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The collapse of Atlantic cod (Gadus morhua) stocks in the northwest Atlantic Ocean ranks among the most dramatic and widely known ecological changes of the 20th century. Less widely known are the systemic changes that occurred within these areas prior to and coincident with the cod collapse. Our analysis of a fishery-independent, long-term, standardized database collected on the eastern Scotian Shelf off Nova Scotia revealed that during the past four decades, coherent, community-level reductions in body size, biomass, and physiological condition have occurred in the resident demersal fish species. The changes occurred over large spatial (>104 km2) and short temporal (<10 years) scales, suggesting a progressive decline in the nature and extent of the energy flow through the benthic system. The unexpected and persistent poor condition of a variety of morphologically and functionally dissimilar demersal fish species, living in an environment of reduced intra- and inter-specific competition, suggests a decoupling of the benthic– pelagic systems. This decoupling appears to be attributable to a cascading series of processes involving the cumulative removal of biomass resulting from commercial fishing, compensatory and self-stabilizing increases in pelagic fish biomass, and a decline in groundfish productivity exacerbated by decadal scale variability in water temperature and stratification.

To anticipate the response of fish populations to climate change, we developed a framework that integrates requirements in all life stages to assess impacts across the entire life cycle. The framework was applied on plaice (Pleuronectes platessa) and Atlantic herring (Clupea harengus) in the North Sea, Atlantic cod (Gadus morhua) in the Norwegian/Barents Seas and European anchovy (Engraulis encrasicolus) in the Bay of Biscay. In each case study, we reviewed habitats required by each life stage, habitat availability, and connectivity between habitats. We then explored how these could be altered by climate change. We documented environmental processes impacting habitat availability and connectivity, providing an integrated view at the population level and in a spatial context of potential climate impacts. A key result was that climate-driven changes in larval dispersion seem to be the major unknown. Our summary suggested that species with specific habitat requirements for spawning (herring) or nursery grounds (plaice) display bottlenecks in their life cycle. Among the species examined, anchovy could cope best with environmental variability. Plaice was considered to be least resilient to climate-driven changes due to its strict connectivity between spawning and nursery grounds. For plaice in the North Sea, habitat availability was expected to reduce with climate change. For North Sea herring, Norwegian cod and Biscay anchovy, climate-driven changes were expected to have contrasting impacts depending on the life stage. Our review highlights the need to integrate physiological and behavioural processes across the life cycle to project the response of specific populations to climate change. © 2012 Blackwell Publishing Ltd

The determinants of the structure, functioning and resilience of pelagic ecosystems across most of the polar regions are not well known. Improved understanding is essential for assessing the value of biodiversity and predicting the effects of change (including in biodiversity) on these ecosystems and the services they maintain. Here we focus on the trophic interactions that underpin ecosystem structure, developing comparative analyses of how polar pelagic food webs vary in relation to the environment. We highlight that there is not a singular, generic Arctic or Antarctic pelagic food web, and, although there are characteristic pathways of energy flow dominated by a small number of species, alternative routes are important for maintaining energy transfer and resilience. These more complex routes cannot, however, provide the same rate of energy flow to highest trophic-level species. Food-web structure may be similar in different regions, but the individual species that dominate mid-trophic levels vary across polar regions. The characteristics (traits) of these species are also different and these differences influence a range of food-web processes. Low functional redundancy at key trophic levels makes these ecosystems particularly sensitive to change. To develop models for projecting responses of polar ecosystems to future environmental change, we propose a conceptual framework that links the life histories of pelagic species and the structure of polar food webs.

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We compared habitat associations of southern Gulf of St. Lawrence Atlantic cod (Gadus morhua) and American plaice (Hippoglossoides platessoides) between the summer feeding season on the Magdalen Shallows and the overwintering period in the Cabot Strait. Data were from bottom trawl surveys conducted in September 1993, 1994, and 1995 and January 1994, 1995, and 1996. Both species occupied much deeper, warmer water in winter than in summer. The effect of cod age on temperature distribution reversed between the two seasons, with younger cod occupying warmer water than older cod in summer and colder water in winter. Selection of both depth and temperature by cod tended to be more significant in September than in January. The reduced statistical significance of habitat selection by cod in winter was associated with a more aggregated distribution in this season. The contrast between seasons in habitat associations was particularly strong for plaice. The median habitats occupied by plaice were 58-67 m and -0.1 to 0.3°C in September and 374-426 m and 5.2-5.4°C in January. Habitat selection by plaice was significant in both seasons, but significance tended to be greater in January. Degree of aggregation in plaice distribution was similar between the two seasons. Female plaice occupied significantly warmer water than males in September but not in January. The ecological and practical implications of this striking seasonal variation in habitat associations are discussed.

The present study examines sources of the interannual variability in salinity on