Community-wide decline in the occurrence of lesser sandeels Ammodytes marinus in seabird chick diets at a North Sea colony

Many pelagic marine ecosystems have a wasp-waist trophic structure characterised by low diversity of mid-trophic species, typically small, shoaling forage fish that are eaten by a wide range of top predators. In the North Sea, this mid-trophic position is occupied by the lesser sandeel Ammodytes marinus. Over the last 30-40 yr, the abundance and length-at-age of sandeels have declined, but information on concurrent changes in the diets of seabird communities is scarce. We used data on chick diet composition, sandeel length-at-age and energy density collected at a colony in the western North Sea from 1973-2015, to test for dietary shifts in this seabird community during a period when a local sandeel fishery opened, operated and was closed. We found a long-term decline in the overall importance (measured as the frequency of occurrence and proportion of biomass in diet samples) of sandeels, particularly 1+ group fish. However, there were species-specific differences such that the overall decline in sandeels was strongest in common guillemots Uria aalge, while the shift from 1+ group to 0 group sandeels was apparent in all species except European shags Phalacrocorax aristotelis. Community-level differences were also apparent in the alternative prey to sandeels, with with common guillemot, razorbill Alca torda, Atlantic puffin Fratercula arctica, and black-legged kittiwake Rissa tridactyla chicks being fed clupeids, predominantly sprat Sprattus sprattus, while shag chicks received a wide range of benthic fish species. There was also evidence for a decline in the quality of sandeels fed to chicks with significant decreases in length-at-age of 0 group and 1+ group. However, there was no significant annual variation in the energy density of sandeels except for 2004, when values were exceptionally low. Neither the opening nor the closing of the sandeel fishery had any detectable effect on chick diet composition, sandeel length or sandeel energy density. Overall, our results suggest marked community-level changes in seabird diet composition over the last 3 decades that may reflect long-term declines in the abundance and quality of their principal prey.