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Evaluation of the Morrison soft turtle excluder device: prawn and bycatch variation in Moreton Bay, Queensland
Fisheries Research, 19 ( 1 9 9 4 ) 2 0 5 - 2 1 7
205
0 1 6 5 - 7 8 3 6 / 9 4 / $ 0 7 . 0 0 © 1994 - Elsevier Science B.V. All rights reserved
Evaluation of the Morrison soft turtle excluder device: prawn and bycatch variation in Moreton Bay, Queensland J.B.
Robins-Troeger
Southern Fisheries Centre, Queensland Department of Primary Industries, PO Box 76, Deception Bay, Qld. 4508, Australia (Accepted 27 October 1993)
Abstract The present study assesses the effects of a Morrison soft turtle excluder device ( T E D ) upon catch rates of commercial prawn, fish and non-commercial bycatch species at an essentially oceanic site and an estuarine site. Effects of the TED upon catch rates were highly variable. Reductions in prawn catch ( Penaeus plebejus, Penaeus esculentus, Metapenaeus bennettae) in the TED-equipped net varied between no significant reduction to a 29% reduction, by weight, depending on location and season. Reductions in prawn catch could not be attributed to the loss of prawns by species or solely to the selective loss of prawns on the basis of size. Fish catches (Sillago sp., Platycephalus sp., Suggrundus sp.) were highly variable and showed no significant difference between control and TED-equipped nets. Catches of marketable sand crabs (Portunus pelagicus) were reduced by more than 50% in the TED-equipped net. Large bycatch animals, such as shovelnose rays (Rhinobatus batillum), sand rays (Amphotistius kuhlii) and turtles (Caretta caretta), were caught occasionally in the control net but were absent from the TED-equipped net. Non-commercial bycatch, composed of fish and crustaceans, was reduced by up to 32% by weight in the TED-equipped net. Results from this study suggest that it is difficult to predict accurately the effect of the Morrison soft TED upon catches of prawns and bycatch for all trawl locations and seasons.
Introduction The coastal waters of Queensland, Australia, support a trawl fishery in which some 900 trawlers land approximately 7000 t of prawns (live weight) and 1000 t of scallop meat annually (Trainor, 1990). These trawlers also take a substantial amount of non-commercial bycatch, occasionally including sea turtles. Henwood and Stuntz (1987 ) estimated that as many as 50 000 sea turtles are caught annually in prawn trawl nets in the south-eastern USA, with SSDI0165-7836(93)00267-M
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J.B. Robins- Troeger / Fisheries Research 19 (1994) 205-217
approximately 20% of these animals drowning. Preliminary studies in northern Australia suggest that sea turtles may not be significantly impacted by trawl fisheries in this area (Poiner et al., 1990), and studies are under way to investigate the impact of trawling on sea turtles on the east coast of Queensland (M. Dredge, personal communication, 1991 ). Turtle excluder devices (TEDs) are effective in conserving sea turtles in areas where trawl fisheries exist (Anonymous, 1990). TEDs were first designed by the US National Marine Fisheries Service (NMFS) in 1978 and at least six designs have been officially certified (Magnuson et al., 1990). The NMFS TED, the Georgia Jumper TED, the Matagorde TED and the Cameron TED are all 'hard' TEDs which incorporate rigid steel or fibreglass structures. 'Soft' TEDs use polypropylene mesh rather than rigid bars or frames to exclude sea turtles and other large bycatch animals. The Morrison soft TED and the Parrish soft TED have been developed on these principles. Different fishing practices and different trawl conditions in Australia suggest that those TEDs most suitable for use in US waters may not necessarily suit Queensland conditions. Goeden ( 1985 ) carried out pilot trials over two nights with a NMFS TED on prawn trawl grounds off Cairns. These trials did not demonstrate significant catch differences between a standard net and a NMFS TED-equipped net, but several points relevant to the use of hard TEDs were highlighted. The design of the NMFS TED was too large for Australian nets, which are longer and narrower in the throat than American nets. Goeden suggested that there would be a loss in efficiency of the device at excluding turtles if the TED was reduced to fit Australian nets. A major difference between Australian and American trawl procedures is the manner in which the cod end is emptied. American trawlers leave the throat and body of the net (which incorporates the TED when in use) in the water while the cod end is spilt on to the back deck. On Australian vessels, the net and cod end are lifted via a central boom and the catch is spilt on to a sorting tray, leaving the TEDs swinging in the air above the back deck. This can make operations hazardous for the crew. Goeden's comments suggest that a soft TED may be more suitable to Queensland fishing practices. A Morrison soft TED was tested at two selected sites within Moreton Bay, Australia, to investigate the efficiency of soft TEDs in Queensland waters. Catches of prawns, fish and bycatch were compared between control and TEDequipped nets. Moreton Bay is one of Queensland's most valuable trawling grounds (Trainor, 1990) and is also a major feeding ground for sea turtles (Marsh and Saalfeld, 1990). Studies on efficiencies of TEDs tested in Moreton Bay are directly relevant to commercial trawl operations in this area. Materials and methods
Trials on the efficiency of trawl nets fitted with a Morrison soft TED were conducted in May 1991 (Phase 1 ) when prawn catches were low, and during
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207
January 1992 (Phase 2) when prawns were abundant (Courtney et al., 1991 ). Catches of prawn, fsh, marketable bycatch and other non-target species were compared between two 9.2 m headrope length sandikan trawl nets (42 m m mesh), which were towed by a 15 m research otterboard trawler. Trials were conducted at two selected sites within Moreton Bay (Fig. 1 ). Site 1 (Moreton Banks) is essentially an oceanic site with a predominantly sandy substrate (Maxwell, 1970; Blaber and Blaber, 1980; Poiner and Kennedy, 1984). Site 2 (Deception Bay) is closer to shore and more estuarine, with a predominantly mud/silt substrate (Maxwell, 1970; Blaber and Blaber, 1980). Prior to the installation of the Morrison soft TED in each phase, a series of preliminary tows of the unmodified nets were conducted. The unmodified nets were towed and tuned until uniform catch rates of prawns, fish and bycatch were observed from the port and starboard nets. The uniformity of the catch rates between port and starboard nets was tested using a balanced analysis of variance. A Morrison soft TED (Fig. 2 ) was constructed of 150 m m monofilament mesh according to installation instructions issued by the Georgia Sea Grant Program (Christian et al., 1989 ). The Morrison soft TED was installed into one net after preliminary tows were completed. In each phase, between 17 and 23 tows were completed at each of the two sites. Bottom times for tows varied between 45 and 100 min. Catches from the control and TED-equipped net were landed and processed separately.
N
Bay
scale nm Sit
o
2
a
Fig. 1. Map of Moreton Bay, Australia showing sites selected for testing of a Morrison soft TED. Site 1: oceanic site, Moreton Banks, mean tow depth 20 m. Site 2: estuarine site, Deception Bay, mean tow depth 5 m.
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J.B. Robins- Troeger / Fisheries Research 19 (1994) 205- 217
~ ~
~
~ h foot rope
=
float _~_~. ~
Morrison soft TED
~
~
close up of turtle escaping through 142 cm exit slit cut in the top panel of the net
Fig. 2. The Morrison soft TED installed into a prawn trawl net. Redrawn from Christian et al. (1989). Large objects, such as stingrays, were removed and weighed first. Commercially important prawns ( Metapenaeus endeavouri, Metapenaeus ensis, Metapenaeus bennettae, Penaeus esculentus, Penaeus plebejus ) were sorted and weighed by species. Weights of these species were pooled to give the total prawn catch weight. Carapace lengths of three prawn species (Penaeus esculentus, Penaeus plebejus, Metapenaeus bennettae) were measured and recorded during Phase 2. Whiting (Sillago maculata and Sillago analis) and flathead (Platycephalus sp. and Suggrundus sp. ) were sorted and weighed. Data on these species were pooled to form the combined fish category. Sandcrabs (Portunus pelagicus) of legal status (male and > 15 cm carapace width ) were removed and recorded. The remainder of the catch was classified as non-commercial bycatch and was weighed before being discarded. Numbers and weights of non-commercial bycatch species were recorded from samples taken from every fourth tow during Phase 2. After each tow, the nets were checked and cleared of any debris or blockages.
Data analyses Catch rates (kg h - ~) for prawns, fish and bycatch were evaluated using a modification of Robson's (1966) m e t h o d for comparison of fishing powers with natural logarithm transformed data. Catch rates were adjusted for mean catch rates for each net, derived from the preliminary, standardization tows. Balanced analyses of variance were conducted, with location and net type as orthogonal factors. The fishing power of the TED-equipped net relative to the control net (95% confidence interval) was then calculated for each site (Robson, 1966). The fishing power analysis was specifically chosen to allow the comparison of catch statistics between nets, whilst taking into account such
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J.B. Robins-Troeger / Fisheries Research 19 (1994) 205-217
factors as individual tow and location variability. If there was no difference in the catch rate between the nets, relative fishing power would be 1.00. Initial analyses were conducted on combined prawn, combined fish and non-commercial bycatch catch rates. During Phase 2, further analyses were conducted to determine if the presence of a TED had significant effects on the individual catch rates of three commercial prawn species (Penaeus esculentus, Penaeus plebejus, Metapenaeus bennettae). Frequency distributions of prawn carapace lengths were compared between nets by testing against a Poisson distribution using an analysis of deviance (Bishop et al., 1977). An analysis of deviance is an extended form ofa chi square contingency table analysis which identifies the effects of various factors, i.e. species, size class, location. Legal sand crab catches were compared between nets using a paired t-test. Noncommercial bycatch species composition was compared between nets for differences in numeric abundance and weight using a balanced analysis of variance. Results Prawn catch
No significant differences were observed in the catch rates of prawns between nets at the oceanic site during Phase 1 (Table 1 ). Catch rates of prawns Table 1 Cross-site analysis of fishing power ( F P O W ) for a Morrison soft TED-equipped trawl net relative to a control trawl net Species/phase
Site
No. of tows
Mean FPOW
95% CI
Prawns/Phase 1
O E O E
23 20 17 20
0.917 0.707* 0.801" 0.826*
0.702-1.197 0.542-0.931 0.674-0.951 0.695-0.982
O E O E
23 20 17 20
0.898 0.722 1.354 1.010
0.579-1.390 0.466-1.119 0.715-2.564 0.533-1.190
O E O E
23 20 17 20
0.786 0.681"* 0.820 0.977
0.625-0.99(I 0.541-0.857 0.660-1.020 0.786-1.215
Prawns/Phase 2
Fish/Phase 1 Fish/Phase 2
Bycatch/Phase 1 Bycatch/Phase 2
*Significant at 0.05 probability level. **Significant at 0.01 probability level. O, oceanic site, Moreton Banks; E, estuarine site, Deception Bay; CI, confidence interval.
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z B. Robins-Troeger / FisheriesResearch 19 (1994) 205-217
Table 2 Cross-site analysis of fishing power ( F P O W ) for three commercial prawn species caught in a Morrison soft TED-equipped trawl net relative to a control trawl net (Phase 2) Species
Site
Mean FPOW
95% CI
Metapenaeus bennettae
0 E O E 0 E
0.863 0.880 0.875 0.837 0.991 0.847
0.617-1.207 0.629-1.231 0.628-1.219 0.601-1.165 0.834-1.179 0.713-1.008
Penaeus esculentus Peneaeus plebejus
Abbreviations as in Table 1.
at this site were significantly reduced in the TED-equipped net during Phase 2. Prawn catch rates at the estuarine site were significantly reduced in the TED-equipped net in both phases of the study (Table 1 ). Catch rates of individual prawn species ( Metapenaeus bennettae, Penaeus plebejus, Penaeus esculentus) sampled during Phase 2 were not significantly different between nets at either site (Table 2 ).
Size class frequencies for three commercial prawns--Phase 2 Species, size class and location factors all had significant effects on the size distribution of prawns caught ( P < 0.01 ). Interactions between net type and the other main effects were non-significant, except for size class. Whilst there is a statistically significant difference in size distribution of prawns between nets ( P < 0.01 ), in practical terms (Fig. 3) there is only a slight trend towards reduced numbers of large prawns in the TED-equipped net.
Fish catches Combined fish catch rates were not significantly different between nets at either site (Table 1 ). Wide confidence intervals associated with the fishing power estimates reflect the highly variable nature of whiting and flathead catches, which fluctuated between 15% and 2% by weight of the total catch.
Sand crab catches Legal sand crab catches were significantly reduced in the TED-equipped net at both sites during Phase 1 (oceanic site P < 0.01, estuarine site P < 0.05 ). Legal sand crab catches were low during Phase 2 and were not significantly different between nets at either site ( P > 0.05 ).
J.B. Robins- Troeger / Fisheries Research 19 (1994) 205-217
211
80
7o
I~
~
~ control net
~ ~o g
g 30 2O 10 0 < 165
185 205 225 245 255 285 305 325 345 3fiS > Carapace
Size
Class
(turn)
Fig. 3. Mean size class frequency of prawns caught in a control net and a Morrison soft TEDequipped net.
Non-commercial bycatch Large non-commercial bycatch animals, such as stingrays (Amphotistius kuhlii), shovelnose rays (Rhinobatus batillum) and loggerhead turtles (Caretta caretta), were caught occasionally in the control net but were absent from catches of the TED-equipped net. Most of the non-commercial bycatch was composed of fish and crustaceans, and accounted for between 65% and 80% of the total catch weight. Reductions in non-commercial bycatch in the TEDequipped net were variable at both sites (Table 1). Significant reductions in non-commercial bycatch ranged from 21% by weight at the oceanic site to 32% by weight at the estuarine site in Phase 1. Reductions in non-commercial bycatch were not significantly different between nets during Phase 2. Twelve species of invertebrates and 43 species of fish were observed in the non-commercial bycatch samples. Taxa of low numerical abundance or low catch weight were pooled into groups of similar body form before analysis (Table 3 ). The species composition of non-commercialbycatch was not significantly different for pooled taxonomic groups between nets at either site, for both numerical abundance (oceanic site F12, 72=0.35, estuarine site Fs, 48=0.26) and weight based analyses (oceanic site F12, 72=0.72, estuarine site /78,48=1.18).
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J.B. Robins-Troeger/ FisheriesResearch19(1994)205-217
Table 3 Phylogenetic list of non-commercial bycatch species with pooled taxa groupings for analysis Pooled taxa
Family
Species
Crustaceans Alpheids
Alpheidae
Alphenus distinguendus Alphenus stephensi
Portunus spp.
Portunidae
Portunus pelagicus (non-legal) Portunus sanguinolentus
Trawl crabs
Portunidae
Charybdis jaubertensis Charybdis callianassa Charybdis helleri Thalamita sirna
Squillids
Squillidae
Squilla laevis Oratosquilla woodmasoni Oratosquilla mephoroi Harpiosquilla raphidia
Teleost fishes Apogonids
Apogonidae
Apogon fasciata Apogon poecilopterus Siphamia roiegaster
Bothids
Bothidae
Pseudorhombus arsius Pseudorhombus jenynsii
Callionymids
Callionymidae
Callionymus limiceps Callionymus grossi Callionymus belcheri
Cynoglossids
Cynoglossidae
Cynoglossus bilineatus
Leoignathids
Leoignathidae
Leiognathus decorus Leiognathus moretoniensis
Monacanthids
Monacanthidae
Monacanthus chinensis Pseudomonacanthus peroni
Polynemids
Polynemidae
Polydactylus multiradiatus
Sydontids
Syndontidae
Saurida undosquamis
Combined Group A (small, poor swimming )
Atherinidae Chaetodontidae Clupeidae
Antennarius striatus Parachaetodon ocellatus Herklotischthys castelnaui Hyperlophus translucidus Thryssa aestuaria Thryssa hamiltoni Stolephorus carpentariae Acentrogobius caninus Favonigobius sp. Brachyamblyopus coecus Centropogon marmoratus Minous versicolor
Engraulididae Gobioidae Scorpaenidae
.LB. Robins-Troeger / FisheriesResearch 19 (1994)205-217
21 3
Pooled taxa
Family
Species
Combined Group B (elongate body form, relatively strong swimmers)
Carangidae
Carangoides uui Carangoides malabraicus Gerres oyeana Parapercis diplospilus Upeneus tragula Priacanthus macracanthus Johnius vogleri Scomberomorus queenslandicus Trachorus declivis Centrogenys vaigiensis Sphyraena obtusata Terapon theraps Pelates quadrilineatus Lagocephalus spadiceus Lageocephalus inmeris
Gerreidae Mugiloididae Mullidae Priaeanthidae Sciaenidae Scombridae Serranidae Sphyraenidae Teraponidae Tetraodontidae
Discussion Penaeid prawns are the main target species of commercial otterboard trawlers in Moreton Bay. Once the Morrison soft TED was fitted to a net, average prawn catch was reduced by up to 29% depending on the phase and site. The combined prawn catch in the TED-equipped net was reduced by 20% at the oceanic site and by 17% at the estuarine site in Phase 2. This contrasts with the results of Kendall (1990) who reported no significant difference in total prawn catch for a Morrison soft TED. Reductions in total prawn catch in the present study could not be attributed to a significant reduction in catch weight of individual species. There was only a slight trend towards fewer large prawns being caught in the TED-equipped net (Fig. 3) and it would be difficult to suggest that this factor alone was responsible for the 17% and 20% reduction in prawn catch rates observed during Phase 2. Kendall (1990) suggested that the size of shrimp caught did not appear to be significantly impacted by a Morrison soft TED. Recent studies in southern Australia, while based on only a few nights' work, reported no effect on the size distribution of Penaeus plebejus catches in nets equipped with a Morrison soft TED (Andrew et al., 1993 ). The loss in efficiency of the TED-equipped net in retaining target penaeid species in the present study may be because of a random loss of prawns rather than the loss of certain species or the extensive loss of certain sized individuals. Catches of prawns in a TED-equipped net may be influenced also by the type of sea bottom on which the tow is conducted. Bottom type and abundance of debris (e.g. algae) may to account for variable efficiencies of TEDs (Magnuson et al., 1990). Estuarine sites generally have higher levels of debris and weed than oceanic sites. Nets equipped with a Morrison soft TED might be expected to have greater reductions in the catch rates of prawns at
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J.B. Robins-Troeger /Fisheries Research 19 (1994) 205-217
estuarine sites as a result of clogging, compared with 'cleaner', oceanic sites. This was the case in Phase 1 of the present study. However, in the second phase, prawn catch rates were reduced in the TED-equipped net at both the estuarine and oceanic sites. Variability in prawn catch efficiency has been reported previously for the Morrison soft TED as well as for other certified TEDs (Christian and Harrington, 1987; Renaud et al., 1993 ). Kendall ( 1990 ) suggested that, because of variability, it was difficult to determine the exact effect of the TED on prawn catches with any precision. Clark et al. ( 1991 ) have previously pointed out that TED capabilities vary with experiments because of gear and net differences associated with individual studies. Thus, it appears difficult to predict accurately the effect of the Morrison soft TED on penaeid catches for all trawl locations and seasons, and for different gear configurations. However, at least if the nets are known to be fishing equally prior to installation of experimental gear, as in the present study, variability in catches is not a result of inherent net differences. Alternatively, interchanging experimental gear randomly between nets would reduce variability in catches as a result of inherent differences between nets but would increase the risk of variability as a consequence of changing net and otterboard configurations. Whiting and flathead are commonly sold as trawl bycatch at local Queensland markets (N. Trainor, personal communication, 1992). No significant differences were observed in fish catch rates in the present study, but confidence intervals associated with fishing power estimates were wide, reflecting the high variability of whiting and flathead catches. TED studies from the USA report finfish catches to be lower in TED-equipped nets than control nets (Renaud et al., 1993 ). Anecdotal evidence from commercial trials of the Morrison soft TED suggest a loss of marketable size fin fish, especially whiting (Menticirrhus sp. ) and flounder (Paralicthys lethostigma) (Kendall, 1990). Andrew et al. ( 1993 ) reported significant reductions in 'discarded commercial species of finfish'. These included undersized flathead (Platycephalus spp. ), flounders (Pseudorhombus spp. ) and whiting (Sillago bassensis). Results obtained in the present study were too variable to base comments upon for the exclusion of fin fish. Sand crabs are a valuable bycatch for commercial trawl operators in Moreton Bay between September and May, supplying approximately 30% of the sand crab market (Sumpton et al., 1989). Catches of legal sand crabs were reduced by 50% in the net equipped with a Morrison soft TED when sand crabs were abundant (Phase 1 ). The anatomy and behaviour of Portunuspelagicus may account for their elimination from the TED-equipped net as the crabs have a tendency to entangle or hold on to webbing within a trawl net. An increase in the mesh size of the TED webbing to 300 m m may increase the number of sand crabs retained by the TED-equipped net, reducing the loss of this valuable bycatch species. This increase in mesh size would proba-
J.B. Robins-Troeger / Fisheries Research 19 (1994) 205-217
2 15
bly not affect turtle exclusion rates, as the smallest turtles caught are 45 cm curved carapace length (CCL) for Moreton Bay and 28 cm CCL for the Queensland coast (J. Robins-Troeger, unpublished data, 1992). A major attribute of TEDs is the reduction of non-commercial bycatch (Naamin and Sujastani, 1984; Renaud et al., 1990). Reductions in non-commercial bycatch for the Morrison soft TED have been reported at an average of 24% per tow (Kendall, 1990) and at 32% (Andrew et al., 1993). Noncommercial bycatch was reduced in the present study by at best 32% by weight; at worst, reductions in non-commercial bycatch were non-significant. Variability in bycatch reduction may be influenced by the type of sea bottom trawled as well as by the species composition of the area. Apart from the exclusion of large bycatch animals such as stingrays, little comment can be made upon the exclusion of particular non-commercial bycatch species. The non-significant difference in bycatch species composition probably reflects the non-significant difference in fishing power between nets observed for bycatch during Phase 2 (Table 1 ). Reductions in non-commercial bycatch generally associated with TEDs would reduce sorting times and may increase product quality, which is one of the benefits which may make TEDs more attractive to the commercial trawling industry.
Conclusion A Morrison soft TED was tested at two sites within a major trawl ground in south-eastern Queensland. The TED had variable effects on catch rates of prawns, fish and non-commercial bycatch, depending on the season and location. This variability may have been influenced by the type of trawl ground the tests were conducted on, i.e. oceanic versus estuarine. To introduce the concepts of TEDs and implement their usage in Australian waters, it will need to be shown that TEDs impose few economic costs on fishermen and that TEDs do indeed have some direct benefits, e.g. the exclusion of large bycatch animals. Whilst the results of the present study are directly applicable to the Moreton Bay trawl fishery, it would be difficult to predict how the Morrison soft TED would affect prawn catches in other Queensland trawl grounds with different trawling conditions, i.e. bottom type, amount of debris, net configurations. The continuation of research into the suitability of different TEDs for different trawl regimes is necessary if TEDs are ever to be considered as a solution to conflicts between commercial trawling operations and sea turtle populations in Australia.
Acknowledgements I gratefully acknowledge Dave Trama, Peter Pardoe and other Fisheries Services staff for their assistance during field and laboratory work. I am also
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J.B. Robins- Troeger / F isheries Research 19 (1994) 205-217
grateful to David Mayer and David Die for their statistical assistance and to staff from the Southern Fisheries Centre for their comments towards improving the manuscript. This study was funded by the Queensland Fish Management Authority. References Andrew, N.L., Kennelly, S.J. and Broadhurst, M.K., 1993. An application of the Morrison soft TED to the offshore prawn fishery in New South Wales, Australia. Fish. Res., 16:101-111. Anonymous, 1990. Hearing before the Subcommittee on Fisheries and Wildlife Conservation and the Environment of the Committee on Merchant Marine Fisheries, House of Representatives: Discussion on the Effectiveness of Federal Efforts to Protect Endangered Species of Sea Turtle and the Impact of Turtle Excluder Devices (TEDs) upon the Shrimp Fishing Industry. Serial No. 101-83, US Government Printing Office, Washington, DC, 326 pp. Bishop, Y.M.M., Fienberg, S.E. and Holland, P.W., 1977. Discrete Multivariate Analysis: Theory and Practice. MIT Press, Cambridge. Blaber, S.J.M. and Blaber, T.G., 1980. Factors affecting the distribution of juvenile estuarine and inshore fish. J. Fish Biol., 17: 143-162. Christian, P. and Harrington, D., 1987. Loggerhead turtle, finfish and shrimp retention studies on 4 turtle excluder devices (TEDs). In: Proceedings of the Nongame and Endangered Wildlife Symposium, 8-l0 September 1987, Georgia DNR, Social Circle, GA, pp. 114-127. Christian, P., Rivers, J. and Morrison, S., 1989. Morrison Soft TED Installation Instructions. Georgia Sea Grant College Program, University of Georgia, Athens, 4 pp. Clark, J., Griffin, W., Clark, J. and Richardson, J., 1991. Simulated economic impacts of TED regulations on selected vessels in the Texas shrimp fishery. Fish. Rev., 53: 1-8. Courtney, A.J., Masel, J.M. and Die, D.J., 1991. An Assessment of Recently Introduced Seasonal Prawn Trawl Closures in Moreton Bay, Queensland. Qld. DPI Fisheries Publication, 84 pp. Goeden, G., 1985. Preliminary Report: An Evaluation of the Trawl Efficiency Device in Queensland Waters. Unpublished Qld. DPI Fisheries Report, 16 pp. Henwood, T.A. and Stuntz, W.E., 1987. Analysis of sea turtle captures and mortalities during commercial shrimp trawling. Fish. Bull., 85:813-817. Kendall, D., 1990. Shrimp retention characteristics of the Morrison soft TED: a selective webbing exclusion panel inserted in a shrimp trawl net. Fish. Res., 9:13-21. Magnuson, J.J., Bjorndal, K.A., DuPaul, W.D., Graham, G.L., Owens, D.W., Peterson, C.H., Pritchard, P.C.H., Richardson, J.I., Saul, G.E. and West, C.W., 1990. Decline of the Sea Turtles. Causes and Prevention. National Academy Press, Washington, DC, 259 pp. Marsh, H. and Saalfeld, W.K., 1990. Distribution and Relative Abundance of Sea Turtles in Southern Queensland Waters. Unpublished Qld. DPI Fisheries Management Branch Report, 27 pp. Maxwell, W.G.H., 1970. The sedimentary framework of Moreton Bay, Queensland, Australia. Aust. J. Mar. Freshwater Res., 21: 71-88. Naamin, N. and Sujastani, T., 1984. The by-catch excluder device. Experiments in Indonesia. In: FAO/AUSTRALIA: Workshop on the Management of Penaeid Shrimp/Prawns in the Asia Pacific Region, 29 October-2 November 1984. Poiner, I.R. and Kennedy, R., 1984. Complex patterns of change in the macrobenthos of a large sandbank following dredging. Mar. Biol., 78: 335-352. Poiner, I.R., Buckworth, R.C. and Harris, A.N.H., 1990. Incidental capture and mortality of sea turtles in Australia's northern prawn fishery. Aust. J. Mar. Freshwater Res., 41:97-110.
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Renaud, M., Gitschlag, G., Klima, E., Shah, A., Nance, J., Caillouet, C., Zein-Eldin, Z., Koi, D. and Patella, F., 1990. An Evaluation of the Impact of Turtle Excluder Devices (TEDs) on Shrimp Catch Rates in the Gulf of Mexico and South Atlantic, March 1988 through July 1989. NOAA Technical Memorandum NMFS-SERC-254, 165 pp. Renaud, M., Gitschlag, G., Klima, E., Shah, A., Koi, D. and Nance, J., 1993. Loss of shrimp by turtle excluder devices (TEDs) in coastal waters of the United States North Carolina 1o Texas, March 1988-August 1990. Fish. Bull., 91: 129-137. Robson, D.S., 1966. Estimation of Fishing Power of Individual Ships. ICNAF Research Bulletin No. 3, ICNAF, Dartmouth, N.S., pp. 1-13. Sumpton, W.D., Potter, M.A. and Smith, G.S., 1989. The commercial pot and trawl fishery for sand crabs (Portunus pelagicus) in Moreton Bay. Proc. R. Soc. Queensl., 100: 89-100. Trainor, N., 1990. Review of the east coast otter trawl fishery. Queensland Fisherman, September (Appendix II): 8-11.
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0 1 6 5 - 7 8 3 6 / 9 4 / $ 0 7 . 0 0 © 1994 - Elsevier Science B.V. All rights reserved
Evaluation of the Morrison soft turtle excluder device: prawn and bycatch variation in Moreton Bay, Queensland J.B.
Robins-Troeger
Southern Fisheries Centre, Queensland Department of Primary Industries, PO Box 76, Deception Bay, Qld. 4508, Australia (Accepted 27 October 1993)
Abstract The present study assesses the effects of a Morrison soft turtle excluder device ( T E D ) upon catch rates of commercial prawn, fish and non-commercial bycatch species at an essentially oceanic site and an estuarine site. Effects of the TED upon catch rates were highly variable. Reductions in prawn catch ( Penaeus plebejus, Penaeus esculentus, Metapenaeus bennettae) in the TED-equipped net varied between no significant reduction to a 29% reduction, by weight, depending on location and season. Reductions in prawn catch could not be attributed to the loss of prawns by species or solely to the selective loss of prawns on the basis of size. Fish catches (Sillago sp., Platycephalus sp., Suggrundus sp.) were highly variable and showed no significant difference between control and TED-equipped nets. Catches of marketable sand crabs (Portunus pelagicus) were reduced by more than 50% in the TED-equipped net. Large bycatch animals, such as shovelnose rays (Rhinobatus batillum), sand rays (Amphotistius kuhlii) and turtles (Caretta caretta), were caught occasionally in the control net but were absent from the TED-equipped net. Non-commercial bycatch, composed of fish and crustaceans, was reduced by up to 32% by weight in the TED-equipped net. Results from this study suggest that it is difficult to predict accurately the effect of the Morrison soft TED upon catches of prawns and bycatch for all trawl locations and seasons.
Introduction The coastal waters of Queensland, Australia, support a trawl fishery in which some 900 trawlers land approximately 7000 t of prawns (live weight) and 1000 t of scallop meat annually (Trainor, 1990). These trawlers also take a substantial amount of non-commercial bycatch, occasionally including sea turtles. Henwood and Stuntz (1987 ) estimated that as many as 50 000 sea turtles are caught annually in prawn trawl nets in the south-eastern USA, with SSDI0165-7836(93)00267-M
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approximately 20% of these animals drowning. Preliminary studies in northern Australia suggest that sea turtles may not be significantly impacted by trawl fisheries in this area (Poiner et al., 1990), and studies are under way to investigate the impact of trawling on sea turtles on the east coast of Queensland (M. Dredge, personal communication, 1991 ). Turtle excluder devices (TEDs) are effective in conserving sea turtles in areas where trawl fisheries exist (Anonymous, 1990). TEDs were first designed by the US National Marine Fisheries Service (NMFS) in 1978 and at least six designs have been officially certified (Magnuson et al., 1990). The NMFS TED, the Georgia Jumper TED, the Matagorde TED and the Cameron TED are all 'hard' TEDs which incorporate rigid steel or fibreglass structures. 'Soft' TEDs use polypropylene mesh rather than rigid bars or frames to exclude sea turtles and other large bycatch animals. The Morrison soft TED and the Parrish soft TED have been developed on these principles. Different fishing practices and different trawl conditions in Australia suggest that those TEDs most suitable for use in US waters may not necessarily suit Queensland conditions. Goeden ( 1985 ) carried out pilot trials over two nights with a NMFS TED on prawn trawl grounds off Cairns. These trials did not demonstrate significant catch differences between a standard net and a NMFS TED-equipped net, but several points relevant to the use of hard TEDs were highlighted. The design of the NMFS TED was too large for Australian nets, which are longer and narrower in the throat than American nets. Goeden suggested that there would be a loss in efficiency of the device at excluding turtles if the TED was reduced to fit Australian nets. A major difference between Australian and American trawl procedures is the manner in which the cod end is emptied. American trawlers leave the throat and body of the net (which incorporates the TED when in use) in the water while the cod end is spilt on to the back deck. On Australian vessels, the net and cod end are lifted via a central boom and the catch is spilt on to a sorting tray, leaving the TEDs swinging in the air above the back deck. This can make operations hazardous for the crew. Goeden's comments suggest that a soft TED may be more suitable to Queensland fishing practices. A Morrison soft TED was tested at two selected sites within Moreton Bay, Australia, to investigate the efficiency of soft TEDs in Queensland waters. Catches of prawns, fish and bycatch were compared between control and TEDequipped nets. Moreton Bay is one of Queensland's most valuable trawling grounds (Trainor, 1990) and is also a major feeding ground for sea turtles (Marsh and Saalfeld, 1990). Studies on efficiencies of TEDs tested in Moreton Bay are directly relevant to commercial trawl operations in this area. Materials and methods
Trials on the efficiency of trawl nets fitted with a Morrison soft TED were conducted in May 1991 (Phase 1 ) when prawn catches were low, and during
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January 1992 (Phase 2) when prawns were abundant (Courtney et al., 1991 ). Catches of prawn, fsh, marketable bycatch and other non-target species were compared between two 9.2 m headrope length sandikan trawl nets (42 m m mesh), which were towed by a 15 m research otterboard trawler. Trials were conducted at two selected sites within Moreton Bay (Fig. 1 ). Site 1 (Moreton Banks) is essentially an oceanic site with a predominantly sandy substrate (Maxwell, 1970; Blaber and Blaber, 1980; Poiner and Kennedy, 1984). Site 2 (Deception Bay) is closer to shore and more estuarine, with a predominantly mud/silt substrate (Maxwell, 1970; Blaber and Blaber, 1980). Prior to the installation of the Morrison soft TED in each phase, a series of preliminary tows of the unmodified nets were conducted. The unmodified nets were towed and tuned until uniform catch rates of prawns, fish and bycatch were observed from the port and starboard nets. The uniformity of the catch rates between port and starboard nets was tested using a balanced analysis of variance. A Morrison soft TED (Fig. 2 ) was constructed of 150 m m monofilament mesh according to installation instructions issued by the Georgia Sea Grant Program (Christian et al., 1989 ). The Morrison soft TED was installed into one net after preliminary tows were completed. In each phase, between 17 and 23 tows were completed at each of the two sites. Bottom times for tows varied between 45 and 100 min. Catches from the control and TED-equipped net were landed and processed separately.
N
Bay
scale nm Sit
o
2
a
Fig. 1. Map of Moreton Bay, Australia showing sites selected for testing of a Morrison soft TED. Site 1: oceanic site, Moreton Banks, mean tow depth 20 m. Site 2: estuarine site, Deception Bay, mean tow depth 5 m.
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~ ~
~
~ h foot rope
=
float _~_~. ~
Morrison soft TED
~
~
close up of turtle escaping through 142 cm exit slit cut in the top panel of the net
Fig. 2. The Morrison soft TED installed into a prawn trawl net. Redrawn from Christian et al. (1989). Large objects, such as stingrays, were removed and weighed first. Commercially important prawns ( Metapenaeus endeavouri, Metapenaeus ensis, Metapenaeus bennettae, Penaeus esculentus, Penaeus plebejus ) were sorted and weighed by species. Weights of these species were pooled to give the total prawn catch weight. Carapace lengths of three prawn species (Penaeus esculentus, Penaeus plebejus, Metapenaeus bennettae) were measured and recorded during Phase 2. Whiting (Sillago maculata and Sillago analis) and flathead (Platycephalus sp. and Suggrundus sp. ) were sorted and weighed. Data on these species were pooled to form the combined fish category. Sandcrabs (Portunus pelagicus) of legal status (male and > 15 cm carapace width ) were removed and recorded. The remainder of the catch was classified as non-commercial bycatch and was weighed before being discarded. Numbers and weights of non-commercial bycatch species were recorded from samples taken from every fourth tow during Phase 2. After each tow, the nets were checked and cleared of any debris or blockages.
Data analyses Catch rates (kg h - ~) for prawns, fish and bycatch were evaluated using a modification of Robson's (1966) m e t h o d for comparison of fishing powers with natural logarithm transformed data. Catch rates were adjusted for mean catch rates for each net, derived from the preliminary, standardization tows. Balanced analyses of variance were conducted, with location and net type as orthogonal factors. The fishing power of the TED-equipped net relative to the control net (95% confidence interval) was then calculated for each site (Robson, 1966). The fishing power analysis was specifically chosen to allow the comparison of catch statistics between nets, whilst taking into account such
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factors as individual tow and location variability. If there was no difference in the catch rate between the nets, relative fishing power would be 1.00. Initial analyses were conducted on combined prawn, combined fish and non-commercial bycatch catch rates. During Phase 2, further analyses were conducted to determine if the presence of a TED had significant effects on the individual catch rates of three commercial prawn species (Penaeus esculentus, Penaeus plebejus, Metapenaeus bennettae). Frequency distributions of prawn carapace lengths were compared between nets by testing against a Poisson distribution using an analysis of deviance (Bishop et al., 1977). An analysis of deviance is an extended form ofa chi square contingency table analysis which identifies the effects of various factors, i.e. species, size class, location. Legal sand crab catches were compared between nets using a paired t-test. Noncommercial bycatch species composition was compared between nets for differences in numeric abundance and weight using a balanced analysis of variance. Results Prawn catch
No significant differences were observed in the catch rates of prawns between nets at the oceanic site during Phase 1 (Table 1 ). Catch rates of prawns Table 1 Cross-site analysis of fishing power ( F P O W ) for a Morrison soft TED-equipped trawl net relative to a control trawl net Species/phase
Site
No. of tows
Mean FPOW
95% CI
Prawns/Phase 1
O E O E
23 20 17 20
0.917 0.707* 0.801" 0.826*
0.702-1.197 0.542-0.931 0.674-0.951 0.695-0.982
O E O E
23 20 17 20
0.898 0.722 1.354 1.010
0.579-1.390 0.466-1.119 0.715-2.564 0.533-1.190
O E O E
23 20 17 20
0.786 0.681"* 0.820 0.977
0.625-0.99(I 0.541-0.857 0.660-1.020 0.786-1.215
Prawns/Phase 2
Fish/Phase 1 Fish/Phase 2
Bycatch/Phase 1 Bycatch/Phase 2
*Significant at 0.05 probability level. **Significant at 0.01 probability level. O, oceanic site, Moreton Banks; E, estuarine site, Deception Bay; CI, confidence interval.
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Table 2 Cross-site analysis of fishing power ( F P O W ) for three commercial prawn species caught in a Morrison soft TED-equipped trawl net relative to a control trawl net (Phase 2) Species
Site
Mean FPOW
95% CI
Metapenaeus bennettae
0 E O E 0 E
0.863 0.880 0.875 0.837 0.991 0.847
0.617-1.207 0.629-1.231 0.628-1.219 0.601-1.165 0.834-1.179 0.713-1.008
Penaeus esculentus Peneaeus plebejus
Abbreviations as in Table 1.
at this site were significantly reduced in the TED-equipped net during Phase 2. Prawn catch rates at the estuarine site were significantly reduced in the TED-equipped net in both phases of the study (Table 1 ). Catch rates of individual prawn species ( Metapenaeus bennettae, Penaeus plebejus, Penaeus esculentus) sampled during Phase 2 were not significantly different between nets at either site (Table 2 ).
Size class frequencies for three commercial prawns--Phase 2 Species, size class and location factors all had significant effects on the size distribution of prawns caught ( P < 0.01 ). Interactions between net type and the other main effects were non-significant, except for size class. Whilst there is a statistically significant difference in size distribution of prawns between nets ( P < 0.01 ), in practical terms (Fig. 3) there is only a slight trend towards reduced numbers of large prawns in the TED-equipped net.
Fish catches Combined fish catch rates were not significantly different between nets at either site (Table 1 ). Wide confidence intervals associated with the fishing power estimates reflect the highly variable nature of whiting and flathead catches, which fluctuated between 15% and 2% by weight of the total catch.
Sand crab catches Legal sand crab catches were significantly reduced in the TED-equipped net at both sites during Phase 1 (oceanic site P < 0.01, estuarine site P < 0.05 ). Legal sand crab catches were low during Phase 2 and were not significantly different between nets at either site ( P > 0.05 ).
J.B. Robins- Troeger / Fisheries Research 19 (1994) 205-217
211
80
7o
I~
~
~ control net
~ ~o g
g 30 2O 10 0 < 165
185 205 225 245 255 285 305 325 345 3fiS > Carapace
Size
Class
(turn)
Fig. 3. Mean size class frequency of prawns caught in a control net and a Morrison soft TEDequipped net.
Non-commercial bycatch Large non-commercial bycatch animals, such as stingrays (Amphotistius kuhlii), shovelnose rays (Rhinobatus batillum) and loggerhead turtles (Caretta caretta), were caught occasionally in the control net but were absent from catches of the TED-equipped net. Most of the non-commercial bycatch was composed of fish and crustaceans, and accounted for between 65% and 80% of the total catch weight. Reductions in non-commercial bycatch in the TEDequipped net were variable at both sites (Table 1). Significant reductions in non-commercial bycatch ranged from 21% by weight at the oceanic site to 32% by weight at the estuarine site in Phase 1. Reductions in non-commercial bycatch were not significantly different between nets during Phase 2. Twelve species of invertebrates and 43 species of fish were observed in the non-commercial bycatch samples. Taxa of low numerical abundance or low catch weight were pooled into groups of similar body form before analysis (Table 3 ). The species composition of non-commercialbycatch was not significantly different for pooled taxonomic groups between nets at either site, for both numerical abundance (oceanic site F12, 72=0.35, estuarine site Fs, 48=0.26) and weight based analyses (oceanic site F12, 72=0.72, estuarine site /78,48=1.18).
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Table 3 Phylogenetic list of non-commercial bycatch species with pooled taxa groupings for analysis Pooled taxa
Family
Species
Crustaceans Alpheids
Alpheidae
Alphenus distinguendus Alphenus stephensi
Portunus spp.
Portunidae
Portunus pelagicus (non-legal) Portunus sanguinolentus
Trawl crabs
Portunidae
Charybdis jaubertensis Charybdis callianassa Charybdis helleri Thalamita sirna
Squillids
Squillidae
Squilla laevis Oratosquilla woodmasoni Oratosquilla mephoroi Harpiosquilla raphidia
Teleost fishes Apogonids
Apogonidae
Apogon fasciata Apogon poecilopterus Siphamia roiegaster
Bothids
Bothidae
Pseudorhombus arsius Pseudorhombus jenynsii
Callionymids
Callionymidae
Callionymus limiceps Callionymus grossi Callionymus belcheri
Cynoglossids
Cynoglossidae
Cynoglossus bilineatus
Leoignathids
Leoignathidae
Leiognathus decorus Leiognathus moretoniensis
Monacanthids
Monacanthidae
Monacanthus chinensis Pseudomonacanthus peroni
Polynemids
Polynemidae
Polydactylus multiradiatus
Sydontids
Syndontidae
Saurida undosquamis
Combined Group A (small, poor swimming )
Atherinidae Chaetodontidae Clupeidae
Antennarius striatus Parachaetodon ocellatus Herklotischthys castelnaui Hyperlophus translucidus Thryssa aestuaria Thryssa hamiltoni Stolephorus carpentariae Acentrogobius caninus Favonigobius sp. Brachyamblyopus coecus Centropogon marmoratus Minous versicolor
Engraulididae Gobioidae Scorpaenidae
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21 3
Pooled taxa
Family
Species
Combined Group B (elongate body form, relatively strong swimmers)
Carangidae
Carangoides uui Carangoides malabraicus Gerres oyeana Parapercis diplospilus Upeneus tragula Priacanthus macracanthus Johnius vogleri Scomberomorus queenslandicus Trachorus declivis Centrogenys vaigiensis Sphyraena obtusata Terapon theraps Pelates quadrilineatus Lagocephalus spadiceus Lageocephalus inmeris
Gerreidae Mugiloididae Mullidae Priaeanthidae Sciaenidae Scombridae Serranidae Sphyraenidae Teraponidae Tetraodontidae
Discussion Penaeid prawns are the main target species of commercial otterboard trawlers in Moreton Bay. Once the Morrison soft TED was fitted to a net, average prawn catch was reduced by up to 29% depending on the phase and site. The combined prawn catch in the TED-equipped net was reduced by 20% at the oceanic site and by 17% at the estuarine site in Phase 2. This contrasts with the results of Kendall (1990) who reported no significant difference in total prawn catch for a Morrison soft TED. Reductions in total prawn catch in the present study could not be attributed to a significant reduction in catch weight of individual species. There was only a slight trend towards fewer large prawns being caught in the TED-equipped net (Fig. 3) and it would be difficult to suggest that this factor alone was responsible for the 17% and 20% reduction in prawn catch rates observed during Phase 2. Kendall (1990) suggested that the size of shrimp caught did not appear to be significantly impacted by a Morrison soft TED. Recent studies in southern Australia, while based on only a few nights' work, reported no effect on the size distribution of Penaeus plebejus catches in nets equipped with a Morrison soft TED (Andrew et al., 1993 ). The loss in efficiency of the TED-equipped net in retaining target penaeid species in the present study may be because of a random loss of prawns rather than the loss of certain species or the extensive loss of certain sized individuals. Catches of prawns in a TED-equipped net may be influenced also by the type of sea bottom on which the tow is conducted. Bottom type and abundance of debris (e.g. algae) may to account for variable efficiencies of TEDs (Magnuson et al., 1990). Estuarine sites generally have higher levels of debris and weed than oceanic sites. Nets equipped with a Morrison soft TED might be expected to have greater reductions in the catch rates of prawns at
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J.B. Robins-Troeger /Fisheries Research 19 (1994) 205-217
estuarine sites as a result of clogging, compared with 'cleaner', oceanic sites. This was the case in Phase 1 of the present study. However, in the second phase, prawn catch rates were reduced in the TED-equipped net at both the estuarine and oceanic sites. Variability in prawn catch efficiency has been reported previously for the Morrison soft TED as well as for other certified TEDs (Christian and Harrington, 1987; Renaud et al., 1993 ). Kendall ( 1990 ) suggested that, because of variability, it was difficult to determine the exact effect of the TED on prawn catches with any precision. Clark et al. ( 1991 ) have previously pointed out that TED capabilities vary with experiments because of gear and net differences associated with individual studies. Thus, it appears difficult to predict accurately the effect of the Morrison soft TED on penaeid catches for all trawl locations and seasons, and for different gear configurations. However, at least if the nets are known to be fishing equally prior to installation of experimental gear, as in the present study, variability in catches is not a result of inherent net differences. Alternatively, interchanging experimental gear randomly between nets would reduce variability in catches as a result of inherent differences between nets but would increase the risk of variability as a consequence of changing net and otterboard configurations. Whiting and flathead are commonly sold as trawl bycatch at local Queensland markets (N. Trainor, personal communication, 1992). No significant differences were observed in fish catch rates in the present study, but confidence intervals associated with fishing power estimates were wide, reflecting the high variability of whiting and flathead catches. TED studies from the USA report finfish catches to be lower in TED-equipped nets than control nets (Renaud et al., 1993 ). Anecdotal evidence from commercial trials of the Morrison soft TED suggest a loss of marketable size fin fish, especially whiting (Menticirrhus sp. ) and flounder (Paralicthys lethostigma) (Kendall, 1990). Andrew et al. ( 1993 ) reported significant reductions in 'discarded commercial species of finfish'. These included undersized flathead (Platycephalus spp. ), flounders (Pseudorhombus spp. ) and whiting (Sillago bassensis). Results obtained in the present study were too variable to base comments upon for the exclusion of fin fish. Sand crabs are a valuable bycatch for commercial trawl operators in Moreton Bay between September and May, supplying approximately 30% of the sand crab market (Sumpton et al., 1989). Catches of legal sand crabs were reduced by 50% in the net equipped with a Morrison soft TED when sand crabs were abundant (Phase 1 ). The anatomy and behaviour of Portunuspelagicus may account for their elimination from the TED-equipped net as the crabs have a tendency to entangle or hold on to webbing within a trawl net. An increase in the mesh size of the TED webbing to 300 m m may increase the number of sand crabs retained by the TED-equipped net, reducing the loss of this valuable bycatch species. This increase in mesh size would proba-
J.B. Robins-Troeger / Fisheries Research 19 (1994) 205-217
2 15
bly not affect turtle exclusion rates, as the smallest turtles caught are 45 cm curved carapace length (CCL) for Moreton Bay and 28 cm CCL for the Queensland coast (J. Robins-Troeger, unpublished data, 1992). A major attribute of TEDs is the reduction of non-commercial bycatch (Naamin and Sujastani, 1984; Renaud et al., 1990). Reductions in non-commercial bycatch for the Morrison soft TED have been reported at an average of 24% per tow (Kendall, 1990) and at 32% (Andrew et al., 1993). Noncommercial bycatch was reduced in the present study by at best 32% by weight; at worst, reductions in non-commercial bycatch were non-significant. Variability in bycatch reduction may be influenced by the type of sea bottom trawled as well as by the species composition of the area. Apart from the exclusion of large bycatch animals such as stingrays, little comment can be made upon the exclusion of particular non-commercial bycatch species. The non-significant difference in bycatch species composition probably reflects the non-significant difference in fishing power between nets observed for bycatch during Phase 2 (Table 1 ). Reductions in non-commercial bycatch generally associated with TEDs would reduce sorting times and may increase product quality, which is one of the benefits which may make TEDs more attractive to the commercial trawling industry.
Conclusion A Morrison soft TED was tested at two sites within a major trawl ground in south-eastern Queensland. The TED had variable effects on catch rates of prawns, fish and non-commercial bycatch, depending on the season and location. This variability may have been influenced by the type of trawl ground the tests were conducted on, i.e. oceanic versus estuarine. To introduce the concepts of TEDs and implement their usage in Australian waters, it will need to be shown that TEDs impose few economic costs on fishermen and that TEDs do indeed have some direct benefits, e.g. the exclusion of large bycatch animals. Whilst the results of the present study are directly applicable to the Moreton Bay trawl fishery, it would be difficult to predict how the Morrison soft TED would affect prawn catches in other Queensland trawl grounds with different trawling conditions, i.e. bottom type, amount of debris, net configurations. The continuation of research into the suitability of different TEDs for different trawl regimes is necessary if TEDs are ever to be considered as a solution to conflicts between commercial trawling operations and sea turtle populations in Australia.
Acknowledgements I gratefully acknowledge Dave Trama, Peter Pardoe and other Fisheries Services staff for their assistance during field and laboratory work. I am also
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grateful to David Mayer and David Die for their statistical assistance and to staff from the Southern Fisheries Centre for their comments towards improving the manuscript. This study was funded by the Queensland Fish Management Authority. References Andrew, N.L., Kennelly, S.J. and Broadhurst, M.K., 1993. An application of the Morrison soft TED to the offshore prawn fishery in New South Wales, Australia. Fish. Res., 16:101-111. Anonymous, 1990. Hearing before the Subcommittee on Fisheries and Wildlife Conservation and the Environment of the Committee on Merchant Marine Fisheries, House of Representatives: Discussion on the Effectiveness of Federal Efforts to Protect Endangered Species of Sea Turtle and the Impact of Turtle Excluder Devices (TEDs) upon the Shrimp Fishing Industry. Serial No. 101-83, US Government Printing Office, Washington, DC, 326 pp. Bishop, Y.M.M., Fienberg, S.E. and Holland, P.W., 1977. Discrete Multivariate Analysis: Theory and Practice. MIT Press, Cambridge. Blaber, S.J.M. and Blaber, T.G., 1980. Factors affecting the distribution of juvenile estuarine and inshore fish. J. Fish Biol., 17: 143-162. Christian, P. and Harrington, D., 1987. Loggerhead turtle, finfish and shrimp retention studies on 4 turtle excluder devices (TEDs). In: Proceedings of the Nongame and Endangered Wildlife Symposium, 8-l0 September 1987, Georgia DNR, Social Circle, GA, pp. 114-127. Christian, P., Rivers, J. and Morrison, S., 1989. Morrison Soft TED Installation Instructions. Georgia Sea Grant College Program, University of Georgia, Athens, 4 pp. Clark, J., Griffin, W., Clark, J. and Richardson, J., 1991. Simulated economic impacts of TED regulations on selected vessels in the Texas shrimp fishery. Fish. Rev., 53: 1-8. Courtney, A.J., Masel, J.M. and Die, D.J., 1991. An Assessment of Recently Introduced Seasonal Prawn Trawl Closures in Moreton Bay, Queensland. Qld. DPI Fisheries Publication, 84 pp. Goeden, G., 1985. Preliminary Report: An Evaluation of the Trawl Efficiency Device in Queensland Waters. Unpublished Qld. DPI Fisheries Report, 16 pp. Henwood, T.A. and Stuntz, W.E., 1987. Analysis of sea turtle captures and mortalities during commercial shrimp trawling. Fish. Bull., 85:813-817. Kendall, D., 1990. Shrimp retention characteristics of the Morrison soft TED: a selective webbing exclusion panel inserted in a shrimp trawl net. Fish. Res., 9:13-21. Magnuson, J.J., Bjorndal, K.A., DuPaul, W.D., Graham, G.L., Owens, D.W., Peterson, C.H., Pritchard, P.C.H., Richardson, J.I., Saul, G.E. and West, C.W., 1990. Decline of the Sea Turtles. Causes and Prevention. National Academy Press, Washington, DC, 259 pp. Marsh, H. and Saalfeld, W.K., 1990. Distribution and Relative Abundance of Sea Turtles in Southern Queensland Waters. Unpublished Qld. DPI Fisheries Management Branch Report, 27 pp. Maxwell, W.G.H., 1970. The sedimentary framework of Moreton Bay, Queensland, Australia. Aust. J. Mar. Freshwater Res., 21: 71-88. Naamin, N. and Sujastani, T., 1984. The by-catch excluder device. Experiments in Indonesia. In: FAO/AUSTRALIA: Workshop on the Management of Penaeid Shrimp/Prawns in the Asia Pacific Region, 29 October-2 November 1984. Poiner, I.R. and Kennedy, R., 1984. Complex patterns of change in the macrobenthos of a large sandbank following dredging. Mar. Biol., 78: 335-352. Poiner, I.R., Buckworth, R.C. and Harris, A.N.H., 1990. Incidental capture and mortality of sea turtles in Australia's northern prawn fishery. Aust. J. Mar. Freshwater Res., 41:97-110.
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Renaud, M., Gitschlag, G., Klima, E., Shah, A., Nance, J., Caillouet, C., Zein-Eldin, Z., Koi, D. and Patella, F., 1990. An Evaluation of the Impact of Turtle Excluder Devices (TEDs) on Shrimp Catch Rates in the Gulf of Mexico and South Atlantic, March 1988 through July 1989. NOAA Technical Memorandum NMFS-SERC-254, 165 pp. Renaud, M., Gitschlag, G., Klima, E., Shah, A., Koi, D. and Nance, J., 1993. Loss of shrimp by turtle excluder devices (TEDs) in coastal waters of the United States North Carolina 1o Texas, March 1988-August 1990. Fish. Bull., 91: 129-137. Robson, D.S., 1966. Estimation of Fishing Power of Individual Ships. ICNAF Research Bulletin No. 3, ICNAF, Dartmouth, N.S., pp. 1-13. Sumpton, W.D., Potter, M.A. and Smith, G.S., 1989. The commercial pot and trawl fishery for sand crabs (Portunus pelagicus) in Moreton Bay. Proc. R. Soc. Queensl., 100: 89-100. Trainor, N., 1990. Review of the east coast otter trawl fishery. Queensland Fisherman, September (Appendix II): 8-11.