Foraging ecology of grey-headed mollymawks at Marion Island, southern Indian Ocean, in relation to longline fishing activity

Incidental mortality due to longline fishing has been implicated as the mai n cause for the global population decline in grey-headed mollymawks (Thalas sarche chrysostoma). Two of these fisheries, within the potential foraging range of grey-headed mollymawks breeding on Marion Island, have increased d rastically over the past 5-10 years. In order to understand the impacts of these fisheries on the grey-headed mollymawk population breeding on Marion Island, we studied their foraging ecology by tracking their foraging trips and sampling their diets. During the incubation stage, birds made long fora ging trips, mostly towards the subtropical convergence and sub-Antarctic zo nes, bringing them into contact with areas of intense southern blue-fin tun a (Thunnus maccoyii) longline fishing. Females spent a higher proportion of their time within these areas than males, thus exposing themselves to a hi gher risk of incidental mortality from this fishery. During the early chick -rearing stage, foraging trips were shorter and to the southwest of the isl and in the Polar frontal and Antarctic zones, thus avoiding any contact wit h the southern blue-fin tuna industry. However, short foraging trips (< 2 d ays) were made within the boundary of known Patagonian toothfish (Dissostic hus eleginoides) longline sets around Marion Island. Males made a higher pr oportion of short foraging trips and spent more time within the boundaries of the toothfish fishery than females. These differences may account for th e male-biased mortality of grey-headed mollymawks observed in the toothfish fishery around Marion Island. Although a decrease in the annual breeding p opulation has not been detected on Marion Island as yet, we warn that the m ethods used to detect these changes are inaccurate in measuring short term population changes (< 10 years) and that the impacts of these fisheries may already have altered the demographic structure of this population. (C) 200 0 Elsevier Science Ltd. All rights reserved.