Factors affecting the number and mortality of seabirds attending trawlers and longliners in the Kerguelen area

The factors affecting the number and the mortality of seabirds attending long-liners and trawlers fishing in the Kerguelen area were studied during four successive seasons (1994-1997), based on observations carried out onboard by dedicated observers. Twenty-four species of seabirds were observed attending fishing vessels representing 591 birds / census. The total numbers attending varied mainly according to the year, the cloud cover and the presence of offal for long-liners. The dumping of offal increased the numbers of birds attending the vessel, especially when offals can be handled easily by birds. The activity of the vessels also affected the numbers attending, birds being more abundant during line setting and during trawl hauling. The white-chinned petrel was the most abundant ship-following seabird followed by Black-browed albatrosses, giant petrels and cape pigeon. The number of white-chinned petrels, black-browed and grey headed albatrosses attending fishing vessels increased through the season whereas it was the reverse situation for giant petrels and cape petrels. Four species of birds were caught by fishing gears mainly by long-lines, in order of importance white-chinned petrels, black-browed, grey-headed and wandering albatrosses. Taking into account the number of birds from each species attending long-liners and known to be potential by-catch, it appears that some species appear to be more susceptible of being caught than others. White-chinned and grey-headed albatrosses appear to be caught in much larger proportion than the number of potential by-catch present, whereas black-browed are caught in lower numbers. Giant petrels are abundant around long-liners but were never caught. In long-liners, most birds were killed when the lines were set during the day or when the deployment of the scaring device was not successful with an overall figure of 0.47 birds / 1.000 hooks. No albatross except one was caught when the lines were set during the night. The white-chinned petrel represented 92.2 % of all birds killed by long-liners. The number of birds caught varied significantly between months and between 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 were caught by the netsonde cable. The efficiency of mitigation measures in order to reduce seabird mortality is discussed and it is stressed that night setting is the most efficient method way to reduce mortality and should be enforced everywhere when possible. However further methods should be developed to reduce the mortality of species active at night, especially the white-chinned petrel whose populations in the Indian ocean may by threatened by long-line fisheries.