Report of the ICES/GLOBEC Working Group on Life Cycle and Ecology of Small Pelagic Fish (WGLESP)

A conceptual biological framework is suggested to envisage changes in fish spatial distributions. Environmental forcing alone cannot be expected to explain changes in distribution, unless changes are so strong that they over-ride all other drivers. Changes in the distributions of pelagic fish were understood as resulting from the interaction between external factors to the population such as climate or ecosystem change and internal factors to the population. Internal factors to the population include total abundance, population structure, individual fish condition, aggregation behaviour, density dependence. Internal factors result in the population to be innovative or conservative in occupying its potential habitats. Change in spatial distributions can then occur because the distribution of potential habitats changed under climate change and or because the population’s internal behaviour changed under internal factors such as demographic change.