A VALIDATION OF SHORT-DURATION SCIENTIFIC TOWS AS A REPRESENTATION OFLONG COMMERCIAL-LENGTH TOWS - COMPARING THE CATCH RATES, SIZE COMPOSITION AND SPECIES COMPOSITION OF PRAWN TRAWLER BYCATCH IN THE FAR NORTHERN GREAT-BARRIER-REEF, AUSTRALIA

The duration of tows for scientific trawl surveys in northern Australi a is generally 30 min. Commercial trawls may be up to 200 min long. In this study we compare the catch rates, size of fish and species compo sition for 10 short (30 min) tows paired with 10 commercial length (16 5 min-not in a straight line) tows at both an inshore and an offshore location in the far northern Great Barrier Reef. Evaluations of this k ind have not been reported in tropical Australian multi-species fisher ies. Overall, catch rates of fish and invertebrates differed between s hort and long tows both inshore and offshore. Inshore, the mean catch rate for the fish and the invertebrates was significantly greater in s hort duration tows (P < 0.05). Offshore, the mean catch rate for all f ish, and all invertebrates was not significantly different between sho rt and long tows, although higher catch rates of invertebrates were re corded in long tows. Inshore, 12 fish species out of 49 had significan tly different catch rates, most being greater in short tows. Only 2 in vertebrates out of 16 had significantly different catch rates (P < 0.0 5). The combined weight of species showing significant differences in catch rates represented less than 10% of the total weight in both insh ore and offshore samples. Principal component analysis indicated that catch composition was similar between short and long tows both inshore and offshore. The results from our sampling are that the species comp osition of 30-min trawls is similar to that of commercial length tows, and that size frequencies are equivalent. The implications are that s hort tows (30 min) can be used in scientific surveys to give a true re presentation of both size and species composition of commercial length tows, but may overestimate catch rates. (C) 1998 Elsevier Science B.V . All rights reserved.