Factors affecting the number and mortality of seabirds attending trawlers and long-liners in the Kerguelen area |
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Authors: | Henri Weimerskirch Didier Capdeville Guy Duhamel |
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Institution: | (1) Centre National de la Recherche Scientifique, Centre Biologique de Chizé, F-79360 Villiers en Bois, France e-mail: henriw@cebc.cnrs.fr Fax: +33-549-096526, FR;(2) Muséum National d'Histoire Naturelle, Ichtyologie générale et appliquée, rue Cuvier, F-75005 Paris, France, FR |
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Abstract: | The factors affecting the number and the mortality rates 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 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 increased 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
trawl hauling. White-chinned petrels were the most abundant ship-following seabirds, followed by black-browed albatrosses,
giant petrels and cape petrels. The number of white-chinned petrels, black-browed and grey-headed albatrosses attending fishing
vessels increased in the time between spring and autumn, whereas it was the reverse situation for giant petrels and cape petrels.
Four species of seabirds were caught by fishing gear, mainly by long-lines: white-chinned petrels, and 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, some species appear to be more susceptible to being caught than others. White-chinned and grey-headed
albatrosses are caught in much higher proportions than the numbers present, whereas black-browed albatrosses 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/1000 hooks. Only one albatross was caught when the lines were set during the night. White-chinned petrels represented
92.2% of all birds 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 were entangled 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 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 white-chinned petrels whose populations in the Indian Ocean may by threatened
by long-line fisheries.
Accepted: 15 October 1999 |
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