Diet dynamics and trophic relations of Laysan and Black-footed albatrosses associated with pelagic longline fishing

Abstract

Commercial fishing is a source of food for many types of seabirds, including albatrosses, but the importance of commercial fishing may vary between fishing methods, locations and affected species. Longline fishing in the north Pacific has expanded and affects Laysan Albatrosses and Black-footed Albatrosses through incidental bycatch, but the importance of longline fishing to the diets of these albatrosses is unknown. I analyzed fishery observer data to determine whether there are differences in fishery association behavior between Laysan and Black-footed Albatrosses and whether they differ in their amount of catch scavenging effort. I analyzed the digestive tract contents of tuna and swordfish longline-associated albatrosses to determine the fishery component of the albatross diets. Last, I analyzed the stable nitrogen isotopic compositions of longline associated birds and birds from colonies, as well as prey, to determine whether there might be broader, time-averaged effects on the albatross diets. Black-footed Albatrosses were more abundant around vessels than were Laysan Albatrosses and swordfish longline fishing attracted more birds than did tuna longline fishing. Black-footed Albatrosses also scavenged more swordfish catch, but not necessarily in greater proportion than expected. Laysan Albatrosses ate more fish bait and these two species also differed in their naturally acquired diets, with Laysan Albatrosses consuming more T. borealis. Albatross δ^15N values differed between species and Black-footed Albatrosses differed between fishing methods, but it is unclear whether these differences in nitrogen isotopic compositions were fishery related because of overlap in the δ^15N values among fishery and naturally acquired prey. Differential fishery resource use was evident, even when the albatrosses were exploiting the same resource, indicating that these species should not be given equal treatment in management decisions. Managers should consider the effect of longline fishing on the diets of Lay san and Black-footed albatrosses separately when making decisions that may change fishing practice. Black-footed Albatrosses should receive the most attention because they are the most affected species, while swordfish operations appear to have the greatest affect on the albatross diets.