Incidental capture and mortality of Sea Turtles in Australia's northern Prawn Fishery

Abstract

Species composition, catch and mortality rates of sea turtles captured in the tiger prawn segment of Australia's northern prawn fishery were estimated from six prawn research surveys and three commercial catch monitoring programmes. Four species of turtles were captured in the research surveys: the flatback (Natator depressa, 43%) was the dominant species, although the loggerhead (Caretta caretta, 19%) and the olive Ridley (Lepidochelys olivacea, 15%) were common and the green turtle (Chelonia mydas, 4%) was occasionally captured. The size of the turtle catches varied with the duration of the trawl and water depth: the highest catch rates (turtles per standard net-h) were from trawls of 90 min or more in water less than 25 m deep: no turtles were captured in water deeper than 43 m. The rate of mortality amongst captured turtles also varied with trawl duration; there was no mortality recorded in trawls of less than 90 min, 5% mortality in trawls of 165 min, and 7% in trawls of 180 min. The incidence of capture in the commercial fishery was 0.045 (±0.006) turtles per 180-min trawl, with 0.0027 (±0.0014) turtles per 180-min trawl drowning in the nets. If it is assumed that these rates have been constant over the history of the fishery, then on the basis of the annual fishing effort, an average of 5730 (±1907) turtles have been caught per year [of which an average of 344 (±125) drowned]. Since the introduction of management measures in 1987 to reduce effort in the fishery, the number captured declined to about 4114 (±1369) turtles in 1988, of which an estimated 247 (k90) turtles drowned. It is concluded that the impact of trawl-induced drownings on the turtle populations is probably not of such proportions as to create immediate concern.