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Effects of Trawling Subprogram: monitoring the impact of trawling on sea turtle populations of the Queensland East Coast

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Department of Agriculture and Fisheries (QLD)

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Final Report - 1993/229 - Monitoring the Impact of Trawling on Sea Turtle Populations of the Queensland East Coast

Final Report
Date Published:December 2010
Principal Investigator: M. Dredge, J.B. Robins and D.G. Mayer

Key Words: Coral Reef, turtle, bycatch, TED, BRD, fisheries management, loggerhead turtles, green tultles, hawksbill turtles, Pacific Ridley turtles, flatback turtles, Torres Strait Prawn Fishery.

Six species of sea turtle inhabit the waters of northern Australia. Significant trawl fisheries for penaeid prawns and scallops also occur in these areas. The overlap between the distribution of sea turtles and the distribution of trawling effort allows sea turtles to be caught in trawl nets.

Catching a turtle in a trawl net is a relatively infrequent occurrence with overall catch rates averaging less than I turtle per 20 days of trawling. Low frequency of capture and ethical considerations limit the research of turtle bycatch to observational studies. The most feasible approach to measure turtle catch rates under current research budgets is to monitor turtle bycatch through participants in the commercial fishery. This can take the form of a logbook program (either compulsory, voluntary or selective) or an observer-based sampling program. Most Australian fisheries use compulsory logbooks to monitor the effort expended to take commercial catch. Research trawls, though limited in time and space, can be used to validate logbook information. The wide geographic distribution of trawl fisheries in Australia makes voluntary monitoring the only feasible method, in terms of both cost and coverage, to obtain information on the number of turtles caught and killed in these fisheries.

Turtle bycatch data are limited for trawl fisheries in New South Wales and Torres Strait. However, even less is known about the size or extent of turtle bycatch in trawl fisheries of Western Australia, including the North West Shelf. Information on turtle bycatch has been collected for limited periods of time within the Northern Prawn Fishery. About 6,000 turtles are estimated to be caught annually in the tiger prawn sector of the Northern Prawn Fishery, of which an estimated 350 die. A program to monitor the incidental capture of sea turtles in the Queensland Trawl Fishery was initiated in 1991 by the Queensland Department of Primary Industries. The Queensland Fisheries Management Authority funded the program between 1991 and 1993. It utilised voluntary data recording by selected commercial fishers. The project was extended until 1996 with funding from the Fisheries Research and Development Corporation. The extension of the project aimed to provide a long-term database on turtle-trawl interactions throughout the Queensland east coast by collecting information continuously for 6 years.

The success of the voluntary turtle monitoring program relied heavily on the participation of individual commercial fishers. Over the 6 years, 106 different vessels took part in the program, representing the involvement of 12% of the Queensland trawling industry. In total 1,527 turtles were reported caught over 23,906 days fished. Stratified, weighted analysis of the data resulted in an annual estimated turtle catch of 5,901 for the Queensland Trawl Fishery (95% Confidence Interval 5,199 - 6,604) given an average total fleet effort of 84,876 days fished. The catch was comprised of 2,938 loggerhead turtles (95% C.l. 2,390 - 3,487), 1,562 green tultles (95% C.l. 1,223 - 1,902), 80 hawksbill turtles (95% C.l. 42 - 119), 323 Pacific Ridley turtles (95% C.l. 240 - 406) and 968 flatback turtles (95% C.l. 770 - 1,165). A similar analysis for the Torres Strait Prawn Fishery resulted in an annual estimated catch of 652 turtles (95% C.l. 537 - 788), given an average total fleet effort of 8,634 days fished. This was comprised of 85 loggerhead turtles (95% C.l. 50 - 131), 145 green turtles (95% C.l. 95 - 203), 6 hawksbill turtles (95% C.l. 0 - 15), 18 Pacific Ridley turtles (95% C.l. 6 - 32) and 400 flatback turtles (95% C.l. 304 - 518).

Greater than 90% of all turtles reported caught in the Queensland Trawl Fishery were healthy when first landed on the boat. Four percent were reported as comatose and 1% were reported as dead. Mortality rates of trawl-caught turtles were similar in the Torres Strait Prawn Fishery, where 96% of turtles were reported as healthy. Three percent were reported as comatose and 1% were reported as dead. These mortality rates translate to an estimated trawl related mortality of between 72 and 94 turtles for the Queensland Trawl Fishery. If comatose turtles are considered to die as a consequence of a trawl capture (i.e. dead + comatose turtles) then between 306 and 468 turtles are estimated die as a consequence of a trawl capture. Trawl related mortality for the Torres Strait Prawn Fishery was estimated to be between five and eight turtles per year (i.e. dead turtles only) or between 21 and 32 turtles if comatose turtles are considered to die as a consequence of a trawl capture. These mortality rates are considerably lower than that reported for the Northern Prawn Fishery, which were 10% dead in 1989 and 18% dead in 1990, and 39% if comatose turtles were assumed to die in 1990.

There are a number of factors that may explain the difference in mortality rates between the Northern Prawn Fishery and the two fisheries reported here. It has been suggested that mortality rates in a fishery are the consequence of the average duration of the trawls as well as the susceptibility to drowning of the dominant species caught. It has been speculated that flatback turtles have a greater tolerance to trawl-capture than other species. Flatback turtles were the dominant species caught in the Torres Strait (66%) and this combined with an average tow duration of 144 minutes may account for the lower mortality rates in the Torres Strait Prawn Fishery than in the Northern Prawn Fishery, where average tow duration has been reported as 186 minutes. Mortality rates of turtles in the Queensland Trawl Fishery are markedly lower than the Northern Prawn Fishery most likely as a consequence of short tow durations (i.e. 60 to 90 minutes) in the areas where turtles are caught predominantly, i.e. the Moreton Bay fishery. Another possible cause of the low mortality rates in this study could be under-reporting of dead turtles by fishers involved in the program. However, the incidence of a low mortality rate of trawl-caught turtles is supported by tow duration data and levels of mortality similar to the Northern Prawn Fishery were reported in some areas of the Queensland Trawl Fishery where tow durations are longer (i.e. 129 minutes, tiger and endeavour prawn fisheries of north Queensland). The degree of inaccurate reporting should be variable, as different fishers would repOli differently. It would take a concerted effort from the majority of commercial fishers involved in this study (some 106 individuals) to have a major effect on data accuracy.

It is difficult to speculate what impact the estimated turtle bycatch has on sea turtle populations of eastern Australia. There is limited quantitative information available about the population status of the six species of sea turtle that inhabit the waters of eastern Australia. The exception to this is the loggerhead turtle, for which a 50% to 80% decline in the number of nesting female turtles has been observed since the mid 1980's. Determining the numbers and the status of sea populations has intrinsic difficulties because of: i) the paucity of census data, ii) the difficulties in estimating abundance and determining trends in localised feeding grounds, iii) the mixture of stocks in feeding grounds, iv) the lack of quantification of life history parameters and the longevity of turtle life cycles, and v) the dispersed nature of the population between feeding grounds and nesting beaches and our incomplete understanding of the migration patterns. Sea turtles are long-lived, have delayed sexual maturity and high survivorship of adults. Species with these life history traits are particularly susceptible to human impacts that can result in population declines. Hypothetical modelling of the Queensland east coast loggerhead turtle population suggests that an annual loss of only a few hundred adult and sub-adult female turtles would have a profound effect on the population and would result in a declining population size.

The turtle bycatch and trawl related mortality estimated for the Queensland Trawl Fishery and the Torres Strait Prawn Fishery would contribute to a decline in the loggerhead turtle population, if the model reflects the true situation. It is likely that bycatch in trawl nets is only one factor contributing to the declining numbers of sea turtles in eastern Australia. This is especially so for species such as green and hawksbill turtles, that are the target of commercial and traditional harvest, or flatback turtles whose eggs are at risk to feral animal predation in northern Australia. Nevertheless, measures that the trawl industry can take to minimise its impact upon sea turtle populations of eastern Australia should be investigated.

The fate of turtles post-release from a trawl capture was also investigated during the research project. Seven trawl-caught turtles were monitored after release from the trawler using realtime tracking systems and data-logging equipment. The data-logging equipment (Temperature Depth Recorders or TDRs) provided the most complete picture of dive profiles of trawlcaught turtles. All turtles displayed a distinctive "escape" response upon release. The data recorded indicates that trawl capture resulted in appreciable behavioural changes, i.e. an increased number of surfacings. It appeared that small turtles took longer to recover than large turtles. No delayed post-trawl mortalities were observed, as would be expected with the small sample size and a reported trawl mortality of 0.6% in Moreton Bay, the location where field work was undertaken.

The participation of commercial fishers in the voluntary turtle monitoring program had a significant impact on raising the industry's awareness of the issues associated with the incidental capture of turtles in trawl nets. Visits by research staff to the ports and wharfs of the Queensland east coast resulted in energetic discussions on these issues between boat owners, skippers, decldlands and research staff. Recovery treatments for trawl-caught turtles and a code of fishing ethics, covering turtle captmes, were developed in conjunction with the Queensland Commercial Fisherman's Organisation. A four page leaflet, including recovery procedures, species identification guide and code of fishing ethics was produced with support from the Queensland Commercial Fisherman's Organisation, the Australian Fisheries Management Authority, the Australian Prawn Promotion Association and the Australian Nature Conservation Agency (= Environment Australia). It was distributed to all master fishermen from the Queensland East Coast, Torres Strait and the Northern Prawn Fishery. Anecdotal reports from commercial fishers provide encouraging information that these recovery techniques are being employed in the industry and that many turtles can recover from trawl captures.

Limited quantitative information is available about the current status of turtle populations from the Queensland east coast. Current indices of population trends (i.e. nesting beach surveys) are only available for loggerhead turtles. Turtle catch per Unit effort (CPUE) was investigated as an alternate means of monitoring turtle populations only in areas where sampling effort and turtle catch were continuous throughout time. Only two of the 133 QFISH grids in which turtle bycatch occurred, had sufficient data to provide a continuous picture of abundance. These grids were Moreton Bay (W88) and BU11daberg (U32). Turtle CPUE was still highly variable in these grids. It is likely that U11less sampling effort is highly concentrated and continuous throughout time, turtle CPUE will not be able to detect changes in population size unless dramatic changes occur. The use of turtle CPUE as an index of abundance may be possible if accurate turtle bycatch is recorded by the majority of the trawl fleet as information collected through the compulsory trawl fishery logbooks. Turtle CPUE was most useful as an overall, wide-scale, in-water survey of the distribution of sea turtles throughout Queensland waters. The turtle CPUE by species has provided insights into potential areas where sea turtles are aggregated and may provide fruitful areas for research into sea turtle biology and population dynamics by conservation agencies.

The assessment of sea turtle bycatch in Australian prawn trawl fisheries is necessary to support the conservation of threatened sea turtle species. The voluntary turtle monitoring program has developed a long-term database on the frequency and location of turtle captures. The data is being used in fisheries management for the identification of priority areas where the issue of how to abate threats to turtles from trawling is being negotiated. This includes the identification of areas where TEDs are to become compulsory. The commercial fishing industry has input to these negotiations through the Queensland Trawl Management Plan via TrawIMAC. The Queensland Department of Environment and the Great Barrier Reef Marine Park Authority also have input into determining these priority areas through the joint analysis of the turtle CPUE data via a collaborative risk assessment.

The process of conducting a voluntary turtle monitoring program over 6 years has helped to develop a responsible attitude by commercial fishers to environmentally sensitive issues such as sea turtle conservation. The positive relationship established between commercial fishers and research staff has been of considerable value in assisting with the introduction and adoption of measures to mitigate turtle bycatch (i.e. Turtle Excluder Devices) in Queensland east coast trawl fisheries. This project has demonstrated the value of involving commercial fishers in research projects, especially when there is continuity in the research staff. This enables contacts with the fishing industry to be established and developed over an extended period of time.


1. To provide detailed information on the turtle-trawl interactions over an extended period along the Queensland coast and in the Torres Strait.

2. To determine the fate of turtles which suffer repeated trawl capture.

3. To liaise with industry on the issue of turtle-trawl interactions and to educate fishermen on treatment of trawl-captured turtles.

4. To investigate an alternative population monitoring method for sea turtles using catch and effort information from the trawl fleet