Great Shearwater (Ardenna gravis) attendance at commercial fisheries in the Argentine economic fishing zone

Abstract

The Great Shearwater (Ardenna gravis) is a pelagic seabird that forages in waters of the southwestern Atlantic Ocean mainly during the pre-laying and chick rearing periods. There, the species have been reported in the bycatch of longline and trawl fisheries. The aim of this study was to evaluate the effect of fishing effort on the foraging behavior of shearwaters, analysing the distribution and behavior of birds and fishing effort, and using evidence from isotope analysis to determine the dietary information to assess the use of fishery discards and facilitated prey. Tracking data of immature and adult shearwaters and fishing effort of different Argentine commercial fishing fleets were used to determine the effect of fishing effort in the foraging behaviour of the species through generalized additive mixed models. Adult and immature shearwaters are more likely to forage when the fishing effort of demersal high-seas ice trawlers increases and when coastal demersal ice-trawlers decreases (and mid-water ice-trawlers for immatures). The isotope analysis showed higher contribution of zooplanktonic species and mid-water fish, followed by demersal species (which can be only available through the consumption of discards and offal). These results are related to the common use of highly productive waters and the attraction of shearwaters generated by prey captured in nets and by discards as a predictable source of food. Understanding the impact of fisheries on seabird behaviour is essential for implementing measures aimed at reducing the incidental capture of seabirds by fishing fleets.