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Aim Marine conservation often prioritizes measures based on ecosystem diversity, missing important information on the size and structure of communities such as that provided by body size-spectra ( SS). While most studies have focussed on temporal or spatial patterns of SS, there is no research investigating their spatial heterogeneity, which is a key community trait revealing mechanisms of resilience. We aimed to comparatively determine the main patterns of variation of fish SS and their spatial heterogeneity across geographic and bathymetric gradients. Location Shelf and slope Atlantic and Mediterranean demersal ecosystems. Methods Using the information obtained from the experimental surveys, we calculated fish SS at four bathymetric strata and investigated the influence of the biomass of small-sized and large-sized fish on their temporal and spatial variation. We also assessed the functional relationship between the mean pattern and the spatial heterogeneity of SS as a characteristic structural trait of a community. Results The variation in SS demonstrates that Atlantic communities were geographically and bathymetrically more connected, while those in the Mediterranean were more fragmented. The relationship between mean SS and its spatial heterogeneity was a community fingerprint differentiating two contrasting pathways of community recovery between Atlantic and Mediterranean ecosystems. Atlantic communities were recovering because of the temporal increasing contribution of large fish and a 'rescue effect' of neighbouring areas, which have spatially homogenized size structure. Mediterranean communities were, by contrast, spatially clustered and highly dependent on small-sized fish variability that hampers the capacity of ecosystems to recover after a chronic degradation. Main conclusions Patterns of SS variation depend on the spatial complexity of communities, which contrasts between Atlantic and Mediterranean systems. Integrative analyses of size-based indicators (mean and variance) provide a complete assessment of the structural properties of communities and characterize the resilience mechanisms of recovery against perturbations under contrasting ecological contexts. [ABSTRACT FROM AUTHOR]

Aim The life history of a species is determined by trade-offs between growth, survival and reproduction to maximize fitness in a given environment. Following a theoretical model, we investigate whether the composition of marine fish communities can be understood in terms of a set of life-history strategies and whether the prevalence of the strategies follows specific spatial patterns that can be related to the environment. Location European seas. Time period 1980-2014. Major taxa studied Fish. Methods An extensive set of scientific bottom trawl surveys was collected to obtain the species composition of fish communities across European seas. We complemented these data with species-specific information regarding six life-history traits, reflecting reproductive, growth and feeding modes. We then calculated the optimal number of strategies needed to summarize the information contained in the traits by using archetypal analysis. The proportion of each obtained strategy in the communities and their spatial patterns were explained as a function of the environment and their temporal changes were investigated. Results The species could be decomposed into a continuum of three life-history strategies-opportunistic, periodic and equilibrium-resulting from trade-offs between traits. The marked spatial patterns of these strategies could be explained by depth, temperature and its seasonality, chlorophyll and fishing effort. In recent years, opportunistic and equilibrium strategies significantly increased, probably due to an increase in temperature and decrease in fishing effort. Main conclusions Our empirical analysis supports a theoretical framework outlining three life-history strategies of fish. The strategies vary predictably in space and time in response to the environment. This highlights the underlying process whereby fitness is optimized through trade-offs between growth, feeding and reproduction under different environmental conditions. Due to their response to the environment, life-history strategies provide a suitable tool for monitoring and understanding community changes in response to natural and anthropogenic stressors, including fishing and climate change. [ABSTRACT FROM AUTHOR]

• Community Weighted Mean Temperature (CWMT) depicts fish communities' thermal preferences. • CWMT responds to the water temperature reproducing its space–time trends. • CWMT allows to classify the fish community in cold, temperate and warm communities. • Extension of warm communities is increasing, while cold communities are decreasing. Under current levels of global warming most demersal species in the Northeast Atlantic are experiencing tropicalization, meridionalization or borealization of their distributions, leading to profound changes in demersal communities. We explore these changes using the Community Weighted Mean Temperature (CWMT), an index to link the thermal preference of demersal fish communities and temperature. The CWMT is calculated as the summation of the mean temperature of each fish species distribution weighted by its relative abundance in the community. The relative abundance is based on the community composition data obtained by the International Bottom Trawl Surveys (IBTS) in the Southern Bay of Biscay between 1983 and 2015. Our analyses show that the CWMT responds to the actual temperature of the water column reproducing its space–time trends in the study area: (i) an increase from SW to NE, towards the inner Bay of Biscay, (ii) a decrease with depth, except in the SW area characterized by an intense upwelling, (iii) a general increase along the time series. Applying a k-means classification to the CWMT data we identified warm-, temperate- and cold-communities over the shelf and slope and their spatial changes in the last decades. The area occupied by warm communities has expanded 268.4 km2/yr since the 80 s, while the cold communities have retracted at a speed of −155.4 km2/yr. The CWMT was able to capture the community dynamics in relation to environmental temperature at different temporal and spatial scales, highlighting the potential of this index to explore and anticipate the effects of climate change in demersal communities under different scenarios of global warming. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]