The probable fate of discards from prawn trawlers fishing near coral reefs A study in the northern Great Barrier Reef, Australia

摘要

We investigated the composition of the bycatch from research prawn (shrimp) trawling in an area closed to fishing in the northern Great Barrier Reef. By weight, fish made up 69% of the bycatch, sponges 11% and crustaceans 10%. A third of the fish and 45% of the cephalopods floated when discarded. Over 90% of all other groups sank. Floating fish had a modal length of 10 cm. The only animals seen to scavenge floating discards at night were dolphins and these were uncommon. During the day, Crested Terns, Frigatebirds and Brown Boobies as well as dolphins and sharks scavenged discards on the surface. The seabirds took fish mostly smaller than 12cm total length, these made up 60% of floating fish. Using the proportion of fish in the discards (69%) and the proportion of fish that floated (33%) with the proportion of fish (60%) that are of the preferred size for scavenging seabirds, we estimated that only 14% of trawling discards are available to seabirds. Thus seabirds consume only a minor amount of prawn trawler discards in this region. The remaining floating discards are presumably either eaten by sharks or dolphins or they may wash upon reefs or the mainland coast. Over 80% by weight of discards sink, some will survive but the majority are scavenged on the seabed mainly by fish, small sharks and possibly cirolanid isopods. The low survival rate of discards indicates that trawlers in the region are killing large numbers of animals that are caught incidentally in nets. Modifications to trawl nets being introduced to reduce bycatch should result in an improvement in this situation.