Bycatch is the term used for the non-target species captured in fishing operations and typically discarded. Pauly and Christensen (1995) calculated that some 25-35% of the primary productivity of continental shelves (which yield 95% of global catches) is required to sustain reported catches plus the discarded bycatch. The amount of bycatch, and how it is treated, varies between the kinds of operation and the gear used. Prawn trawl fisheries generate 6-10 kg of bycatch for every kg of prawns caught (Poiner et al. 1998); thus, each prawn trawler in northern Australia catches and discards 300-500 kg of bycatch per night (see review by Dayton et al. 1995). Andrew and Pepperell (1992) describe the effects of shrimp bycatch on populations of the non-target organisms caught and the effects that the discards have in increased levels of scavenging and changes in community structure. Hill and Wassenberg (1990) discuss the fate of discards from prawn trawlers in Torres Strait and note that the fishing operations make large quantities of benthic material available at the surface for scavengers. Liggins et al. (1996) report on bycatch from prawn trawling in Botany Bay and Port Jackson, although they mainly focus on finfish. Probert et al. (1997) describe the benthic invertebrate bycatch from a deep-water (662-1524 m) trawl fishery for Orange Roughy (Hoplostethus atlantina) on the Chatham Rise, New Zealand. Despite the large mesh size used, 82% of the tows included large benthic invertebrates comprising 96 species (Ophiuroidea, 12 spp., Natantia, 11 spp., Asteroidea, 11 spp., Gorgonacea, 11 spp., Holothuroidea, 7 spp., and Porifera, 6 spp.). The composition of the bycatch from different topographies differed significantly.

 

While the bycatch of trawling operations have generated most attention, pelagic fisheries can also have impacts on non-target taxa. While the impacts on species such as dolphins, seabirds and sharks have generated the most publicity, some larger pelagic invertebrates such as cephalopods could also be affected. Owing to the generally great reproductive potential and dispersive ability of this latter group of species, this is unlikely to pose a serious threat, but the issue requires further research to clarify impacts.



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Department of Environment and Heritage