Temporal and spatial variation in the diet of a marine top predator-links with commercial fisheries

The huge quantities of waste produced by commercial fisheries worldwide attract large numbers of scavengers. Reducing this wasteful practice is desirable, but may have implications for marine ecosystems as scavengers will face a major shortfall in food. Predicting the impact of reduced discarding requires information on the strength of the link between scavengers and fisheries. We analysed sagittal otoliths regurgitated by great skuas Stercorarius skua over 5 yr from 8 different colonies in Shetland, UK, and over 18 yr from 1 of these sites, in relation to spatial and temporal variation in fisheries activity. The proportions of 2 demersal fish not normally available to skuas, haddock Melanogrammus aeglefinus and whiting Merlangius merlangus, are positively correlated with annual variation in discard estimates. The proportion of whiting at 8 colonies is positively correlated with spatial differences in fish biomass estimates, and blue whiting Micromesistus potassou are only eaten in colonies close to the continental shelf edge, where this species is fished. Non-breeding great skuas tend to eat smaller and significantly more variable discarded fish compared with breeders. Our results indicate that great skuas rely heavily on fishery discards and are also sensitive to spatial and temporal changes in fisheries activity. Differences between breeding status indicate that the non-breeding component of the population may respond differently to changing discard policy. The strength of the link between fisheries and scavengers appear to be species-specific, and testing the strength of these relationships should be an important avenue of future research to interpret the response of scavengers to changes in fisheries management.