Factors affecting the number and mortality of seabirds attending trawlers and long-liners in the Kerguelen area

The factors affecting the number and the mortality rates of seabirds attend ing long-liners and trawlers fishing in the Kerguelen area were studied dur ing four successive seasons (1994-1997), based on observations carried out onboard by dedicated observers. Twenty-four species of seabirds were observ ed attending fishing vessels, representing an average of 591 birds/census. The total numbers attending varied mainly according to the year, the cloud cover and the presence of offal from long-liners. The dumping of offal incr eased the numbers of birds attending the vessel. especially when the offal could be easily handled by birds. The activity of the vessels also affected the numbers attending, birds being more abundant during line setting and t rawl hauling. White-chinned petrels were the most abundant ship-following s eabirds, followed by black-browed albatrosses. giant petrels and cape petre ls. The number of while-chinned petrels, black-browed and grey-headed albat rosses attending fishing vessels increased in the time between spring and a utumn. whereas it was the reverse situation for giant petrels and cape petr els. Four species of seabirds were caught by fishing gear, mainly by long-l ines: white-chinned petrels, and black-browed, grey-headed and wandering al batrosses. Taking into account the number of birds from each species attend ing long-liners and known to be potential by-catch, some species appear to be more susceptible to being caught than others. White-chinned and grey-hea ded albatrosses are caught in much higher proportions than the numbers pres ent, whereas black-brow-ed albatrosses are caught in lower numbers. Giant p etrels are abundant around long-liners but were never caught. In long-liner s, most birds were killed when the lines were set during the day or when th e deployment of the scaring device was not successful, with an overall figu re of 0.47 birds/1000 hooks. Only one albatross was caught when the lines w ere set during the night. White-chinned petrels represented 92.2% of all bi rds killed by long-liners. The number of birds caught varied significantly among months and among years. The type of bait used also affected the catch rate. The catch rate was related to the number of birds attending the long -liner only for black-browed albatrosses. Most birds killed by trawlers wer e entangled by the netsonde cable. The efficiency of mitigation measures in order to reduce seabird mortality is discussed and it is stressed that nig ht setting is the most efficient way to reduce mortality and should be enfo rced everywhere when possible. However, further methods should be developed to reduce the mortality of species active at night, especially white-chinn ed petrels whose populations in the Indian Ocean may by threatened by long- line fisheries.