INCIDENTAL CAPTURE, DIRECT MORTALITY AND DELAYED MORTALITY OF SEA-TURTLES IN AUSTRALIAS NORTHERN PRAWN FISHERY

The species composition, catch and mortality rates of sea turtles capt ured incidentally by the tiger prawn fishery on Australia's northern c oast in 1989 and 1990 were estimated by monitoring the fishery's catch . In 1990, the delayed rate of mortality from damage was estimated and the size composition was measured. Five species of turtles were captu red: the flatback (Natator depressa, 59% of the total), loggerhead (Ca retta caretta, 10%), olive ridley (Lepidochelys olivacea, 12%), green turtle (Chelonia mydas, 8%) and hawksbill (Eretmochelys imbricata, 5%) . The turtle catches varied with water depth: the highest catch rates (0.068 +/- 0.006 turtles per trawl) were from trawls in water between 20 and 30 m deep, relatively few turtles (10%) were captured in water deeper than 40 m (25% of trawls). Catch rates varied with time of year : the highest catch rates were 0.098 (+/- 0.013) turtles per trawl in winter. There was no significant difference in the overall catch rate (chi(2) = 0.047; p = 0.8111, df = 1) but a significant difference in m ortality rate (chi(2) = 3.99; p < 0.05; df = 1) between the two years. The incidence of capture in the commercial fishery was 0.051 (+/- 0.0 03) turtles per trawl towed for about 180 min, with 0.007 ( +/- 0.001) turtles per trawl drowning in the nets. There were no significant dif ferences in the catch and mortality rates between the two years for an y of the turtle species except the loggerhead, which had a significant ly (chi(2) = 11.029; p = 0.0013; df = 1) lower catch rate in 1990 (0.0 02 +/- 0.001 turtles per trawl) than in 1989 (0.008 +/- 0.002 turtles per trawl), and a significantly higher mortality in 1990 (33%) than in 1989 (19%). Catch rates and mortality varied between the species: the flatback had the highest catch rate (0.030 +/- 0.002 turtles per traw l) but the lowest mortality (10.9%); the loggerhead had a catch rate o f 0.005 +/- 0.001 turtles per trawl, and high mortality (21.9%); the o live ridley had a catch rate of 0.006 +/- 0.001 turtles per trawl and a low mortality (12.5%); the green turtle's catch rate was 0.004 +/- 0 .001 per trawl and mortality 12.0%; the hawksbill had the lowest catch rate (0.002 +/- 0.001 turtles per trawl) but highest mortality (26.4% ). Based on the fishing effort (27 049 d for 1989 and 25 746 d for 199 0), we estimate that 5 503 (+/- 424) turtles were caught and returned to the sea in 1989 and 5 238 (+/- 404) in 1990, of which 567 +/- 140 d rowned in 1989 and 943 +/- 187 in 1990. In 1990, an estimated 25% of a ll captured turtles suffered some non-lethal damage; an estimated 21% of turtles were captured comatose and 4% were injured. We conclude tha t, considering other threats, trawl-induced drowning is not the major impact on turtle populations in northern Australia, but that measures to reduce drowning and delayed mortality would be desirable.