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Practical and applied knowledge of local fishers can help to improve our understanding of target species ecology and fisheries management decisions. In the Western Baltic Sea (WBS), the spatio-temporal distribution of cod is still largely unknown despite decades of research. We studied changes in cod distribution by obtaining information on temporal depth and habitat use of cod from commercial gillnet fishers using semi-directed interviews supplemented by at-sea observer data. Linear and non-linear regression analyses revealed significant relationships between depth use of cod and sea surface temperature (SST) as well as thermal stratification. Moreover, habitat use was related to SST and residence depth of cod. Areas deeper than 15 m were favoured from late December until March during low SST and a mixed water column (spawning) and also from July until August during high SST and strong thermal stratification (summer aestivation). Shallower areas were favoured during the rest of the year. The depth and habitat use displayed distinct seasonal up- and downslope movements of cod. This study highlights the importance of shallow-water and structured habitats for cod in the WBS and the value of local knowledge held by fishers for a better understanding of the distributional dynamics of important marine resource populations. [ABSTRACT FROM AUTHOR]

The stock–recruitment relationship is the basis of any stock prediction and thus fundamental for fishery management. Traditional parametric stock–recruitment models often poorly fit empirical data, nevertheless they are still the rule in fish stock assessment procedures. We here apply a multi-model approach to predict recruitment of 20 Atlantic cod (Gadus morhua) stocks as a function of adult biomass and environmental variables. We compare the traditional Ricker model with two non-parametric approaches: (i) the stochastic cusp model from catastrophe theory and (ii) multivariate simplex projections , based on attractor state-space reconstruction. We show that the performance of each model is contingent on the historical dynamics of individual stocks, and that stocks which experienced abrupt and state-dependent dynamics are best modelled using non-parametric approaches. These dynamics are pervasive in Western stocks highlighting a geographical distinction between cod stocks, which have implications for their recovery potential. Furthermore, the addition of environmental variables always improved the models' predictive power indicating that they should be considered in stock assessment and management routines. Using our multi-model approach, we demonstrate that we should be more flexible when modelling recruitment and tailor our approaches to the dynamical properties of each individual stock. [ABSTRACT FROM AUTHOR]

Fisheries and marine ecosystem-based management requires a holistic understanding of the dynamics of fish communities and their responses to changes in environmental conditions. Environmental conditions can simultaneously shape the spatial distribution and the temporal dynamics of a population, which together can trigger changes in the functional structure of communities. Here, we developed a comprehensive framework based on complementary multivariate statistical methodologies to simultaneously investigate the effects of environmental conditions on the spatial, temporal and functional dynamics of species assemblages. The framework is tested using survey data collected during more than 4000 fisheries hauls over the Baltic Sea between 2001 and 2016. The approach revealed the Baltic fish community to be structured into three sub-assemblages along a strong and temporally stable salinity gradient decreasing from West to the East. Additionally, we highlight a mismatch between species and functional richness associated with a lower functional redundancy in the Baltic Proper compared with other sub-areas, suggesting an ecosystem more susceptible to external pressures. Based on a large dataset of community data analysed in an innovative and comprehensive way, we could disentangle the effects of environmental changes on the structure of biotic communities—key information for the management and conservation of ecosystems. [ABSTRACT FROM AUTHOR]

Predictive maps of biodiversity patterns are pivotal to marine conservation and marine spatial planning alike, yet mapping of biodiversity indicators at the community-level is neither straightforward nor well-tested empirically. Two principle approaches exist. A direct approach involves calculation of indices for each sample, followed by interpolation to estimate values at unsampled locations. An indirect approach first interpolates individual species distributions and then determines indices based on the stacked distribution maps. We compared the appropriateness of both approaches to provide management-relevant information by mapping the distribution of demersal fish biodiversity in the German North Sea Exclusive Economic Zone using species richness, Hill's N1 and a novel traits-based community sensitivity to fishing index (CSI). To substitute zero-inflated species with up to 95% zeros in the sample data, we applied each species' mean abundance value as a flat surface. Spatial patterns between indicators varied, but certain hot- and cold-spots were revealed, which, under current legislation, might suggest that the present level of biodiversity protection is insufficient. Despite both approaches generating similar main patterns, the direct approach predicted a narrower range of index values and only depicted the most dominant patterns. Contrary to that the indirect approach better reproduced the variability in the data, along with additional information on species distributions and a theoretical advantage pertaining to sampling issues. Although the choice over the mapping approach is context dependent, for our study area featuring a community with relatively few species, we consider the indirect approach to provide the more reliable information for implementing marine environmental legislation. [ABSTRACT FROM AUTHOR]

Theory behind ecosystem-based management (EBM) and ecosystem-based fisheries management (EBFM) is now well developed. However, the implementation of EBFM exemplified by fisheries management in Europe is still largely based on single-species assessments and ignores the wider ecosystem context and impact. The reason for the lack or slow implementation of EBM and specifically EBFM is a lack of a coherent strategy. Such a strategy is offered by recently developed integrated ecosystem assessments (IEAs), a formal synthesis tool to quantitatively analyse information on relevant natural and socio-economic factors, in relation to specified management objectives. Here, we focus on implementing the IEA approach for Baltic Sea fish stocks. We combine both tactical and strategic management aspects into a single strategy that supports the present Baltic Sea fish stock advice, conducted by the International Council for the Exploration of the Sea (ICES). We first review the state of the art in the development of IEA within the current management framework. We then outline and discuss an approach that integrates fish stock advice and IEAs for the Baltic Sea. We intentionally focus on the central Baltic Sea and its three major fish stocks cod (Gadus morhua), herring (Clupea harengus), and sprat (Sprattus sprattus), but emphasize that our approach may be applied to other parts and stocks of the Baltic, as well as other ocean areas. [ABSTRACT FROM PUBLISHER]

An ecosystem approach to management (EAM) aims to secure a healthy ecosystem along with sustainable use of its goods and services. Although the main principles of EAM are agreed upon and desirable, wider implementation of EAM is still a challenge. The difficulties stem from unclear definition and communication of the EAM, lack of routines or protocols to develop ecosystem-based advice, inappropriate institutional structures, and communication issues between scientists, advisers, and managers. Integrated ecosystem assessment (IEA) is an instrument that has proven to help in the implementation of EAM. For successful implementation of EAM and IEA in the European regional seas context, an international forum is required that develops tailor-made EAM tools specific to the different regional ecosystems to overcome fragmented national strategies. We describe a multinational peer network of working groups developed within the International Council for the Exploration of the Sea (ICES) under the auspice of Science Steering Group on Regional Sea Programmes. Available is a wealth of data, expertise, scientific methods, and models for each regional sea. This network can be instrumental in advancing IEA for the implementation of EAM in the North Atlantic seas. [ABSTRACT FROM PUBLISHER]

Gårdmark, A., Nielsen, A., Floeter, J., and Möllmann, C. 2011. Depleted marine fish stocks and ecosystem-based management: on the road to recovery, we need to be precautionary. – ICES Journal of Marine Science, 68: 212–220. Precautionary management for fish stocks in need of recovery requires that likely stock increases can be distinguished from model artefacts and that the uncertainty of stock status can be handled. Yet, ICES stock assessments are predominantly deterministic and many EC management plans are designed for deterministic advice. Using the eastern Baltic cod (Gadus morhua) stock as an example, we show how deterministic scientific advice can lead to illusive certainty of a rapid stock recovery and management decisions taken in unawareness of large uncertainties in stock status. By (i) performing sensitivity analyses of key assessment model assumptions, (ii) quantifying the uncertainty of the estimates due to data uncertainty, and (iii) developing alternative stock and ecosystem indicators, we demonstrate that estimates of recent fishing mortality and recruitment of this stock were highly uncertain and show that these uncertainties are crucial when combined with management plans based on fixed reference points of fishing mortality. We therefore call for fisheries management that does not neglect uncertainty. To this end, we outline a four-step approach to handle uncertainty of stock status in advice and management. We argue that it is time to use these four steps towards an ecosystem-based approach to fisheries management. [ABSTRACT FROM PUBLISHER]

Shin, Y-J., Shannon, L. J., Bundy, A., Coll, M., Aydin, K., Bez, N., Blanchard, J. L., Borges, M. F., Diallo, I., Diaz, E., Heymans, J. J., Hill, L., Johannesen, E., Jouffre, D., Kifani, S., Labrosse, P., Link, J. S., Mackinson, S., Masski, H., Möllmann, C., Neira, S., Ojaveer, H., ould Mohammed Abdallahi, K., Perry, I., Thiao, D., Yemane, D., and Cury, P. M. 2010. Using indicators for evaluating, comparing, and communicating the ecological status of exploited marine ecosystems. 2. Setting the scene. – ICES Journal of Marine Science, 67: 692–716.Background is provided to the selection of ecological indicators by the IndiSeas Working Group, and the methodology adopted for analysis and comparison of indicators across exploited marine ecosystems is documented. The selected indicators are presented, how they are calculated is explained, and the philosophy behind the comparative approach is given. The combination of selected indicators is intended to reflect different dynamics, tracking processes that display differential responses to fishing, and is meant to provide a complementary means of assessing marine ecosystem trends and states. IndiSeas relied on inputs and insights provided by the local experts from participating ecosystems, helping to understand state and trend indicators and to disentangle the effect of other potential ecosystem drivers, such as climate variability. This project showed that the use of simple and available indicators under an ecosystem approach can achieve a real, wide-reaching evaluation of marine ecosystem status caused by fishing. This is important because the socio-economics of areas where fishing activities develop differs significantly around the globe, and in many countries, insufficient data are available for complex and exhaustive analyses. [ABSTRACT FROM PUBLISHER]

Rijnsdorp, A. D., Peck, M. A., Engelhard, G. H., Möllmann, C., and Pinnegar, J. K. 2009. Resolving the effect of climate change on fish populations. – ICES Journal of Marine Science, 66: 1570–1583.This paper develops a framework for the study of climate on fish populations based on first principles of physiology, ecology, and available observations. Environmental variables and oceanographic features that are relevant to fish and that are likely to be affected by climate change are reviewed. Working hypotheses are derived from the differences in the expected response of different species groups. A review of published data on Northeast Atlantic fish species representing different biogeographic affinities, habitats, and body size lends support to the hypothesis that global warming results in a shift in abundance and distribution (in patterns of occurrence with latitude and depth) of fish species. Pelagic species exhibit clear changes in seasonal migration patterns related to climate-induced changes in zooplankton productivity. Lusitanian species have increased in recent decades (sprat, anchovy, and horse mackerel), especially at the northern limit of their distribution areas, while Boreal species decreased at the southern limit of their distribution range (cod and plaice), but increased at the northern limit (cod). Although the underlying mechanisms remain uncertain, available evidence suggests climate-related changes in recruitment success to be the key process, stemming from either higher production or survival in the pelagic egg or larval stage, or owing to changes in the quality/quantity of nursery habitats. [ABSTRACT FROM PUBLISHER]

Kraus, G., Pelletier, D., Dubreuil, J., Möllmann, C., Hinrichsen, H-H., Bastardie, F., Vermard, Y., and Mahévas, S. 2009. A model-based evaluation of Marine Protected Areas: the example of eastern Baltic cod (Gadus morhua callarias L.). – ICES Journal of Marine Science, 66: 109–121. The eastern Baltic cod stock collapsed as a consequence of climate-driven adverse hydrographic conditions and overfishing and has remained at historically low levels. Spatio-temporal fishing closures [Marine Protected Areas (MPAs)] have been implemented since 1995, to protect and restore the spawning stock. However, no signs of recovery have been observed yet, either suggesting that MPAs are an inappropriate management measure or pointing towards suboptimal closure design. We used the spatially explicit fishery simulation model ISIS-Fish to evaluate proposed and implemented fishery closures, combining an age-structured population module with a multifleet exploitation module and a management module in a single model environment. The model is parameterized based on (i) the large amount of biological knowledge available for cod and (ii) an analysis of existing spatially disaggregated fishery data. As the population dynamics of eastern Baltic cod depend strongly on the climate-driven hydrographic regime, we considered two production regimes of the stock. MPAs were only effective for stock recovery when they reduced overall fishing effort. The performance of MPAs needs to be evaluated relative to environmental regimes, especially for stocks facing strong environmental variability. [ABSTRACT FROM PUBLISHER]

Möllmann, C., Müller-Karulis, B., Kornilovs, G., and St John, M. A. 2008. Effects of climate and overfishing on zooplankton dynamics and ecosystem structure: regime shifts, trophic cascade, and feeback loops in a simple ecosystem. – ICES Journal of Marine Science, 65: 302–310.The Central Baltic Sea is the largest brackish waterbody in the world ocean, containing a highly productive but low-diversity ecosystem. Climate-induced changes in hydrography recently caused an ecosystem regime shift with changes at all trophic levels. The most pronounced changes in the ecosystem occurred at the zooplankton and fish trophic levels. In the zooplankton, dominance changed between the copepods Pseudocalanus acuspes and Acartia spp., a result of reduced salinities and increased temperatures. The change in hydrography also affected the reproductive success of the major fish species, resulting in a change in dominance from the piscivorous cod (Gadus morhua) to the planktivorous sprat (Sprattus sprattus). First, we investigate statistically the occurrence of regime shifts in time-series of key hydrographic variables and the biomass time-series of key species. Second, we demonstrate a three-level trophic cascade involving zooplankton. Finally, we model the ecosystem effects of the abiotic and biotic changes on copepod biomass and recruitment of fish stocks. Our results demonstrate the linkage between climate-induced zooplankton and fish regime changes, and how overfishing amplified the climate-induced changes at both trophic levels. Hence, our study demonstrates (i) the multiple pathways along which climatic and anthropogenic pressures can propagate through the foodweb; (ii) how both effects act synergistically to cause and stabilize regime changes; and (iii) the crucial role of zooplankton in mediating these ecosystem changes. [ABSTRACT FROM PUBLISHER]

Large-scale climatic conditions prevailing over the central Baltic Sea resulted in declining salinity and oxygen concentrations in spawning areas of the eastern Baltic cod stock. These changes in hydrography reduced the reproductive success and, combined with high fishing pressure, caused a decline of the stock to the lowest level on record in the early 1990s. The present study aims at disentangling the interactions between reproductive effort and hydrographic forcing leading to variable recruitment. Based on identified key processes, stock dynamics is explained using updated environmental and life stage-specific abundance and production time-series. Declining salinities and oxygen concentrations caused high egg mortalities and indirectly increased egg predation by clupeid fish. Low recruitment, despite enhanced hydrographic conditions for egg survival in the mid-1990s, was due to food limitation for larvae, caused by the decline in the abundance of the copepod Pseudocalanus sp. The case of the eastern Baltic cod stock exemplifies the multitude effects climatic variability may have on a fish stock and underscores the importance of knowledge of these processes for understanding stock dynamics. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]

Throughout the 1980s, reproductive success of the top-predator cod declined and stock sizes of the main prey species herring and especially sprat, important planktivorous predators in the system, increased substantially. Although the hydrographic conditions conducive for survival of early life stages improved during the 1990s, recruitment success of cod remained far below average. As clupeids have been identified as major predators on cod eggs and larvae in the Baltic, increased predation may be an important factor hampering stock recovery. Results from stomach content analysis of herring and sprat during the spawning season of 1988–1995 and ichthyoplankton surveys in the Bornholm Basin (the only important spawning area of cod in the Central Baltic in this period) allow a comparison of estimated consumption rates by the predator populations with standing stocks and production rates of cod eggs and larvae. Despite uncertainties in the estimation procedure, the findings confirm substantial predation on cod eggs by both clupeid species. Especially at the beginning of the cod-spawning season, characterized by low zooplankton availability, sprat consumed a considerable proportion of the eggs produced. In 1993, the relative importance of zooplankton as prey increased, while fish eggs were encountered more rarely. In contrast, predation by herring remained on the same level or even increased, especially late in the spawning season. The larval stage of cod is not substantially affected by predation owing to limited spatial overlap between prey and predator. [ABSTRACT FROM PUBLISHER]