Survivorship of sea turtles after capture in trawls (2.5.3)

Task leader: Dr J Robins, Queensland Department of Primary Industries and Fisheries (QDPI&F)

Task Associate: NA

By-catch in prawn trawl fisheries of Australia includes marketable (prawns, bugs, scallops and squid) and non-marketable species (fish, sharks, sea turtles, rays and sponges). Fish and crustaceans dominate the majority of non-marketable by-catch with 6 to 10kg being caught for each 1kg of prawns (Harris and Poiner, 1990; Pender and Willing, 1989). Sea turtles are protected under the Australian Endangered Species Protection Act 1992 (ESPA) and are a "rare" but controversial catch of prawn trawlers for fishers, fisheries managers, conservationists and politicians. Sea turtles are isted by the Australian Nature Conservation Agency as either vulnerable or endangered with extinction (Edgar and Stephens, 1993). Prawn trawling is suggested as the major cause for the decline of the east Australian nesting population of Caretta caretta (Limpus and Reimer, 1994), however the effect of trawl-related mortality on sea turtle population numbers is mostly speculative and unquantified.

Trawling has been recently nominated under ESPA legislation as a key threatening process to the sea turtles. If trawling is a key threatening process to sea turtles, then management measures, such as the closure of fishing grounds, the restriction to tow durations or the requirement of turtle excluder devices (TEDs) in all trawl nets could be introduced. The failure of Australia to demonstrate the adequate protection of sea turtles from trawling oculd also result in the embargo of Australia's marine exports to the United States of America.

The prawn trawling industry of northern Australia is worth around $130m annually to the Australian economy (Kailola et al., 1993). Before decisions and management measures are introduced to the industry, the problem of sea turtle captures in trawl nets should be assessed based upon sound quantitative data. This should incluse catch and effort data for trawl captures of turtles, and estimates of total turtle mortality as a result of trawl capture for each of the six sea turtle species found in Australia. This fishery related data then needs to incorporated into stock assessment models of sea turtles on a species by species basis. Unfortunately, there are gaps in the knowledge of life history data for many sea turtle populations, and many of these parameters take a long time to collect.

Trawl fisheries in Australia that catch sea turtles include:

• the NSW Trawl Fishery (both inshore and offshore)
• the Torres Strait Trawl Fishery
• the North West Shelf Trawl Fishery
• the West Australian Trawl Fishery, including Exmouth Gulf and Shark Bay
• the Northern Prawn Fishery in the Gulf of Carpentaira (127 degrees East to 142 degreees East)
• and the Queensland East Coast Otter Trawl Fishery (10 degrees 30' S, 142 degrees 30'E to 28 degrees S).

Limited data on turtles captures has been collected for trawl fisheries in New South Wales (S. Kennelly, NSW Fisheries, personal communication) and Torres Strait (QDPI&F, J. Robins unpublished data; CSIRO A. Harris, personal communication) but not analysed. Little information is available about the size or extent of trawl capture of turtles in Western Australia or on the North West Shelf.

Data on turtle catch and mortality has been collected since 1988 in the Northern Prawn Fishery (Pointer et al. 1990; Poiner and Harris, 1994), and since 1991 for the Queensland East coast Otter Trawl Fishery (Robins, in press). About 6,000 sea turtles are caught annually by the Northern Prawn Fishery, of which an estimated 350 die (Poiner et al., 1990). About 5,000 sea turtles are estimated to be caught in the Queensland East Coast Otter Trawl Fishery, of which an estimated 60 die (Robins, in press). These mortality figures refer to the number of turtles that die in the trawl net. Kemmerer (1989) suggested that comatose turtles returned to the water most probably drown. By assuming that all comatose turtles die, mortality for the East Coast Otter Trawl Fishery increased from 1% to about 7% (Robins, in press) further estimated the potential mortality of turtles caught in trawl nets. This increased trawl related mortality from 1% to about 7%.

It has also been suggested that some apparently healthy turtles may die when returned to the water (Shoop and Schwartz, 1992), but the causes and magnitude of post-trawl mortality is unknown. The causes of post-trawl mortality oculd include internal injuries or acute metabolic stress. If this occurs frequently, post-trawl mortality could significantly alter current estimates of the turtle deaths associated with trawl captures and could be important when assessing of the effects of trawling on sea turtle populations of eastern Australia.

The aims of the current project proposal are to assess methods of quantifying post-trawl mortality; to estimate catch and mortality of sea turtles by trawl fisheries of eastrn Australia more accurately and in greater detail than in published studies; and to develop representations of the catch and mortality of sea turtles within the sub-component fisheries of eastern Australia. This information has been identified as being highly useful in the assessment of the impact of trawling on sea turtle populations of Australia (FRDC Effects of Trawling Subprogram meeting, 1995).