Seabird mortality in the Japanese tuna longline fishery around Australia,1988-1995

摘要

Large numbers of seabirds are killed each year within the Australian Fishing Zone (AFZ) by Japanese longline vessels targeting tuna. In recent years the estimated rate of seabird bycatch in the AFZ has been in the order of 0.15 birds/1000 hooks, translating to mortalities of 1000-3500 birds per year. These estimates are absolute minima because not all birds killed remain on hooks to be observed hauled aboard the vessels. The observed seabird catch rate varies annually, seasonally and spatially. Most birds are killed: (a) during the summer fishing season (October to March), even though most fishing effort occurs in winter; (b) when longlines are set during the day; (c) in the waters around southern Australia. Uncertainties in the observed catch rates prevent confident assessment of trends, but seabird catch rates do not appear to be showing a sustained decrease. The process of the incidental collection of seabird bycatch data (by observers whose priority is to fish sampling tasks) renders the seabird bycatch data inadequate for reliable assessment of trends in total numbers of birds killed over time. Sixteen seabird species of birds killed on longlines in the AFZ have been identified. These include black-browed (Diomedea melanophris), shy (D. cauta), grey-headed (D. chrysostoma), yellow-nosed (D. chlororhynchos) and wandering albatrosses (D. exulans), flesh-footed shearwaters (Puffinus carneipes) and white-chinned petrels (Procellaria aequinoctialis). Seventy-four per cent of birds killed were albatrosses and the species composition of the bycatch varied with seasons and areas. Most species of birds killed were characterised by unequal representation of sex and age cohorts, and these unequal representations were not consistent between fishing grounds or seasons.