production. Thus, climate and fishing pressures can be expected to influence these regional fisheries in very different ways. Overall, the results indicate very strong spatial patterns in the fish foodweb structure and function, which will be important considerations in the establishment of regional management plans for fisheries.

Hunt, George L.; Megrey, Bernard A.

doi: 10.1016/j.icesjms.2005.04.008pmid: N/A

The eastern Bering Sea and the Barents Sea share a number of common biophysical characteristics. For example, both are seasonally ice-covered, high-latitude, shelf seas, dependent on advection for heat and for replenishment of nutrients on their shelves, and with ecosystems dominated by a single species of gadoid fish. At the same time, they differ in important respects. In the Barents Sea, advection of Atlantic Water is important for zooplankton vital to the Barents Sea productivity. Advection of zooplankton is not as important for the ecosystems of the southeastern Bering Sea, where high levels of diatom production can support production of small, neritic zooplankton. In the Barents Sea, cod are the dominant gadoid, and juvenile and older fish depend on capelin and other forage fish to repackage the energy available in copepods. In contrast, the dominant fish in the eastern Bering Sea is the walleye pollock, juveniles and adults of which consume zooplankton directly. The southeastern Bering Sea supports considerably larger fish stocks than the Barents. In part, this may reflect the greater depth of much of the Barents Sea compared with the shallow shelf of the southeastern Bering. However, walleye pollock is estimated to occupy a trophic level of 3.3 as compared to 4.3 for Barents Sea cod. This difference alone could have a major impact on the abilities of these seas to support a large biomass of gadoids. In both seas, climate-forced variability in advection and sea-ice cover can potentially have major effects on the productivity of these Subarctic seas. In the Bering Sea, the size and location of pools of cold bottom waters on the shelf may influence the likelihood of predation of juvenile pollock.

Megrey, Bernard A.; Lee, Yong-Woo; Macklin, S. Allen

doi: 10.1016/j.icesjms.2005.05.018pmid: N/A

Many of the factors affecting recruitment in marine populations are still poorly understood, complicating the prediction of strong year classes. Despite numerous attempts, the complexity of the problem often seems beyond the capabilities of traditional statistical analysis paradigms. This study examines the utility of four statistical procedures to identify relationships between recruitment and the environment. Because we can never really know the parameters or underlying relationships of actual data, we chose to use simulated data with known properties and different levels of measurement error to test and compare the methods, especially their ability to forecast future recruitment states. Methods examined include traditional linear regression, non-linear regression, Generalized Additive Models (GAM), and Artificial Neural Networks (ANN). Each is compared according to its ability to recover known patterns and parameters from simulated data, as well as to accurately forecast future recruitment states. We also apply the methods to published Norwegian spring-spawning herring (Clupea harengus L.) spawner–recruit–environment data. Results were not consistently conclusive, but in general, flexible non-parametric methods such as GAMs and ANNs performed better than parametric approaches in both parameter estimation and forecasting. Even under controlled data simulation procedures, we saw evidence of spurious correlations. Models fit to the Norwegian spring-spawning herring data show the importance of sea temperature and spawning biomass. The North Atlantic Oscillation (NAO) did not appear to be an influential factor affecting herring recruitment.

Möllmann, Christian; Kornilovs, Georgs; Fetter, Marina; Köster, Friedrich W.

doi: 10.1016/j.icesjms.2005.04.021pmid: N/A

Oceanographic conditions in the brackish central Baltic Sea are strongly linked to atmospheric forcing and the unusual period of persistently strong westerlies that, since the late 1980s, have resulted in an increase in average water temperatures and decreasing salinity. These changes in temperature and salinity resulted in a change in the dominance of the mesozooplankton community from Pseudocalanus sp. to Temora longicornis and Acartia spp. Similar to the copepod community, the central Baltic fish community shifted from cod (Gadus morhua), dominant during the 1980s, to sprat (Sprattus sprattus), dominant during the 1990s. Further, the commercially important pelagic fish species herring (Clupea harengus) and sprat exhibited reductions in growth. Using Principal Component and Correlation Analyses we investigated the temporal variability in the importance of the food supply as well as competition on condition of central Baltic pelagic fish species. Our results indicate that herring condition results from a combined effect of changes in the food environment and increased competition with sprat, while sprat condition appeared to be primarily determined by intra-specific competition.

Orlova, E.L.; Boitsov, V.D.; Dolgov, A.V.; Rudneva, G.B.; Nesterova, V.N.

doi: 10.1016/j.icesjms.2005.05.020pmid: N/A

On the basis of data from cold (1982 and 1987) and warm summers (1983, 1984, 1990, and 1992), we explore the relationship between the phytoplankton bloom and the timing and intensity of the zooplankton bloom. In warm years, there is more overlap in the time between the zooplankton and the phytoplankton bloom. In northern areas (76–78°N) with seasonal ice, the phytoplankton bloom and reproductive processes in Calanus finmarchicus and Calanus glacialis continue well into August, evidenced by the presence of an abundance of nauplii and younger copepodites. We analyse feeding intensity of capelin and its distribution relative to food availability and capelin abundance. The extent to which feeding areas of cod and capelin, its major prey, overlap is subject to the abundance of these species, distribution of zooplankton, and sea temperature in a given year.

Skreslet, Stig; Borja, Angel; Bugliaro, Luca; Hansen, Georg; Meerkötter, Ralf; Olsen, Ketil; Verdebout, Jean

doi: 10.1016/j.icesjms.2005.05.019pmid: N/A

Zooplankton sampling in 1997 identified the frontal zone of the Norwegian Coastal Current as a reproduction habitat for Calanus finmarchicus in June–August. This area is subject to considerable ultraviolet radiation (UVR), as calculated from satellite observations of ozone and cloudiness. While in situ experiments indicated UVR-induced mortality in reproducing C. finmarchicus, monthly UVR doses during the actual reproduction period did not appear to affect the abundance of the resulting generation of adolescent copepodites (CIV-V) that accumulated in a fjord habitat during October 1983–2000. Local UVR in the spawning grounds of Arcto-Norwegian cod at the Lofoten Islands in March–May was positively correlated with the stock's 0-group index, which resulted in the rejection of the hypothesis that local UVR leads to high mortality of cod eggs or reduces the abundance of prey for cod larvae. Rather, the result suggests an indirect positive effect of UVR on the survival of cod eggs and larvae, possibly by controlling harmful microbes.