Effects of changes in sandeel availability on the reproductive output of seabirds

The lesser sandeel Ammodytes marinus is a key prey species for many marine birds in the North Sea. This fish is currently the target of the largest single species fishery in the area, and this has led to concern about the potential impact of the fishery on seabirds. There are 2 critical issues: does the breeding success of seabirds depend on sandeel availability and does the fishery reduce sandeel availability to a level at which avian reproductive output is affected? This paper investigates the first question in detail and briefly touches on the second by testing for correlations between productivity, breeding effort and diet in 3 species of seabird with contrasting foraging and dietary characteristics (common guillemot Uria aalge, black-legged kittiwake Rissa tridactyla, and European shag Phalacrocorax aristotelis) and an index of availability of 1 group and older sandeels derived from catch per unit effort statistics from the Danish sandeel fishery. Breeding success in all 3 species was significantly reduced when sandeel availability to the fishery in June was low. There was also evidence that the timing of peak sandeel availability influenced reproductive output such that success was lower when availability peaked early. We speculate that these effects are linked to annual variations in sandeel life history events and, in particular, to the onset of burying behaviour of 1+ group fish and the arrival of 0 group sandeels on the seabirds’ feeding grounds. Although the timing of these events is unlikely to be directly influenced by the sandeel fishery, since most catches are taken in June, it is possible that the fishery could exacerbate a difficult situation for seabirds by further reducing the biomass of available 1+ group fish. We suggest that this may have occurred in one of the years of the study.