Prey harvests of seabirds reflect pelagic fish and squid abundance on multiple spatial and temporal scales

Abstract

Many studies have demonstrated relationships between seabird prey harvests and fishenes catches. These correlations have for the most part been found at scales from 10s to 100s of kilometers within foraging ranges around seabird breeding colonies. In the present study, we investigated associations between the prey harvests of northern gannets Sula bassana at a large breeding colony off the northeast coast of Newfoundland and the catches of the inshore Newfoundland fishery at different spatial scales and time intervals. Significant correlations occurred between the seabirds' and the humans' catches of mackerel Scomber scombrus and short-finned squid Illex illecebrosus from 1977 through 1992. The relationships for squid were stronger over larger geographic areas than were those for mackerel. The associations for both squid and mackerel reflected abundance/availability around the colony, at a larger scale near the gannets' maximum foraging range (e.g. -200 km), and for the entire Newfoundland region (1000s of kilometers). These correlations were significant at August vs August and August vs annual time intervals. The gannets' landings of squid were also associated with fishery-independent, research survey indices of squid abundance over thousands of kilometers. The robustness of these relationships indicates that levels of pelagic prey harvest by seabirds can provide reliable indices of prey abundance within and outside reproductive seasons and foraging ranges around breeding colonies. Similar relationships are predicted between seabird and human fisheries that are directed at migratory 'warm-water' pelagic prey that move into cold and high latitude oceanographic regions.