Long term trends in great skua Catharacta skua diets and the role of commercial fishery discards

It is well known that waste fish discarded by commercial fisheries are an important food resource for many seabird species. The impact of discards on the biology of seabird communities is complex and not fully understood however. Discard availability affects scavenging species in terms of their population size, reproductive success and also over-winter body condition. Furthermore changes in discard availability may also influence seabird species that are not scavengers themselves. There is evidence that reductions in discards at Mediterranean colonies has prompted an increase in predation and kleptoparasitism rates by yellow-legged gulls on other seabird species.
Great skuas Catharacta skua in the northern North Sea have shown a marked population increase over the last century, which may in part be due to the availability of discards. Recently seabird predation rates by this species have increased causing concern for other seabird species. Part of the reason for this increase in predation is believed to be due to a reduction in the availability of sandeels and discards.
In this study we present data on great skua diets over the past 30 years which provides evidence that great skuas closely track the availability of fishery discards and furthermore that that the availability of discards, and to a lesser degree sandeels, are an important predictor of the amount of seabird prey in the diet of this species. Plans through the Common Fisheries Policy to reduce discard rates in European waters is likely to reduce the availability to great skuas and therefore this may have a detrimental affect on seabird communities in the northern North Sea.