A scientific basis for a comprehensive approach to managing sea turtle by-catch: the Queensland east coast as a case study

Robins, Julie Belinda (2002) A scientific basis for a comprehensive approach to managing sea turtle by-catch: the Queensland east coast as a case study. PhD thesis, James Cook University.

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Abstract

The Australian continental shelf is one of the few remaining areas of the world where sea turtle populations have been subject to relatively small levels of direct harvest and where nesting- and feeding-ground habitats remain essentially intact. The area supports six of the world’s seven sea turtle species (i.e., Chelonia mydas, Caretta caretta, Eretmochelys imbricata, Natator depressus, Lepidochelys olivacea and Dermochelys coriacea), contains significant nesting populations of three species (i.e., Chelonia mydas, Caretta caretta, Eretmochelys imbricata), the world’s only nesting populations of N. depressus and is the main feeding-grounds area of Natator depressus. The incidental capture of sea turtles in the trawl fisheries of northern Australia was a significant threat to sea turtle populations of Australia. Trawl by-catch mortality of Caretta caretta is considered to have contributed to the 80% decline in numbers of nesting sea turtles in eastern Australia, but this speculation is based on limited or unpublished data relating to mortality associated with trawl fisheries. There is limited information on the potential impact of trawl fisheries on other sea turtle species that occur in Australian waters. Turtle Excluder Devices (TEDs) were legislated for use in trawl fisheries of northern Australia in 1999. TEDs allow sea turtles to escape from trawl nets whilst enabling the trawl fishery to continue to operate and catch prawns. TEDs can be an effective solution to sea turtle by-catch, but the adoption and use of TEDs in a fishery needs to be monitored to ensure that the devices are having the desired outcome i.e., sea turtle exclusion. The scale and geographic extent of Australian trawl fisheries have the potential to result in a poor capacity to monitor the effective use of TEDs by fishers. TED compliance strategies (i.e., monitoring and enforcement) should be focused in areas where the effective use of TEDs would have the greatest benefit to sea turtle conservation i.e., areas where sea turtle by-catch or mortality is greatest. The present study adopted a comprehensive approach to understanding the interaction between trawling and endangered sea turtle species in waters adjacent to the Queensland east coast, by collecting and utilising baseline data on the size and distribution of sea turtle by-catch. Aspects of compliance strategies for TEDs that would contribute to the sustainable management of sea turtle by-catch in trawl fisheries are also considered. In this context, the following objectives are addressed in this thesis: (i) To estimate the number and species composition of sea turtles caught and killed in a multiple sector trawl fishery using spatial stratification; (ii) To examine the behavioural responses of sea turtles to trawl capture in order to investigate the potential for post-trawl mortality; (iii) To investigate the factors that influence the distribution pattern of sea turtles, and from this analysis, predict the relative in-water densities of sea turtles at broad spatial scales; and (iv) To develop a spatially explicit strategy for TED compliance (i.e., monitoring and enforcement), based on an assessment of the interaction between sea turtles and fishing effort. The thesis concentrates on sea turtle by-catch in the Queensland East Coast Trawl Fishery, but has broader implications for the management of sea turtles in their feeding-grounds. Baseline information on sea turtle by-catch was collected from select commercial fishers who voluntarily reported information on sea turtles caught in the Queensland East Coast Trawl Fishery. About 1,500 sea turtles were reported caught during ~24,000 days of trawling by 105 fishers who participated in a voluntary program to monitor sea turtle by-catch between 1991 and 1996. Stratified, weighted analysis of the data resulted in an annual estimated sea turtle catch of about 5,900 for the Queensland East Coast Trawl Fishery, given a mean annual total fleet effort of about 85,000 days fished per year. The catch was comprised of Caretta caretta (~50%), Chelonia mydas (27%), Natator depressus (16%) and Lepidochelys olivacea (6)%. The fishery had minimal catches of Eretmochelys imbricata and Dermochelys coriacea. Sea turtle by-catch was dominated by immature individuals, with between 60 and 80% of individuals caught being immature, based on approximate size-at-maturity for each species. Estimates of total mortality were based on observed rates as well as published mortality rates from USA shrimp trawl fisheries. In general, mortality rates of sea turtles caught in the Queensland East Coast Trawl Fishery were lower than mortality rates reported for other trawl fisheries in northern Australia and the USA. Lower mortality rates were probably a function of the tow duration associated with various sectors of the Queensland East Coast Trawl Fishery. However, the combined by-catch of sea turtles in the three major trawl fisheries of northern Australia (i.e., the Queensland East Coast Trawl Fishery, the Northern Prawn Fishery and Torres Strait Prawn Fishery) was estimated to be of sufficient magnitude to have contributed to the observed declines in nesting numbers of east Australian Caretta caretta. Estimates of the mortality of sea turtles in the trawl fisheries of northern Australia raise concerns about the likely impacts of these fisheries on other species of sea turtle, particularly Australian sub-populations of Natator depressus and Lepidochelys olivacea. These species frequent feeding-ground habitats typical of trawl grounds and are poorly monitored through nesting-ground surveys. The scale of impact estimated in this thesis indicates that the use of TEDs in the trawl fisheries of northern Australia is warranted. Mortality rates of trawl-caught sea turtles could be higher than currently estimated if sea turtles die after release as a consequence of the delayed effects of capture or secondary mortality resulting from changes in diving behaviour. Six trawl-caught sea turtles were monitored post-release using ultrasonic transmitters and Temperature Depth Recorders in order to assess their diving patterns for signs of modified behaviour. Two rodeo-caught sea turtles were monitored post-release as controls to the trawl-caught individuals. All sea turtles swam rapidly away from the point of release and displayed a period of frequent surfacing behaviour that was speculated to represent swimming and hyperventilation. The sea turtles then settled into a steady pattern where dive intervals were long and regular. This was speculated to represent recovery behaviour. ‘Normal’ activity patterns, as documented in the literature, were not apparent in the dive profiles of the trawl-caught sea turtles within the post-release monitoring period (i.e., 66 hours). Rodeo-caught sea turtles displayed ‘normal’ activity patterns at about 85 and 111 hours post-release. There was no evidence of delayed post-release mortality in the limited number of individuals monitored. However, the trawl-caught sea turtles displayed modified diving patterns that potentially made them more susceptible to secondary mortality such as boat strike or predation. Recovering sea turtles did not appear to undertake normal feeding activities, suggesting that sea turtles exposed to non-lethal interactions with human activities on a frequent basis may have lower growth rates. The results suggest that sea turtles are affected by interactions with humans to a much greater extent than previously thought and that the recovery period of such interactions can take several days The relative spatial distribution of sea turtles is poorly known and is insufficient for developing management plans, such as monitoring and enforcement strategies for TEDs. Sea turtle catch per unit effort from trawl captures and sea turtle sightings from aerial surveys were used to estimate the relative density of sea turtles in the waters adjacent to the Queensland east coast. As expected, sea turtles were not evenly distributed, with several areas having exceptionally high relative densities. In trawled areas, the relative density of sea turtles was significantly correlated with the benthic species trawled (e.g., species of prawn) as well as water-depth. Natator depressus and Lepidochelys olivacea had high relative densities in inshore, tropical waters less than 40 m deep, where tiger prawns (Penaeus esculentus, P. semisulcatus) and endeavour prawns (Metapenaeus endeavouri, M. ensis) were caught most commonly. Caretta caretta had high relative densities in inshore, sub-tropical waters less than 30 m deep, where banana prawns (Fenneropenaeus merguiensis) or bay prawns (Metapenaeus bennettae i.e., Moreton Bay) were caught most commonly. The relative density of sea turtles was predicted for the majority of waters adjacent to the Queensland east coast based on the mean sea turtle CPUE and information on target species caught most commonly and mean-depth trawled. The relative distribution of sea turtle density developed in this thesis provides the first broad scale maps that quantitatively identify areas that appear to be important as sea turtle habitat. This information could be used to assist the conservation management of sea turtles populations of eastern Australia. Sea turtle by-catch in northern Australia has been addressed through the mandatory use of Turtle Excluder Devices (TEDs) in otter trawl fisheries. However, the use of TEDs in a fishery needs to be supported by monitoring or enforcement to ensure that TEDs are used effectively, but this is difficult in fishery that operates over a large geographic scale. A pragmatic solution would be to target the monitoring and enforcement of TEDs in critical areas where reductions in sea turtle by-catch will make the greatest contribution to recovery of impacted populations of sea turtles. The spatial distribution of effort for the Queensland East Coast Trawl Fishery in the year 2001 was integrated with the relative density of sea turtles to identify critical areas for sea turtle by-catch. Critical areas for sea turtle by-catch were similar despite the use of qualitative or quantitative methods. The results suggest that effective TED use is most critical in the inshore waters of the Queensland east coast. Monitoring and enforcing TEDs in these critical areas would enable fisheries managers to measure progress towards the stated target of the 95% reduction in sea turtle by-catch and contribute to the sustainable management of the fishery. The use of TEDs in non-critical areas should also be monitored, but because of the lower contribution of these areas to sea turtle by-catch, monitoring and enforcement could take place with less intensity. Critical areas for monitoring the effective use of TEDs may change if the intensity of fishing effort changes and may become unnecessary should it be demonstrated that most fishers comply fully with TED regulations.

Item ID: 1251
Item Type: Thesis (PhD)
Keywords: sea turtles, Turtle Excluder Devices, TEDs, trawl fisheries, by-catch, Queensland, compliance, conservation, management, behaviour, distribution
Date Deposited: 13 Jul 2007
FoR Codes: 05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050202 Conservation and Biodiversity @ 0%
05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050211 Wildlife and Habitat Management @ 0%
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