TRAWL DISCARDS IN THE DIETS OF TROPICAL SEABIRDS OF THE NORTHERN GREAT-BARRIER-REEF, AUSTRALIA

The diets of 12 species of tropical seabirds were investigated in 2 ar eas of the far northern Great Barrier Reef, Australia, with particular reference to the effects of bycatch discarded from trawlers on diets and reproduction. The species whose diets included at least 20% discar d taxa are Sterna bergii, S. dougallii and perhaps S. anaetheta; betwe en 5 and 19% - Hydroprogne caspia, Anous stolidus, Sterna bengalensis, Sula leucogaster and Fregata ariel, less than 5% - Sterna sumatrana; and none - Fregata minor, Sula dactylatra and Sula sula. There was a m arked contrast in the diet of S. bergii in closed and open trawling se asons: in the closed season only 5% of the prey were benthic species, whereas in the open season they made up about 70% of the diet. Differe nces in the diets of birds from areas open and closed to fishing were less marked, probably because birds from the closed zone can forage in adjacent areas open to trawling. About half the bycatch discarded by the trawlers is of a size suitable for one or more of the seabirds, an d most of this half floats, some of it for up to 6 h. S. bergii, S. le ucogaster and F ariel are actively opportunistic, feeding around trawl ers, but S. anaetheta and S. dougallii may be passive discard feeders, feeding on floating discards away from the site of dumping. The provi sion of discards has not changed the size ranges of prey taken by any of the species. It may, however, have increased overlap in the diets o f the various seabirds and changed some feeding strategies. There is l ittle evidence that discards have directly affected breeding cycles or nesting periodicity. Nevertheless, most species breed mainly in late summer and the largest quantities of bycatch are discarded in March, i mmediately post-fledging. This may lead to juvenile birds becoming con ditioned to feeding on discards and may also reduce juvenile mortality rates.