Scale-specific density dependence in North Sea sandeel

Density dependent demographic processes occur in many marine fishes and potentially enhance the stability of local aggregations and regional populations. The degree of density dependence exhibited at the population level is a combination of local density dependent effects acting on different spatial scales. In this study, we searched for density dependence in recruitment, growth and mortality of North Sea sandeel Ammodytes marinus at different spatial scales by analysing data at a spatial resolution specifically reflecting the mosaic of populated habitat patches. Two types of density dependent processes occurred: a shift in spatial distribution towards low mortality areas when overall recruitment was high, and an increase in local mortality as local density increased. When combined, these processes largely compensated for each other and the size of the overall population had little influence on population level mortality. The study points to the necessity of considering both local and regional scale processes in the analyses of density dependence.