Can changes in fishery landings explain long-term population trends in gulls?

Abstract

Capsule: Long-term population trends of gulls on the Isle of Canna, Scotland, showed a correlation to fish tonnage landed in a nearby port. Aims: To assess whether gull numbers and breeding success at Canna have been influenced by the amount of fish discarded in the area. Methods: We examined data on gull breeding numbers, breeding success and diet studied at Canna from 1969 to 2014, and data on fish landings at the nearby port of Mallaig for 1985 to 2014. We examined correlations between gull and fishery data, and performed a detrended analysis of Herring Gull Larus argentatus numbers in relation to demersal fish catch (the latter as a proxy for discard volumes). Results: Gulls fed extensively on discards. Gull breeding numbers declined at Canna, especially between 2000 and 2006, the decline being more pronounced than seen in national totals. Gull breeding numbers correlated with demersal landings, even after detrending for long-term decreases in both. Conclusions: Correlation between detrended Herring Gull breeding numbers and detrended demersal fish landings provided strong evidence for a causal link between fishery discarding and gull breeding numbers.