Regional variation in the role of bottom-up and top-down processes in controlling sandeel abundance in the North Sea

The lesser sandeel Ammodytes marinus is a critically important mid-trophic species in the North Sea ecosystem. Seabirds suffered widespread breeding failures in the North Sea in 2004, due to shortages of sandeels, their principal food. Industrial sandeel fisheries also failed in 2003 to 2005. Explaining why sandeels were in short supply is thus critical to understanding and managing the North Sea ecosystem. Sandeel abundance may be controlled 'bottom-up' by food availability or 'top-down' by predation, including fisheries. The relative importance of these 2 mechanisms may vary over space and time, and failure to take account of such variation may lead to inappropriate management. We summarise the available evidence for top-down or bottom-up control of sandeel abundance in 2 well-studied North Sea regions differing in many biotic and abiotic characteristics. In Shetland, recent low abundance of sandeels coincided with record-high abundance of herring, which may have exerted a top-down predation pressure on sandeels. Off SE Scotland, where adult herrings are scarce, abundance of sandeel larvae was positively correlated with plankton abundance, indicating bottom-up control, Seabird breeding failures in this area in 2004 were linked to extremely low energy content of sandeels. Large-scale sandeel fisheries have not been operating in either area since 2000. Control of food web structure and function in the North Sea is thus likely to be complex, with pronounced regional variation. Improved cooperation between diverse research organisations will be needed to understand this complexity, and future ecosystem-based management of marine bioresources will need to take the results of such research into account.