Prawn Trawl Research Articles

An assessment of Bycatch Reduction Devices in a tropical Australian prawn trawl fishery

Our study assessed 16 different Bycatch Reduction Devices (BRDs) for use in Australia's Northern Prawn Fishery (NPF), during three cruises in the Gulf of Carpentaria. The study was a response to pressure from conservation and community groups to make harvesting of the sea more selective and less damaging to the non-target communities. The approach was to incrementally improve the performance of BRDs after each stage: underwater observations in a flume tank and engineering trials at sea; rigorous and controlled scientific trials on the RV Southern Surveyor; testing under normal commercial fishing conditions. Bycatch Reduction Devices were improved throughout these procedures; only the best-performing devices were tested in the next stage. All four inclined grids (Super Shooter, Nordmøre grid, AusTED and NAFTED) were extremely effective at excluding large sharks, stingrays and sea turtles. They were also effective at excluding some of the unwanted small-fish catch, especially when used in combination with other BRDs, such as a fisheye or square-mesh window. Fish exclusion ranged between 0 and 39%. Exclusion of sea snakes was substantially increased by using BRDs, such as the AusTED or square-mesh windows. The ability of BRDs to maintain the catch of the target species was part of the assessment. The Super Shooter had the best prawn retention rate, losing only between 2 and 12% of prawns in scientific trials and gaining 3.4% during the commercial trials. The Super Shooter also performs well in areas where the other inclined grids tend to clog with sponges or other large objects. The NAFTED, which was used only during the commercial trials, lost only 3.3% of the prawn catch of a standard trawl. There is evidence that the fisheye may increase prawn catches. Other devices lost substantial numbers of prawns, but modifications could greatly improve their performance. Some of these devices could be used successfully in their present form. The performance of most BRDs were also strongly affected by the weather and fishing procedures, such as delay in haulback of the trawl. This study has shown that there are BRDs that can be of major benefit to prawn trawl fishers in the NPF, by excluding much of the unwanted bycatch while maintaining catches of commercially valuable prawns. The main benefits to the NPF fishery of using BRDs are: • fewer or no sea turtles caught or killed; • fewer small fish to be sorted from catches; • fewer large animals (stingrays, sharks and sea turtles) that can damage valuable prawns, resulting in a higher catch value; and • decreased fishing impacts on bycatch species, which helps to maintain ecological biodiversity and resilience of these demersal communities. Failure of the NPF and other Australian prawn trawl fisheries to voluntarily adopt BRDs may result in an involuntary adoption brought on by pressures from community, conservation and trade bodies.

A validation of short-duration scientific tows as a representation of long commercial-length tows: comparing the catch rates, size composition and species composition of prawn trawler bycatch in the far northern Great Barrier Reef, Australia

The duration of tows for scientific trawl surveys in northern Australia is generally 30 min. Commercial trawls may be up to 200 min long. In this study we compare the catch rates, size of fish and species composition for 10 short (30 min) tows paired with 10 commercial length (165 min—not in a straight line) tows at both an inshore and an offshore location in the far northern Great Barrier Reef. Evaluations of this kind have not been reported in tropical Australian multi-species fisheries. Overall, catch rates of fish and invertebrates differed between short and long tows both inshore and offshore. Inshore, the mean catch rate for the fish and the invertebrates was significantly greater in short duration tows ( P<0.05). Offshore, the mean catch rate for all fish, and all invertebrates was not significantly different between short and long tows, although higher catch rates of invertebrates were recorded in long tows. Inshore, 12 fish species out of 49 had significantly different catch rates, most being greater in short tows. Only 2 invertebrates out of 16 had significantly different catch rates ( P<0.05). The combined weight of species showing significant differences in catch rates represented less than 10% of the total weight in both inshore and offshore samples. Principal component analysis indicated that catch composition was similar between short and long tows both inshore and offshore. The results from our sampling are that the species composition of 30-min trawls is similar to that of commercial length tows, and that size frequencies are equivalent. The implications are that short tows (30 min) can be used in scientific surveys to give a true representation of both size and species composition of commercial length tows, but may overestimate catch rates.

Retained and discarded by-catch from oceanic prawn trawling in New South Wales, Australia

The catches and by-catches of oceanic prawn trawlers were identified and quantified in a stratified, randomized, observer survey of four of the most important fleets in NSW, Australia. For each port, catches were sampled from all tows done during replicate fishing trips in each season from winter 1990 to autumn 1992. Data were examined using analyses of variance of relative abundances of individual species and groups of species, multi-dimensional scaling of the full by-catch dataset, estimates of retained by-catches and discards by the four ports (incorporating estimates of fishing effort) and length–frequency distributions of common discards. The results revealed a latitudinal gradient in by-catch and discarding with larger quantities occurring northwards. However, significant species-specific variations in abundances were detected at all spatial and temporal scales sampled such that the occurrences and abundances of particular by-catch species depended on the location, season and year examined. In catching an estimated 1 579t of prawns during the two-year survey, the oceanic prawn trawlers from the four ports were estimated to have caught approx. 16 435t of by-catch (a by-catch-to-prawn ratio of 10.4 : 1). Of this by-catch, 2 952t were estimated to have been retained for sale and 13 458t were discarded – including several million individuals of commercially and recreationally important species (e.g. snapper, eastern blue-spot flathead, red-spot whiting). We discuss the results in terms of the spatial and temporal variations in these discards and the relative effectiveness of various ways to reduce them, including fixed and flexible closures to trawling and selective trawl gears and fishing practices.

Fishing effort in the far northern section cross shelf closure area of the Great Barrier Reef Marine Park: the effectiveness of area-closures

A meta-analysis of available information on the intensity and distribution of illegal prawn (shrimp) trawling is reported from a 10 000 square kilometre cross shelf area-closure in the tropical Great Barrier Reef Marine Park. Northern and southern borders of the closure were subject to edge effects as trawlers over-ran their hauls, legally started in the adjacent open zones. There was considerable illegal trawling inshore along a north–south navigation channel running through the area-closure. The middle and outer regions of the closure were originally denied to trawlers because of «rough ground» but this is probably no longer the case since the general adoption of advanced navigation aids (GPS). The area least likely to have been heavily trawled is the central mid-shelf section of the closure in a region of uncharted shoal reefs. Many trawlers fish in the accessible sections of the closure on a casual or incidental basis but approximately 47 boats do so on a consistent basis. An estimated yearly total of 3260 days illegal trawling occurs, giving an approximate estimate of 69 days illegal trawling per regular offender. The implications of this level of non-compliance are the probable invalidation of the original objectives of conservation and maintenance of a pristine reference area in all but the central section of the closure. Management of marine world heritage areas requires the acknowledgement that the harvesting of fish, prawns and other living resources may be an established use of the Marine Park but it also requires a combination of education plus the adoption of effective monitoring technology, such as satellite vessel monitoring systems, to stop or at least reduce unauthorized fishing.

Simulated escape of juvenile sand whiting ( Sillago ciliata, Cuvier) through square-meshes: Effects on scale-loss and survival

Two laboratory experiments were done to assess effects of simulated escape through square-meshes on the scale-loss and survival of (i) non-fatigued and (ii) fatigued small sand whiting ( Sillago ciliata). In experiment 1, non-fatigued fish that were forced through square-meshes (treatment fish) showed no significant difference in scale-loss compared to fish that did not pass through square-meshes (control fish), although there was a 50% difference in mean scale-loss immediately posterior to their maximum height. In experiment 2, fish were fatigued to exhaustion by swimming against a current of 0.7 to 0.8 knots for 15 min. Fatigued fish that were then forced through square-meshes showed significantly more scale-loss across their entire body than did the fatigued control fish (difference in means of between 67% to 84%). In both experiments the total scale-loss on treatment fish was quite low (1.4–4%) and there were negligible mortalities (only 2 treatment fish died in experiment 1) over the duration of each experiment (30 days). We conclude that the composite square-mesh panel currently used to reduce by-catch in the NSW oceanic prawn trawl fishery is likely to cause negligible damage and mortality of small sand whiting.

Adopting turtle excluder devices in Australia and the United States: What are the differences in technology transfer, promotion, and acceptance?

Turtle excluder devices (TEDs) are being trialed on a voluntary basis in many Australian prawn (shrimp) trawl fisheries to reduce sea turtle captures. Analysis of TED introductions into shrimp trawl fisheries of the United States provided major insights into why conflicts occurred between shrimpers, conservationists, and government agencies. A conflict over the introduction and subsequent regulation of TEDs occurred because the “problem”; and the “solution”; were perceived differently by the various stakeholders. Attempts to negotiate and mediate the conflict broke down, resulting in litigation against the U.S. government by conservationists and shrimpers. Litigation ‐was not an efficient resolution to the sea turtle‐TED‐trawl conflict, but it appears that litigation was the only remaining path of resolution once the issue became polarized. We review two major Australian trawl fisheries to identify any significant differences in circumstances that may affect TED acceptance. Australian trawl fisheries are structured differently and good communication occurs between industry and researchers. TEDs are being introduced as mature technology. Furthermore, bycatch issues are of increasing concern to all stakeholders. These factors, combined with insights derived from previous conflicts concerning TEDs in the United States, increase the possibilities that TEDs will be introduced to Australian fishers with better acceptance.

By-catch from prawn trawling in the Clarence River estuary, New South Wales, Australia

By-catch was surveyed in the commercial prawn trawl fleets of the Clarence River system, the largest estuarine prawn fishery in New South Wales, Australia. The catch was censussed in all tows during replicate fishing trips in each month in the 1989–1990, 1990–1991 and 1991–1992 prawn trawl seasons in the two areas of this fishery: the Clarence River and Lake Woolooweyah. Significant species-specific variabilities in abundances were detected at a variety of spatial and temporal scales. Despite comparatively small ratios of by-catch to catch of prawns (annual estimates in each area of between 0.13:1 and 0.45:1), hundreds of thousands of small recreationally and commercially important fin-fish were caught and discarded by the fleet. The results are discussed in terms of (i) their contrast to common generalisations about by-catch in prawn trawl fisheries, (ii) the accuracy and precision of the methods used to quantify and compare by-catches, (iii) the consequences for conflicts between prawn trawl fisheries and other commercial and recreational fisheries that target by-catch species, and (iv) the various strategies available to fisheries managers to resolve potential conflicts.

Observer-based survey of by-catch from prawn trawling in Botany Bay and Port Jackson, New South Wales

Catches and by-catches were surveyed in the commercial prawn trawl fleets of Botany Bay and Port Jackson, two estuaries in the Sydney metropolitan area (NSW, Australia). Catches were surveyed in all tows during replicate fishing trips in each month during the 1990-91 and 1991-92 prawn trawl seasons in each estuary. Significant species-specific variabilities in abundances were detected between estuaries, between years, and between early and late in the fishing season. The mean annual ratio of by-catch to catch of prawns (by weight) was 2.5 : 1 for Botany Bay and 1.8 : 1 for Port Jackson. A mean annual by-catch (� 1 s.e.) of 142 � 14 t was taken from the Botany Bay fishery and 38 � 3 t from Port Jackson. These by-catches included large numbers of small recreationally and commercially important finfish: a mean annual by-catch of 1.52 �0.20 million fish from Botany Bay and 219� 23 thousand fish from Port Jackson. Despite uncertainties about the impacts of such by-catches on interacting commercial and recreational fisheries, it is recommended that strategies for the reduction of such by-catches be considered.

Sea snake by-catch of prawn trawlers on the northern Australian continental shelf

This paper describes and compares the sea snakes caught by vessels that target tiger prawns and endeavour prawns or that target the banana prawns Penaeus merguiensis and P. indicus. In 1989-90, 5203 sea snakes (14 species; 7 unidentified specimens) were purchased from fishers who trawled between Koolan Island and Cape York and participated in a dedicated carcass-tagging and data-collection programme. Hydrophines (11 species) represented 86.7% of the total catch. Aipysurines (3 species) represented 15.0% of specimens from vessels that targeted tiger prawns or endeavour prawns, but only 1.1% of specimens from vessels that targeted the banana prawns P. merguiensis and P. indicus. Prawn trawlers that operated between Koolan Island and Cape York during 1990 caught approximately 81 080 (� 13 670) sea snakes. Vessels that targeted tiger/endeavour prawns, P. merguiensis and P. indicus caught approximately 69260 (� 8750), 7200 (� 3250) and 4620 (� 1120) sea snakes respectively. The results emphasize the advantages of interactions between fishers and scientists and the need to assess separately the impacts of the three fisheries that constitute the northern prawn fishery.

Population model analysis for the loggerhead sea turtle, Caretta caretta, in Queensland

Worldwide declines of marine turtle populations have forced a need for sound conservation policies to prevent their extinction. Loggerhead turtles, Caretta caretta, are declining rapidly at eastern Australian nesting beaches, which are visited by females from all feeding areas for the stock. In some feeding areas of eastern Australia, loggerheads have been protected from deleterious anthropogenic effects. Using long-term mark-recapture data from one such protected group of turtles feeding on Heron Island Reef, Queensland, we created a matrix model to analyse loggerhead demography. We also produced a model for the females nesting at Mon Repos, Queensland, a major rookery where the annual nesting population has declined at rates approaching 8% per year. As indicated by a similar model for loggerheads in the USA, our models predicted that small declines in annual survival rates of adult and subadult loggerheads can have a profound impact on population dynamics. A loss of only a few hundred subadult and adult females each year could lead to extinction of the eastern Australian loggerheads in less than a century. Survival in the first year of life is relatively less important in these long-lived and slow-maturing animals. At Mon Repos, nesting female survival is apparently so low that even beach protection efforts resulting in 90% hatchling emergence success would not prevent population decline. Our research suggests that continued mortality pressure on subadult and adult turtles in their dispersed feeding areas of eastern and northern Australia is a major threat to the eastern Australian loggerhead turtle population. Measures that protect adult and subadult loggerhead turtles should be supported, including the use of turtle excluder devices (TEDs) on prawn trawls.

Rigid and flexible separator panels in trawls that reduce the by-catch of small fish in the Clarence River prawn-trawl fishery, Australia

Two experiments were done in the Clarence River prawn-trawl fishery that compared catches and by-catches from conventional trawls with those from modified trawls containing rigid and flexible separator panels. The modifications included (1) a soft separator panel, the 'blubber-chute' (used commercially by fishers to exclude jellyfish), (2) the standard Nordm�re grid, containing a guiding panel, and (3) a modified Nordm�re grid that had no guiding panel but included a flexible cover of mesh lying semi-attached over the escape exit. In the first experiment, simultaneous paired comparisons among these designs and their controls showed that although all three modified trawls significantly reduced the by-catch of small fish by between 75% and 90%, the standard Nordm�re grid was the only design that did not significantly reduce catches of prawns. In the second experiment, which compared this standard grid with the control during a period of flooding when the by-catch of non-target species was quite high, the standard grid significantly reduced the by-catch of juvenile bream (Acanthopagrus australis) (by 67%) with no significant reduction in catches of prawns. The potential for the Nordm�re grid to alleviate deleterious effects of prawn trawling in estuaries is discussed.

Development of a trawl efficiency device (TED) for Australian prawn fisheries. II. Field evaluations of the AusTED

Concerns about Australian bycatch issues lead to the development of an innovative bycatch reduction system designed for Australian trawling conditions. The system, known as the Australian trawl efficiency device (AusTED), incorporates features of previous TEDs, but includes a flexible grid. The AusTED was tested at five selected locations, which ranged from shallow, estuarine, mud-bottom sites to deep-water, oceanic, sand-bottom sites. No significant differences were observed in prawn catch rates between control and AusTED equipped nets. Sea turtles and large stingrays were excluded from the AusTED equipped net and non-commercial bycatch was significantly reduced at most sites trawled. The AusTED system has the potential to be developed to suit the bycatch exclusion requirements of prawn trawl fisheries around Australia. Its acceptance by the fishing industry will depend upon the minimisation of prawn losses.

Estimated catch and mortality of sea turtles from the East Coast Otter Trawl Fishery of Queensland, Australia

Trawling for penaeid prawns has been suggested to have major effects on the size of sea turtle populations in Australia and overseas. The numbers of turtles caught and killed in the Queensland East Coast Otter Trawl Fishery were estimated from a 2-year programme monitoring turtle captures in this commercial prawn trawl fishery. A sample of about 50 commercial fishers participated and reported an average catch rate of 0·068 turtles per day fished. Loggerhead Caretta caretta (50·4%), green Chelonia mydas (30·1%) and flatback Natator depressus (10·9%) turtles comprised the main species caught. An estimated 5295 ± 1231 turtles were caught annually by the Queensland East Coast Otter Trawl Fishery, which had an annual fishing effort of 80,558 days. About 1·1% of captured turtles were reported dead when landed. This mortality rate is lower than those reported for other trawl fisheries because in the Queensland East Coast Otter Trawl Fishery tow durations are short (<80 min) where turtles are commonly caught. Potentially, if all comatose turtles are assumed to die, the mortality rate of trawl caught turtles could be as high as 6·8%. However, the true mortality rate is probably somewhere between 1·1% and 6·8% of turtles landed because some commercial fishers resuscitate trawl-caught turtles.

Distribution and seasonality of ichthyofauna associated with commercial prawn trawl catches on the Tugela Bank of Natal, South Africa

The ichthyofaunal by-catch of penaeid prawn trawlers operating on the Tugela Bank of Natal, South Africa, was sampled at irregular intervals from May 1989 to June 1992. A total of 110 samples was collected and 108 teleost species were identified. Six species, of which four were sciaenids, comprised 80% of overall abundance by number. Four poorly separated main sample groups were identified by a multivariate approach, using dendrograms produced by hierarchical agglomerative clustering and ordinations produced by nonmetric multidimensional scaling, based on Bray-Curtis dissimilarity measures. Species which characterised and distinguished the four sample groups were identified by a SIMPER (percentage similarities) procedure, whereby the contribution of each species to similarity measures within sample groups and to dissimilarity measures between sample groups was determined respectively. Ten or fewer species accounted for 80% of the contribution to each within-group similarity measure and intergroup dissimilarities were low. The ordination was then replotted using the environmental data (season, depth and time of day), with samples being stratified according to these variables, irrespective of how they were grouped according to species composition. The plots of environmental data superimposed on the species ordinations suggested that the grouping of samples was attributable to season and depth. A non-parametric equivalent of MANOVA (ANOSIM) was used to test for interactive effects of all three environmental variables. This procedure confirmed that depth and season were highly significant in determining sample groupings and day/night effects were only slightly significant. It was concluded that many of the Tugela Bank prawn trawler by-catch fish species were permanent inhabitants of the region and that the most abundant species exhibited seasonal and distributional trends in abundance.

Prawn trawling in the Gulf of Carpentaria

Prawn trawling in the Gulf of Carpentaria

Incidental capture of elasmobranchs by commercial prawn trawlers on the Tugela Bank, Natal, South Africa

Species composition, sizes, trends in occurrence and catch rates of elasmobranchs caught by prawn trawlers on the Tugela Bank of Natal, South Africa, were examined from May 1989 to June 1992. Seven endemic species were recorded, the remainder having Indo-West Pacific or western Indian Ocean distributions. Most sharks were between 0,5 and 1 m long and most of the Myliobatiformes were 0,5 m in disc width. The small sizes of trawl-caught elasmobranchs indicates that the Tugela Bank functions as a nursery area for several species. None of the species examined exhibited diel patterns in frequency of occurrence, but Gymnura natalensis, Himantura gerrardi, Dasyatis chrysonota chrysonota, Sphyrna lewini and the rhinobatids occurred more frequently in warmer months. One species, Halaelurus lineatus, was more frequently taken during cooler months. Himantura gerrardi, Sphyrna lewini, Mustelus mosis and Rhizoprionodon acutus were recorded more frequently in shallower trawls (20–33 m) and G. natalensis and D. c. chrysonota more frequently in deeper trawls (33–45 m). Trawl-induced mortality was shown to be species specific, S. lewini having the highest mortality (98%). Based on catch rates recorded and trawl fleet effort, it is calculated that 44 600 elasmobranchs were caught by Tugela Bank trawlers from 1989 to 1992. About 57% of these were returned to the water alive. These figures are compared to catches made by recreational anglers and the Natal Sharks Board during the same period.

The impact of commercial prawn trawlers on linefish off the north coast of Natal, South Africa

The by-catch of penaeid prawn trawlers operating on the Tugela Bank off Natal, South Africa, was analysed at irregular intervals for a period of three years, from May 1989 to June 1992. Catch-and-effort data were recorded from 159 trawls, and catch composition data from 110 trawls, all within the depth range 20–45 m. A total of 108 teleosts was identified in the by-catch, six of which constituted 80% of overall teleost abundance by number. Annual average by-catch and discarded quantities were estimated at 400 and 315 tons respectively. Ratios of discards to the retained catch ranged from 4,25 (±1,19) : 1 to 15,48 (±3,86) : 1 for shallow (20–33 m) and deep (33–45 m) trawls respectively. Only one linefish of economic importance, the squaretail kob Argyrosomus thorpei, occurred in prawn trawl catches, and all were juveniles spawned 4 - 6 months prior to being caught and apparently utilizing the Tugela Bank as a nursery area. They were seasonal in the catches, being trawled mostly between January and March. During those months trawling effort was generally low, because the prawns had not yet migrated offshore in quantity. Per-recruit analyses showed that the removal of these age-0 fish by trawlers reduced yield-per-recruit and spawning-biomass-per-recruit values for the A. thorpei fishery.

Mixed-Species Yield-per-Recruit Simulations of the Effect of Seasonal Closure on a Central Queensland Coastal Prawn Trawling Ground

An assessment of the effect of seasonal closure on the central Queensland coastal prawn trawling grounds was carried out by mixed-species yield-per-recruit simulations of the fishery based on parameters determined from field studies and analysis of logbook data. Under a conventional scenario of constant fishing mortality, simulations suggest little positive effect on yield to be achieved from closures of up to 15 wk in duration. Economic gain would have to be made via reduced costs during the closure, rather than through increased value of the postclosure yield. Logbook data clearly showed that the fishery underwent a heavy pulse of effort, immediately after the fishing season opened. Nonlocal boats had entered the fishery to take advantage of the prawns available. Using this more realistic regime of fishing mortality, simulations suggest a significant increase in yield and value of catch. The increase was caused by boosted exploitation rates of the stocks and was determined by the magnitude of the effort pulse rather than as a direct benefit of the closure.

Prawn trawling in the Gulf of Carpentaria, northern Australia: Aspects of occupational health

To conduct an occupational health study of men and women employed on prawn trawlers in the northern prawn fishery (NPF), Australia's major fishery. A retrospective review of attendances by crew members of prawn trawlers at the principal Health Centre on Groote Eylandt, between 1986 and 1989, and a prospective study, including a health and safety questionnaire, of all attendances at the Health Centre from April to June, 1990 (the first half of the annual fishing season). Alyangula Health Centre, Groote Eylandt, Northern Territory. From 1986 to mid-1990, there were over 700 attendances. Those with minor ailments were treated by nursing sisters and the remainder (152) were referred to the doctor and included in the study. In 1990, 29 patients were part of the prospective study. Of those seen by the doctor, the mean ages of men and women were 32 and 18 years respectively. The commonest diagnoses were: lacerations, sprains and fractures (30.9%); skin sepsis and dermatitis (25.7%); and sexually transmitted diseases (STDs) (13.8%). Among women, most of whom were cooks, 23% of consultations were for STDs. Knowledge of STDs and their prophylaxis among the crews was poor. All new employees should be given formal safety instruction before going to sea. To reduce the risk of STD, health education posters and condom dispensers should be placed aboard all trawlers. Pre-employment medical examination for all prawn fishermen, which at present is not compulsory, should be mandatory.

Incidental capture of sea-snakes (Hydrophiidae) by prawn trawlers in the Gulf of Carpentaria, Australia

Sea-snakes were collected from research trawlers and commercial prawn trawlers in the Gulf of Carpentaria during the period from April 1976 to December 1991. The data were analysed on the basis of CPUE (catch per unit effort) for depth, latitude and season. The research trawlers, operating in the eastern Gulf of Carpentaria, and the commercial prawn trawlers, operating in the south-western Gulf of Carpentaria, caught sea-snakes at a rate of 0.028 and 0.026 sea-snakes per metre of headrope length per hour, respectively. Lapemis hardwickii was the sea-snake most commonly caught by the research trawlers-53% of all snakes-and Hydrophis elegans was the sea-snake most commonly caught by commercial trawlers-25% of all snakes. Depth was the most significant factor affecting CPUE, with more than 70% of all sea-snakes being caught in waters less than 15 m deep. When catches of all species were combined, a significant interaction (P&lt;0.05) existed between depth and season. L. hardwickii specimens were caught more frequently in shallow coastal waters (&lt; 15 m deep) in spring but in deeper water further offshore in autumn. A significant interaction between latitude and depth was found for Astrotia stokesii; specimens were caught more frequently in deeper water at 14% Enhydrina schistosa is generally coastal, with 8800 of specimens being caught in water less than 10 m deep. Seasonal movement of sea-snakes between inshore and offshore waters may be linked to their breeding cycles. The estimated number of sea-snakes captured in the Gulf of Carpentaria for the 1991 prawning season ranged from 73 583 to 165 559, with a mean of 119 571. The survival rate of sea-snakes from commercial prawn trawls was about 60% and hence between 29 801 and 67 051 sea-snakes are estimated to have died.