The effectiveness of fish and shrimp trawls for sampling fish communities in tropical Australia

Abstract

Catches of fish in a Frank and Bryce fish trawl during the day and in a Florida Flyer shrimp trawl at night were compared over 2 years (1992, 1993). The effectiveness of a Frank and Bryce demersal trawl as a sampling tool for demersal fish communities was assessed from 122 stations in the far northern section of the Great Barrier Reef. Day-time and night-time catches of the Frank and Bryce net were compared to identify diurnal and nocturnal species. Of a total of 259 species caught, 138 were caught both during the day and at night. An analysis of variance was used to identify species that were caught exclusively or in greater biomass during the day or night; 17 species (mainly small apogonids and mullids) were significantly nocturnal and one (a leiognathid) was significantly diurnal. The overall catch rate of the fish trawl during the day was 395.8 kg h−1 (s.e. 141.3) and that of the shrimp trawl at night was 28.0 kg h−1 (s.e. 1.7). A total of 340 species was caught by both types of net—the fish trawler caught 236 species, the shrimp trawler 243, and 141 were common to both trawl types. An analysis of variance was used to help identify species that were caught in greater biomass by either trawl. Fifty-eight fish species each had a greater biomass in the fish trawl than in the shrimp trawls, for 60 species it was the reverse and for 32 species there was no difference. The fish caught only by the fish trawl were mainly pelagic species (mackerels, carangids) or specimens of large species (lutjanids, sharks and rays), while the fish caught only by the shrimp trawl were small benthic species (apogonids, platycephalids, scorpaenids and flatfish). Over 80% of the fish caught by both nets were small (< 30 cm SL). The length-frequency distributions of only seven species differed significantly between fish and shrimp trawl catches, with the shrimp trawl generally catching the smaller component. No large commercially or recreationally important species of reef fish were caught by the shrimp trawl. The results of this study indicate that each type of trawl net will catch different species and, where they catch the same species, they catch different numbers, sizes and weights. Neither gear type can be used on its own for an adequate description of the fish community.