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Many marine fish species are widely distributed over large areas. Failing to acknowledge that such species may be composed of distinct populations may result in overestimation of the stock's true harvest potential. To avoid overexploitation, ways to identify population structuring are therefore needed. In this study, we developed and applied a statistical approach to identify biologically relevant population boundaries for a widely distributed marine fish species, European sprat (Sprattus sprattus). Specifically, we compiled and standardized multiple trawl-survey data sets and used a range of statistical tools to assess whether the current management boundaries adequately account for potential population structuring. Our results demonstrate regional differences in spatial abundance patterns, temporal dynamics and population demographics. These findings are in line with recent genetic studies of sprat, indicating reproductive isolation between the Baltic Sea/Kattegat and a larger cluster containing the North-, Irish-, Celtic Sea, and Bay of Biscay. Since relying on routinely collected survey data, our statistical approach can be a cost-effective complement to population genetic methods for detecting population structuring. These can be used to guide spatial management efforts and ensure sustainable exploitation, especially under climate change and the expected changes in species distributions across current management borders.

Fish community biomass is generally thought to decline with increasing temperature due to higher metabolic losses resulting in less efficient energy transfer in warm-water food webs. However, whether these metabolic predictions explain observed macroecological patterns in fish community biomass is virtually unknown. Here we test these predictions by examining the variation in demersal fish biomass across 21 productive shelf regions using high-resolution monitoring data from the North Atlantic and Northeast Pacific. We find that biomass per km2 varies 40-fold across regions and is highest in cold waters and areas with low fishing exploitation. We find no evidence that temperature change has impacted biomass within marine regions over time. Yet, the cross-regional patterns suggest that long-term impacts of warming will be negative on biomass. These results provide an empirical basis for predicting future changes in fish community biomass and its associated services for human wellbeing i.e., food provisioning, under global warming.

Non-indigenous species (NIS) pose a major threat to biodiversity and the structure and functioning of ecosystems worldwide. However, our understanding of the underlying assembly processes acting on NIS, as well as the relationship with native species is limited, especially in marine ecosystems. To overcome this knowledge gap we here developed a trait-based approach to assess niche overlap and functional distinctiveness between NIS and native species, using high-resolution data on benthic invertebrate communities in the Baltic Sea as a case study. Our results show a moderate degree of functional distinctiveness of NIS if considering the entire trait space, but significant differences in individual traits, notably adult life span, size and bioturbation. This indicates that NIS may successfully establish by displaying only one or a few singular traits, while retaining a certain degree of similarity with the natives. Furthermore, we demonstrate pronounced spatial patterns in distinctiveness, largely explained by environmental conditions and biotic effects channeled through species richness and evenness. Hence, several community assembly processes, including environmental filtering and limiting similarity may affect the establishment and success of NIS, while their effect and relative importance may be highly context dependent. Finally, our trait-based approach provides a generic framework applicable to other areas and organisms, to better understand and address biological invasions and their impacts on biodiversity and marine ecosystem functioning at large.

Teaching ressources developed for FutureMares Task 1.2. Trait-environment workshop. 7th June 2021 All these ressources are under GNU General Public Licence v3. When using this resource, please cite it as: Frelat, R., Beukhof, E., Weigel, B. & Lindegren, M.(2022).Tutorial for analysing trait-environment relationships, Zenodo DOI: 10.5281/zenodo.6712534 &nbsp

1. The concept of biodiversity-ecosystem functioning (BEF) has been studied over the last three decades using experiments, theoretical models, and more recently observational data. While theoretical models revealed that species richness is the best metric summarizing ecosystem functioning, it is clear that ecosystem function is explained by other variables besides species richness. Additionally, theoretical models rarely focus on more than one ecosystem function, limiting ecosystem functioning to biomass or production. There is a lack of theoretical background to verify how other components of biodiversity and species interactions support ecosystem functioning.2. Here, using simulations from a food web model based on a community assembly process and a trait-based approach, we test how species biodiversity, food web structure and predator-prey interactions determine several ecosystem functions (biomass, metabolism, production, and productivity).3. Our results demonstrate that the relationship between species richness and ecosystem functioning depends on the type of ecosystem function considered and the importance of diversity and food web structure differs across functions. Particularly, we show that dominance plays a major role in determining the level of biomass, and is at least as important as the number of species. We find that dominance occurs in the food web when species do not experience strong predation.4. By manipulating the structure of the food web, we show that species using a wider trait space (generalist communities) result in more connected food webs, and generally reach the same level of functioning with less species. The model shows the importance of generalist versus specialist communities on biodiversity-ecosystem functioning relationships, and as such, empirical studies should focus on quantifying the importance of diet/habitat use on ecosystem functioning.5. Our study provides a better understanding of BEF underlying mechanisms, and generates research hypotheses that can be considered and tested in observational studies. We recommend that studies investigating links between biodiversity and ecosystem functions should include metrics of dominance, species composition, trophic structure and possibly environmental trait space. We also advise that more effort should be made into calculating several ecosystem functions and properties with data from natural multi-trophic systems.

Non-indigenous species (NIS) pose a major threat to biodiversity and the functioning and services of ecosystems. Despite their rapid spread in coastal waters worldwide, biotic invasions are widely disregarded in marine conservation planning. To guide conservation actions, a better understanding of the underlying mechanisms determining the success of NIS are therefore needed. Here we develop a joint modelling approach to identify the key drivers and community assembly processes determining the occurrence of invasive benthic invertebrates, using Danish coastal waters as a case study. To reflect factors affecting the introduction, establishment and spread of NIS throughout the area, we compiled long-term monitoring data on NIS, as well as information on commercial shipping, environmental conditions and estimates of larvae settling densities derived from drift model simulations informed by species traits. We then applied a set of species distribution models to identify the key drivers determining the occurrence of NIS. Our results demonstrate a significant positive effect of vessel activity, a negative effect of depth and bottom salinity, as well as a positive effect of the simulated settling densities on the probability of presence. Taken together, our results highlight the role of commercial shipping, habitat characteristics and passive advection of early-life stages on the success of NIS. Our joint modelling approach provide improved process understanding on the key community assembly processes determining the presence of NIS and may serve to guide monitoring, management and conservation planning in order to limit future invasions and their negative consequences on coastal ecosystems.

Funding Information: This work was supported by the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No 675997. The authors are very thankful to Prof. Dr. Mehmet Aydın from the Ordu University Fatsa Faculty of Marine Sciences for his support and contribution and to anchovy fishermen who answered our questions sincerely and patiently. We would also like to thank constructive comments received by anonymous reviewers during the peer review. Funding Information: This work was supported by the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No 675997. The authors are very thankful to Prof. Dr. Mehmet Aydın from the Ordu University Fatsa Faculty of Marine Sciences for his support and contribution and to anchovy fishermen who answered our questions sincerely and patiently. We would also like to thank constructive comments received by anonymous reviewers during the peer review. Data Availability Statement: The data that is used and support the findings are publicly available. Catch and price statistics of the anchovy are openly available by the Turkish Central Statistical Database via the link: https:// biruni.tuik.gov.tr/medas/?kn=97&locale=tr. Biological parameters of the anchovy stock assessment are available in the report STECF-14-14, EUR 26896 via . Economic survey data of the anchovy fishing fleet can be made available upon request. Publisher Copyright: © 2022, Mediterranean Marine Science. All Rights Reserved. European anchovy (Engraulis encrasicolus) is the main commercially exploited fish stock in the Black Sea region, providing a vital source of livelihood and revenue for local communities and national economies. In recent decades, the Black Sea anchovy stock has faced many human-induced threats, including overfishing, eutrophication, invasive species, and climate change while these threats have raised concerns about the status and long-term productivity of the stock. To ensure sustainable levels of exploitation under potential future changes in stock productivity, we here estimate and compare a suite of biological and economic reference points under different levels of stock productivity and discount rates using an age-structured bioeconomic model setup. Our model simulations showed that optimal fishing mortalities achieving maximum sustainable yield (FMSY) and maximum economic yield (FMEY) increase at higher stock productivity but are always lower than the historically high mean levels of exploitation. Furthermore, we illustrate that the stock biomass at maximum economic yield (BMEY) is larger than the stock biomass at maximum sustainable yield (BMSY) at all stock productivities and discount rates, except at low stock productivity under high levels of discounting (i.e., 10%, 20%). By illustrating the ecological and economic benefits of reducing exploitation rates, we expect that our estimated reference points can add value to the decision-making process for the management of the European anchovy fishery and ensure long-term sustainable management even under future climate-driven changes in stock productivity.

A fundamental challenge in ecology is to understand why species are found where they are and predict where they are likely to occur in the future. Trait-based approaches may provide such understanding, because it is the traits and adaptations of species that determine which environments they can inhabit. It is therefore important to identify key traits that determine species distributions and investigate how these traits relate to the environment. Based on scientific bottom-trawl surveys of marine fish abundances and traits of >1,200 species, we investigate trait-environment relationships and project the trait composition of marine fish communities across the continental shelf seas of the Northern hemisphere. We show that traits related to growth, maturation and lifespan respond most strongly to the environment. This is reflected by a pronounced “fast-slow continuum” of fish life-histories, revealing that traits vary with temperature at large spatial scales, but also with depth and seasonality at more local scales. Our findings provide insight into the structure of marine fish communities and suggest that global warming will favour an expansion of fast-living species. Knowledge of the global and local drivers of trait distributions can thus be used to predict future responses of fish communities to environmental change., SI

Marine zooplankton are among the most diverse and abundant organisms on earth. Despite a great wealth of knowledge and research on their ecology, the processes promoting coexistence and maintaining their high diversity worldwide are poorly known. In order to understand the processes underpinning coexistence among marine zooplankton, we investigated the existence and degree of niche separation within two pairs of taxonomically and ecologically related species of copepods belonging to the widespread, abundant genera Calanus and Oithona. We compared the spatial variation in species abundances to the abiotic and biotic environment using a multimodel approach and vertically resolved survey data on zooplankton composition, abundances, and environmental conditions in the North Sea. Our results demonstrate pronounced spatial differences between species in each pair, primarily in their vertical abundance distributions. These differences can largely be explained by different preferences for temperature and salinity. This supports the occurrence of environmental niche separation in marine zooplankton and highlights its role as a mechanism reducing interspecific competition and promoting coexistence.

Describing the spatial and temporal dynamics of communities is essential for understanding the impacts of global environmental change on biodiversity and ecosystem functioning. Trait-based approaches can provide better insight than species-based (i.e., taxonomic) approaches into community assembly and ecosystem functioning, but comparing species and trait dynamics may reveal important patterns for understanding community responses to environmental change. Here, we used a 33-year database of fish monitoring to compare the spatio-temporal dynamics of taxonomic and trait structure in North Sea fish communities. We found that the majority of variation in both taxonomic and trait structure was explained by a pronounced spatial gradient, with distinct communities in the southern and northern North Sea related to depth, sea surface temperature, salinity and bed shear stress. Both taxonomic and trait structure changed significantly over time, however taxonomically, communities in the south and north diverged toward different species, becoming more dissimilar over time, yet they converged toward the same traits regardless of species differences. In particular, communities shifted toward smaller, faster-growing species with higher thermal preferences and pelagic water column position. Although taxonomic structure changed over time, its spatial distribution remained relatively stable, whereas in trait structure the southern zone of the North Sea shifted northward and expanded, leading to homogenization. Our findings suggest that global environmental change, notably climate warming, will lead to convergence toward traits more adapted for novel environments regardless of species composition. This article is protected by copyright. All rights reserved.

The rate at which biological diversity is altered on both land and in the sea, makes temporal community development a critical and fundamental part of understanding global change. With advancements in trait‐based approaches, the focus on the impact of temporal change has shifted towards its potential effects on the functioning of the ecosystems. Our mechanistic understanding of and ability to predict community change is still impeded by the lack of knowledge in long‐term functional dynamics that span several trophic levels. To address this, we assessed species richness and multiple dimensions of functional diversity and dynamics of two interacting key organism groups in the marine food web: fish and zoobenthos. We utilized unique time series‐data spanning four decades, from three environmentally distinct coastal areas in the Baltic Sea, and assembled trait information on six traits per organism group covering aspects of feeding, living habit, reproduction and life history. We identified gradual long‐term trends, rather than abrupt changes in functional diversity (trait richness, evenness, dispersion) trait turnover, and overall multi‐trait community composition. The linkage between fish and zoobenthic functional community change, in terms of correlation in long‐term trends, was weak, with timing of changes being area and trophic group specific. Developments of fish and zoobenthos traits, particularly size (increase in small size for both groups) and feeding habits (e.g. increase in generalist feeding for fish and scavenging or predation for zoobenthos), suggest changes in trophic pathways. We summarize our findings by highlighting three key aspects for understanding functional change across trophic groups: (a) decoupling of species from trait richness, (b) decoupling of richness from density and (c) determining of turnover and multi‐trait dynamics. We therefore argue for quantifying change in multiple functional measures to help assessments of biodiversity change move beyond taxonomy and single trophic groups., In this study, we identified gradual long‐term trends in functional diversity, trait turnover, and overall multi‐trait community composition spanning a period of 40 years and two key trophic groups: fish and zoobenthos, in three coastal marine areas. The study highlights the need for multiple measures and cross‐trophic level assessments to understand empirical functional (trait) change and serves as a baseline for functional change in the Baltic Sea region and other coastal and estuarine ecosystems worldwide. The findings contribute to the general understanding of biodiversity change and can be useful for developing predictions and models of community change.

Marine ecosystems are exposed to a range of environmental and anthropogenic stressors, including climate change and overexploitation. A promising way towards understanding the impacts of such stressors on community composition is by considering species traits rather than species identity. Here, we describe the spatio-temporal dynamics in fish community traits using >30 yr of species abundance data from the North Sea combined with trait information on body size, life history, growth rate, reproduction and trophic level for demersal fish species in the area. We assessed whether the derived patterns and trends in community-weighted mean traits could be explained by a range of environmental stressors and fishing. Our results revealed strong spatial structuring and long-term changes in the trait composition of North Sea fish, with temporal changes not being uniformly distributed in space. Among the environmental drivers investigated, depth was one of the best predictors, primarily explaining the spatial variation in lifespan, growth rate, trophic level and fecundity. This can be explained by variables that co-vary with depth, e.g. temperature, seasonality, salinity and productivity. Finally, we found only weak relationships between fishing and the spatial variation of traits, suggesting that the spatial trait composition of the community is mostly determined by the environment. Yet, long-term changes in trait composition, primarily in body size, have previously been shown to be affected by size-selective fishing. Our study exemplifies how traits can be used to summarize complex community dynamics and responses to environmental and anthropogenic stressors as well as their usefulness for ecosystembased management

Marine biota are redistributing at a rapid pace in response to climate change and shifting seascapes. While changes in fish populations and community structure threaten the sustainability of fisheries, our capacity to adapt by tracking and projecting marine species remains a challenge due to data discontinuities in biological observations, lack of data availability, and mismatch between data and real species distributions. To assess the extent of this challenge, we review the global status and accessibility of ongoing scientific bottom trawl surveys. In total, we gathered metadata for 283,925 samples from 95 surveys conducted regularly from 2001 to 2019. We identified that 59% of the metadata collected are not publicly available, highlighting that the availability of data is the most important challenge to assess species redistributions under global climate change. Given that the primary purpose of surveys is to provide independent data to inform stock assessment of commercially important populations, we further highlight that single surveys do not cover the full range of the main commercial demersal fish species. An average of 18 surveys is needed to cover at least 50% of species ranges, demonstrating the importance of combining multiple surveys to evaluate species range shifts. We assess the potential for combining surveys to track transboundary species redistributions and show that differences in sampling schemes and inconsistency in sampling can be overcome with spatio‐temporal modeling to follow species density redistributions. In light of our global assessment, we establish a framework for improving the management and conservation of transboundary and migrating marine demersal species. We provide directions to improve data availability and encourage countries to share survey data, to assess species vulnerabilities, and to support management adaptation in a time of climate‐driven ocean changes., While many species are changing their spatial distributions rapidly with climate change, the global capacity to track these changes and then adapt management and policy has been limited. To build this capacity for demersal fishes, we compiled the largest worldwide collection of bottom trawl survey metadata and demonstrate how surveys can be combined to follow species across national and survey boundaries. Open data and transparent scientific assessment can be a key step towards improved management of redistributing transboundary species and stocks.

Biomass limit reference points are widely used in fisheries management and defines the biomass threshold below which stock productivity (i.e. recruitment) is likely to be impaired. Scientifically sound and transparent methods for estimating biomass thresholds are therefore needed together with ways of quantifying uncertainties. The main focus of the study was placed on two methods currently applied to several small-bodied pelagic species in the Northeast Atlantic. These methods have not formerly been described in the scientific literature and are in the present study being compared to some already described methods, of which, one is broadly applied outside the Northeast Atlantic. Using a combination of data simulations and data from 51 small-bodied pelagic fish stocks, we analyzed the sensitivity of estimated biomass thresholds to (i) the choice of method, (ii) time-series length, and (iii) stock development (e.g. rebuilding or declining). It was demonstrated that estimated biomass thresholds are associated with considerable uncertainty not previously quantified. Furthermore, the level of the estimated threshold and the amount of uncertainty depended on choice of method, time-series length, and stock development trends. Hence, this study contributes to improving the quality of future biomass limit reference points by providing guidance regarding choice of method and how to demonstrate stock-specific uncertainties.

Climate effects on marine fish depend on life stage, particularly when life stages differ in habitat utilization. In the present study, we investigated life stage-dependent responses of lesser sandeel (Ammodytes marinus) to temperature at contrasting geographical scales. We related population density and individual growth to temperature and found different temperature responses between the first and the second years of life. During the first year of life, fish size was the single most important factor influencing sandeel abundances, indicating a positive relationship between growth and survival. In contrast, during the second year of life, autumn bottom temperature was negatively correlated with sandeel abundance, suggesting elevated mortality in warm years. Southerly areas, experiencing higher temperatures in general, were also the areas showing the strongest response to temperature. This study sheds light on how warming impacts population dynamics of one of the most important forage fishes in the North Sea and supports the discussion of underlying mechanisms.

Management strategy evaluation (MSE) is the state-of-the-art approach for testing and comparing management strategies in a way that accounts for multiple sources of uncertainty (e.g. monitoring, estimation, and implementation). Management strategy evaluation can help identify management strategies that are robust to uncertainty about the life history of the target species and its relationship to other species in the food web. Small pelagic fish (e.g. anchovy, herring and sardine) fulfil an important ecological role in marine food webs and present challenges to the use of MSE and other simulation-based evaluation approaches. This is due to considerable stochastic variation in their ecology and life history, which leads to substantial observation and process uncertainty. Here, we summarize the current state of MSE for small pelagic fishes worldwide. We leverage expert input from ecologists and modellers to draw attention to sources of process and observation uncertainty for small pelagic species, providing examples from geographical regions where these species are ecologically, economically and culturally important. Temporal variation in recruitment and other life-history rates, spatial structure and movement, and species interactions are key considerations for small pelagic fishes. We discuss tools for building these into the MSE process, with examples from existing fisheries. We argue that model complexity should be informed by management priorities and whether ecosystem information will be used to generate dynamics or to inform reference points. We recommend that our list of considerations be used in the initial phases of the MSE process for small pelagic fishes or to build complexity on existing single-species models. publishedVersion

BONUS XWEBS Deliverable D2.1

As the body of literature on marine climate impacts accumulates the question is no longer whether marine ecosystems and their living resources are affected, but what we as scientists, managers and ...

The degree to which ecosystems are regulated through bottom-up, top-down or direct physical processes represents a long-standing issue in ecology, with important consequences for resource management and conservation. In marine ecosystems, the role of bottom-up and top-down forcing has been shown to vary over spatio-temporal scales, often linked to highly variable and heterogeneously distributed environmental conditions. Ecosystem dynamics in the Northeast Pacific have been suggested to be predominately bottom-up regulated. However, it remains unknown to what extent top-down regulation occurs, or whether the relative importance of bottom-up and top-down forcing may shift in response to climate change. In this study, we investigate the effects and relative importance of bottom-up, top-down and physical forcing during changing climate conditions on ecosystem regulation in the Southern California Current System (SCCS) using a generalized food web model. This statistical approach is based on non-linear threshold models and a long-term data set (~60 year) covering multiple trophic levels from phytoplankton to predatory fish. We found bottom-up control to be the primary mode of ecosystem regulation. However, our results also demonstrate an alternative mode of regulation represented by interacting bottom-up and top-down forcing, analogous to wasp-waist dynamics, but occurring across multiple trophic levels and only during periods of reduced bottom-up forcing (i.e., weak upwelling, low nutrient concentrations and primary production). The shifts in ecosystem regulation are caused by changes in ocean-atmosphere forcing and triggered by highly variable climate conditions associated with El Niño. Furthermore, we show that biota respond differently to major El Niño events during positive or negative phases of the Pacific Decadal Oscillation (PDO), as well as highlight potential concerns for marine and fisheries management by demonstrating increased sensitivity of pelagic fish to exploitation during El Niño. This article is protected by copyright. All rights reserved.

Large teleost (bony) fish are a dominant group of predators in the oceans and constitute a major source of food and livelihood for humans. These species differ markedly in morphology and feeding habits across oceanic regions; large pelagic species such as tunas and billfish typically occur in the tropics, whereas demersal species of gadoids and flatfish dominate boreal and temperate regions. Despite their importance for fisheries and the structuring of marine ecosystems, the underlying factors determining the global distribution and productivity of these two groups of teleost predators are poorly known. Here, we show how latitudinal differences in predatory fish can essentially be explained by the inflow of energy at the base of the pelagic and benthic food chain. A low productive benthic energy pathway favours large pelagic species, whereas equal productivities support large demersal generalists that outcompete the pelagic specialists. Our findings demonstrate the vulnerability of large teleost predators to ecosystem-wide changes in energy flows and hence provide key insight to predict the responses of these important marine resources under global change.

Growth is a fundamental physiological process influencing the state and dynamics of fish stocks, yet the physical and biological conditions affecting individual weight and growth throughout ontogeny are poorly known and often unaccounted for in fisheries management. This is rather surprising given that changes in growth have strong direct effects on the total biomass and potential yield derived from any given stock. In this study, we investigate the underlying factors affecting fish growth throughout the life span of cohorts using statistical modelling and long-term observational data on sprat (Sprattus sprattus), a commercially and ecologically important small-pelagic fish species across European seas. Our results demonstrate a negative relationship between total abundance and weight, as well as a positive and dome-shaped relationship between temperature and zooplankton abundance (i.e. food availability), respectively. Furthermore, we demonstrate how such improved knowledge and understanding of the underlying factors affecting weight and growth could be accounted for in future assessment models, by including these considerations into short-term forecast simulations. This, in turn, would provide a stronger scientific basis for management advice and ensure the sustainability and profitability of fisheries, particularly on small and commercially valuable pelagic species with pronounced spatio-temporal variability in weight and growth.

Large‐scale alterations in marine ecosystems as a response to environmental and anthropogenic pressures have been documented worldwide. Yet, these are primarily investigated by assessing abundance fluctuations of a few dominant species, which inadequately reflect ecosystem‐wide changes. In addition, it is increasingly recognized that it is not species identity per se, but their traits that determine environmental responses, biological interactions and ecosystem functioning. In this study, we investigated long‐term, spatio‐temporal variability in trait composition across multiple organism groups to assess whether functional changes occur in a similar way across trophic levels and whether shifts in trait composition link to environmental change. We combined extensive trait datasets with long‐term surveys (30–40 yr) of four organism groups (phytoplankton, zooplankton, benthic invertebrates and fish) in three environmentally distinct areas of a large marine ecosystem. We found similar temporal trajectories in the community weighted mean trait time‐series of the different trophic groups, revealing ecosystem‐wide functional changes. The traits involved and their dynamics differed between areas, concurrent with climate‐driven changes in temperature and salinity, as well as more local dynamics in nutrients and oxygen. This finding highlights the importance of considering both global climate, as well as local external drivers when studying ecosystem changes. Using a multi‐trophic trait‐based approach, our study demonstrates the importance of integrating community functional dynamics across multiple trophic levels to capture ecosystem‐wide responses which could, ultimately, help moving towards a holistic understanding, assessment and management of marine ecosystems.

Game theory has been an effective tool to generate solutions for decision making in fisheries involving multiple countries and fleets. Here, we use a coupled bio-economic model based on a Baltic Sea dynamic multispecies food web model called BALMAR and, we compare non-cooperative (NC) and cooperative game (grand coalition: GC) solutions. Applications of game theory based on a food web model under climate change have not been studied before and the present study aims to fill this gap in the literature. The study focuses on the effects of climate variability on the biological, harvest and economic output of the game models by examining two different climate scenarios, a first scenario characterized by low temperature and high salinity and a second scenario by high temperature and low salinity. Our results showed that in the first scenario sprat spawning stock biomass (SSB) and harvest dropped dramatically both in the NC and the GC cases whereas, herring and cod SSBs and harvests were higher compared to a base scenario (BS) keeping temperature and salinity at mean historical levels. In the second scenario, the sprat SSB and the harvest was higher for both GC and NC cases while the cod and the herring SSBs and harvests were lower. The total GC payoffs clearly outperformed the NC payoffs across all scenarios. Likewise, the first and second scenario GC payoffs for countries were higher except for Poland. The findings suggested the climate vulnerability of Baltic Sea multi-species fisheries and these results would support future decision-making processes of Baltic Sea fisheries.

The presence and survival of the species in a community depend on their abilities to maximize fitness in a given environment. The study of the processes that control survival and co‑existence, termed ‘assembly rules’, follows various mechanisms, primarily related to biotic or abiotic factors. To determine assembly rules, ecological similarities of co-occurring species are often investigated. This can be evaluated using trait-based indices summarizing the species’ niches in a given community. In order to investigate the underlying processes shaping community assembly in marine ecosystems, we investigated the patterns and drivers of fish community composition in the Baltic Sea, a semi-enclosed sea characterized by a pronounced environmental gradient. Our results showed a marked decline in species- and functional richness, largely explained by decreasing salinities. In addition, habitat complexity and oxygen were found to be significant drivers. Furthermore, we showed that the trait composition of the fish community in the western Baltic Sea is more similar than expected by random chance alone. This implies that environmental filtering, acting along the salinity gradient, is the dominant factor shaping community composition. However, community composition in the eastern part, an area beyond the steep decline in salinity, was characterized by fewer species with largely different trait characteristics, indicating that community assembly is also affected by biotic interactions. Our results add to the knowledge base of key abiotic drivers impacting marine fish communities and their vulnerability to environmental changes, a key concern for fisheries and marine ecosystem management

Throughout much of the world’s oceans, life is organized around seasonal cycles of feast and famine. Here we seek to understand the life-history strategies by which marine organisms contend with seasonal variations through a range of adaptations and traits, including overwintering stages, dormancy, investment in reserves, and migration. Our perspective is broad, spanning across marine food webs, from unicellular plankton to whales, and covering all latitudes, from the equator to the poles. The analysis is organized around a simple mechanistic life history optimization model. The model generates several general hypotheses: (i) small organisms should cope with winters by making resting stages or by dormancy; (ii) medium-sized organisms should build reserves and perform seasonal vertical migration to reduce predation; (iii) large organisms should primarily employ latitudinal migrations to follow seasonal peaks in production. Subsequently, these hypotheses are tested against a large assemblage of observations and data reported in the literature. Body size, trophic level, and the intensity and duration of seasonal highs and lows appear to be closely related to seasonal strategies. Some, but not all of these hypotheses are borne out by our analysis of data. In particular, we find that organisms with a lifespan on the order of the seasonal length employ a multitude of strategies.

The relationship between biodiversity and ecosystem functioning (BEF) is a topic of considerable interest to scientists and managers because a better understanding of its underlying mechanisms may help us mitigate the consequences of biodiversity loss on ecosystems. Our current knowledge of BEF relies heavily on theoretical and experimental studies, typically conducted on a narrow range of spatio-temporal scales, environmental conditions, and trophic levels. Hence, whether a relationship holds in the natural environment is poorly understood, especially in exploited marine ecosystems. Using large-scale observations of marine fish communities, we applied a structural equation modelling framework to investigate the existence and significance of BEF relationships across northwestern European seas. We find that ecosystem functioning, here represented by spatial patterns in total fish biomass, is unrelated to species richness—the most commonly used diversity metric in BEF studies. Instead, community evenness, differences in species composition, and abiotic variables are significant drivers. In particular, we find that high fish biomass is associated with fish assemblages dominated by a few generalist species of a high trophic level, who are able to exploit both the benthic and pelagic energy pathway. Our study provides a better understanding of the mechanisms behind marine ecosystem functioning and allows for the integration of biodiversity into management considerations.

Human action is transforming the species composition, biogeochemistry and habitats of the world’s oceans at unprecedented rates. The cumulative effect of natural and anthropogenic drivers is challenging to measure, in part due to indirect effects and the complexity of marine systems. Building on the theory of complex adaptive systems, this thesis aims to increase our understanding of how complex, heterogeneous marine social-ecological systems (SES) may respond to changing conditions. This thesis integrates resilience research with network science and describes change and structural patterns at several SES scales in order to advance our knowledge on the effects of multiple drivers.Paper I proposes a new, quantitative fish stock collapse definition, that accounts for fish stock dynamics and enables standardization and thus comparability across a large number of commercial fish stocks. Recognizing that substantial ecosystem changes are part of SES dynamics, in Paper II we review marine regime shifts worldwide to specify how co-occurring bundles of drivers are related to degraded ecosystem services for management purposes. A more detailed ecological study on regime shifts was performed in Papers III and IV. Paper III describes the late-1980s central Baltic Sea regime shift based on a food-web model. Paper IV uses a novel structural network analysis approach to detect functional shifts in complex food webs. The results of Paper IV imply that the Baltic Sea regime shift may not be a systemwide shift. Paper V uses a network approach to analyze fishing strategy diversification and social-ecological connectivity among Swedish Baltic Sea fishers, indicating that natural resource management evaluations should not be limited only to ecosystem conditions but also take account of social conditions.Overall, this thesis provides empirical evidence for the emerging perspective that marine resource science and management must account for the complexity of system elements in order to ensure the provision of ecosystem services in the future. The first application of Exponential Random Graph Modeling in ecology and an improved fish stock collapse definition provide new advanced tools for studying oceans from an SES perspective in the future.

The global loss of biodiversity threatens unique biota and the functioning and services of ecosystems essential for human wellbeing. To safeguard biodiversity and ecosystem services, designating protected areas is crucial; yet the extent to which the existing placement of protection is aligned to meet these conservation priorities is questionable, especially in the oceans. Here we investigate and compare global patterns of multiple biodiversity components (taxonomic, phylogenetic and functional), ecosystem services and human impacts, with the coverage of marine protected areas across a nested spatial scale. We demonstrate a pronounced spatial mismatch between the existing degree of protection and all the conservation priorities above, highlighting that neither the world’s most diverse, nor the most productive ecosystems are currently the most protected ecosystems. Furthermore, we show that global patterns of biodiversity, ecosystem services and human impacts are poorly correlated, hence complicating the identification of generally applicable spatial prioritization schemes. However, a hypothetical “consensus approach” would have been able to address all these conservation priorities far more effectively than the existing degree of protection, which at best is only marginally better than a random expectation. Therefore, a holistic perspective is needed when designating an appropriate degree of protection of marine conservation priorities worldwide.

Forage fish occupy a central position in marine food-webs worldwide by mediating the transfer of energy and organic matter from lower to higher trophic levels. The lesser sandeel (Ammodytes marinus) is one of the ecologically and economically most important forage fish species in the North-east Atlantic, acting as a key prey for predatory fish and sea birds, as well as supporting a large commercial fishery. In this case study, we investigate the underlying factors affecting recruitment and how these in turn affect productivity of the North Sea sandeel using long-term data and modelling. Our results demonstrate how sandeel productivity in the central North Sea (Dogger Bank) depends on a combination of external and internal regulatory factors, including fishing and climate effects, as well as density dependence and food availability of the preferred zooplankton prey (Calanus finmarchicus and Temora longicornis). Furthermore, our model scenarios suggest that while fishing largely contributed to the abrupt stock decline during the late 1990s and the following period of low biomass, a complete recovery of the stock to the highly productive levels of the early 1980s would only be possible through changes in the surrounding ecosystem, involving lower temperatures and improved feeding conditions. To that end, we stress the need for ecosystem-based management accounting for multiple internal and external factors occurring within the broader context of the ecosystem in which forage fish species, such as sandeel, play an important and integral part

Summary Sustainably managing natural resources under climate change requires understanding how species distribution shifts can impact ecosystem structure and functioning. While numerous studies have documented changes in species’ distributions and abundances in response to warming [ 1 , 2 ], the consequences for the functional structure of ecosystems (i.e., composition of species’ functional traits) have received less attention. Here, using thirty years of fish monitoring, we show that two connected North Atlantic ecosystems (E. English Channel and S. North Sea) underwent a rapid shift in functional structure triggered by a climate oscillation to a prevailing warm-phase in the late-1990s. Using time-lag-based causality analyses, we found that rapid warming drove pelagic fishes with r-selected life history traits (e.g., low age and size at maturity, small offspring, low trophic level) to shift abruptly northward from one ecosystem to the other, causing an inversion in functional structure between the two connected ecosystems. While we observed only a one-year time-lag between the climate oscillation and the functional shift, indicating rapid responses to a changing environment, historical overfishing likely rendered these ecosystems susceptible to climatic stress [ 3 ], and declining fishing in the North Sea may have exacerbated the shift. This shift likely had major consequences for ecosystem functioning due to potential changes in biomass turnover, nutrient cycling, and benthic-pelagic coupling [ 4 , 5 , 6 ]. Under ongoing warming, climate oscillations and extreme warming events may increase in frequency and severity [ 7 , 8 ], which could trigger functional shifts with profound consequences for ecosystem functioning and services.

Coastal habitats are used by a great variety of organisms during some or all stages of their life cycle. When assessing the link between biological communities and their environment, most studies focus on environmental gradients, whereas the comparison between multiple habitats is rarely considered. Consequently, trait-based aspects of biodiversity in and between habitats have received little attention. Here, we use the biological trait approach in addition to the more common species-based approach to examine trait and taxonomic diversity and composition of invertebrate and fish communities in different coastal habitats, common in the northern Baltic Sea. The habitats include bladderwrack (Fucus), seagrass (Zostera), rock with associated algal species (Rock), and bare sand (Sand). We found distinct differences in community diversity and composition between the habitats. For invertebrates, the sediment of the seagrass meadow had the highest taxonomic and trait richness and diversity, whereas Sand had the highest for fish. The highest dissimilarity in invertebrate community composition was between epifaunal (Rock, Fucus, Zostera Epifauna) and infaunal habitats (Sand, Zostera Infauna) on the one hand, and between vegetated (Zostera Infauna) and unvegetated sediments (Sand) on the other hand, emphasizing the major role vegetation plays in structuring communities. We demonstrate that fish community composition is distinct based on species, and to a lesser degree also distinct based on traits, in the different studied habitats. Both invertebrate and fish communities were more similar on a trait level than taxonomically among the habitats highlighting the presence of similar trait identities in the different habitats. Among the traits examined, Body size contributed most to dissimilarities among habitats for both invertebrates and fish, pointing out the ecological importance of body size for differentiating trait composition of communities. Based on our assessment of biodiversity, using the biological trait approach parallel to the taxonomic approach, we show that trait-based measures clearly provide additional information, such as key functions present in a habitat. This aspect cannot be captured by solely using taxonomic indices, which only shed light on diversity from a species identity point of view. Consequently, to include the ecological role of species, we recommend using biological traits in addition to species-based measures in the assessment of biodiversity, and especially in the management and conservation of coastal habitats, given the important ecosystem goods and services these areas provide.

Collapses and regime changes are pervasive in complex systems (such as marine ecosystems) governed by multiple stressors. The demise of Atlantic cod ( Gadus morhua ) stocks constitutes a text book example of the consequences of overexploiting marine living resources, yet the drivers of these nearly synchronous collapses are still debated. Moreover, it is still unclear why rebuilding of collapsed fish stocks such as cod is often slow or absent. Here, we apply the stochastic cusp model, based on catastrophe theory, and show that collapse and recovery of cod stocks are potentially driven by the specific interaction between exploitation pressure and environmental drivers. Our statistical modelling study demonstrates that for most of the cod stocks, ocean warming could induce a nonlinear discontinuous relationship between fishing pressure and stock size, which would explain hysteresis in their response to reduced exploitation pressure. Our study suggests further that a continuing increase in ocean temperatures will probably limit productivity and hence future fishing opportunities for most cod stocks of the Atlantic Ocean. Moreover, our study contributes to the ongoing discussion on the importance of climate and fishing effects on commercially exploited fish stocks, highlighting the importance of considering discontinuous dynamics in holistic ecosystem-based management approaches, particularly under climate change.

Biodiversity is a multifaceted concept, yet most biodiversity studies have taken a taxonomic approach, implying that all species are equally important. However, species do not contribute equally to ecosystem processes and differ markedly in their responses to changing environments. This recognition has led to the exploration of other components of biodiversity, notably the diversity of ecologically important traits. Recent studies taking into account both taxonomic and trait diversity have revealed that the two biodiversity components may exhibit pronounced temporal and spatial differences. These apparent incongruences indicate that the two components may respond differently to environmental drivers and that changes in one component might not affect the other. Such incongruences may provide insight into the structuring of communities through community assembly processes, and the resilience of ecosystems to change. Here we examine temporal and spatial patterns and drivers of multiple marine biodiversity indicators using the North Sea fish community as a case study. Based on long-term spatially resolved survey data on fish species occurrences and biomasses from 1983 to 2014 and an extensive trait dataset we: (i) investigate temporal and spatial incongruences between taxonomy and trait-based indicators of both richness and evenness; (ii) examine the underlying environmental drivers and, (iii) interpret the results in the context of assembly rules acting on community composition. Our study shows that taxonomy and trait-based biodiversity indicators differ in time and space and that these differences are correlated to natural and anthropogenic drivers, notably temperature, depth and substrate richness. Our findings show that trait-based biodiversity indicators add information regarding community composition and ecosystem structure compared to and in conjunction with taxonomy-based indicators. These results emphasize the importance of examining and monitoring multiple indicators of biodiversity in ecological studies as well as for conservation and ecosystem-based management purposes.

Understanding spatio-temporal dynamics of biotic communities containing large numbers of species is crucial to guide ecosystem management and conservation efforts. However, traditional approaches usually focus on studying community dynamics either in space or in time, often failing to fully account for interlinked spatio-temporal changes. In this study, we demonstrate and promote the use of tensor decomposition for disentangling spatio-temporal community dynamics in long-term monitoring data. Tensor decomposition builds on traditional multivariate statistics (e.g. Principal Component Analysis) but extends it to multiple dimensions. This extension allows for the synchronized study of multiple ecological variables measured repeatedly in time and space. We applied this comprehensive approach to explore the spatio-temporal dynamics of 65 demersal fish species in the North Sea, a marine ecosystem strongly altered by human activities and climate change. Our case study demonstrates how tensor decomposition can successfully (i) characterize the main spatio-temporal patterns and trends in species abundances, (ii) identify sub-communities of species that share similar spatial distribution and temporal dynamics, and (iii) reveal external drivers of change. Our results revealed a strong spatial structure in fish assemblages persistent over time and linked to differences in depth, primary production and seasonality. Furthermore, we simultaneously characterized important temporal distribution changes related to the low frequency temperature variability inherent in the Atlantic Multidecadal Oscillation. Finally, we identified six major sub-communities composed of species sharing similar spatial distribution patterns and temporal dynamics. Our case study demonstrates the application and benefits of using tensor decomposition for studying complex community data sets usually derived from large-scale monitoring programs.

Large highly migratory predators can have major impacts on local marine ecosystems by reducing prey populations and leading to trophic cascades that affect the entire fish community. These trophic interactions are typically non-linear and can alter both the migratory behaviour of the predator and the stability of the fish community. The impact of a migrating top-predator is investigated here for Atlantic bluefin tuna in the North Sea. Bluefin tuna has been absent from the region for half-century, but recent years have seen recovery of migrations and a return of bluefin tuna in the area. We use a size spectrum model to analyse the trophic impact of the returning tuna on the entire fish community, under scenarios with varying levels of tuna consumption and fishing mortality on the prey. We show that with high level of prey fishing mortality in the North Sea, the effect of a tuna re-colonization results in only limited trophic cascades. However, high tuna consumption or changes in fishing mortality may result in a sudden recruitment failure of small-pelagic fish due to cascading effects on the fish community. In present-day conditions, the level of tuna consumption that triggers recruitment failure is lower at increasing fishing mortalities on their prey, providing indications for the future sustainable management of both small-pelagics and bluefin tuna in the area.