Seabird bycatch in the pelagic longline fishery off southern Africa

The waters around South Africa provide rich foraging opportunities for pelagic seabirds. They also support a pelagic longline fleet targeting tunas Thunnus spp. and swordfish Xiphias gladius, which set a total of 41.5 million (average 5.2 million per year) and 10.2 million hooks (average 1.3 million per year) respectively during the period 1998-2005. Fisheries observers collected seabird bycatch data from 2 256 sets (4.4 million hooks) and recorded a total of 1 954 birds killed during that period. In all, 11 species of seabird are confirmed incidentally caught by the fishery, eight of which are considered threatened. Birds were caught at an average rate of 0.44 per 1 000 hooks, resulting in an average of 2 900 seabirds killed per year, decreasing from approximately 5 900 in 1998 to 1 800 in 2005. Three techniques for extrapolating total seabird mortality were investigated and little difference between the estimates found. Generalised linear models were used to explain bycatch patterns and revealed that individual vessel is the most important explanatory variable, followed by vessel flag, moon phase, season, sea state, the use of a tori line, time of set, area and bathymetry. Estimates of the numbers of seabirds killed per year were lower than other studies, an improvement most likely linked to the termination of foreign bilateral agreements, as well as to improved awareness among fishers as a result of ongoing education campaigns. Some of the apparent decreases in catch rate could reflect reduced numbers of seabirds at sea, the result of ongoing population decreases in several key species.