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Comparative analysis of vertical dynamics in cod (Gadus morhua L.) between ecosystems

Atlantic cod is a highly adaptive boreal species with a broad distribution through different ecosystems. The behaviour of cod will always be constrained by physiology, be it temperature tolerance, swimming speeds or buoyancy control. However, diet, environmental factors and population dynamics vary enormously throughout its geographic range, and behaviours are therefore extremely plastic. The vertical movements observed in different ecosystems reflected this plasticity and so vertical movements varied significantly in frequency and scale as well as between individuals, season, years and ecosystems. Clear patterns were therefore difficult to detect from the comparative analyses: the data appear more chaotic than ordered.

However it was clear that, in all ecosystems, cod vertical movements were influenced by the migratory strategy, tidal flow and level of illumination. Coastal or resident cod displayed a smaller range of behaviours than migratory cod. Cod in all ecosystems typically varied their behaviour seasonally, as they switched from reproductive behaviour to feeding behaviour. For example, cod in the North Sea appear to reduce their vertical movements in summer to feed on benthic prey mainly at night. In contrast, migratory cod in the Barents Sea and Iceland undertake large vertical migrations throughout the day in the vicinity of frontal systems, where pelagic preys are abundant.

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