ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on October 8, 2007
ICES Journal of Marine Science: Journal du Conseil 2007 64(8):1598-1602; doi:10.1093/icesjms/fsm123
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Assessment of a juvenile and trash excluder device in a Vietnamese shrimp trawl fishery
1 Australian Maritime College, PO Box 21, Beaconsfield, Tasmania 7270, Australia
2 University of Fisheries, Nha Trang City, Khanh Hoa Province, Vietnam
Correspondence to S. Eayrs: current address: Gulf of Maine Research Institute, 350 Commercial Street, Portland, ME 04101, USA; tel: +1 207 228 1659; fax: +1 207 772 6855; e-mail: steve{at}gmri.org
Eayrs, S., Hai, N. P., and Ley, J. 2007. Assessment of a juvenile and trash excluder device in a Vietnamese shrimp trawl fishery. – ICES Journal of Marine Science, 64: 1598–1602.In this study, we (i) identified why Vietnamese shrimp fishers land juvenile fish illegally; (ii) identified groups of fishers that would use a bycatch reduction device (BRD) to exclude these fish; (iii) studied the hydrodynamic performance of a juvenile and trash excluder device (JTED) in a flume tank; and (iv) assessed the performance of this device under commercial fishing conditions. Based on the responses of 65 fishers to a questionnaire, we found that juvenile fish are now an important economic component of the total catch, and that fishers operating larger boats were more willing to use a JTED to exclude these fish than fishers operating smaller boats. The hydrodynamic study of a JTED identified the location of low-velocity regions around the device and codend, and this information can be used to identify the location of a secondary BRD to allow more fish to escape. The at-sea assessment of this device found that 73% of juvenile fish, 16% of valuable fish, and 8% of shrimp were excluded by the JTED, although most valuable fish and shrimp were smaller than the minimum legal landing size. Overall, this loss represents a 9% reduction in revenue. Yield-per-recruit analysis indicated that this could be offset by not catching fish less than the minimum legal landing size.
Keywords: bycatch reduction device, fishery, flume tank, hydrodynamic performance, juvenile and trash excluder device, shrimp trawl, Vietnam
Received 1 September 2006; accepted 21 June 2007; advance access publication 8 October 2007.
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Introduction |
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In many developing tropical countries, fishers in shrimp-trawl fisheries focus primarily on catching shrimp, but retain a diversity of incidentally caught finfish to supplement their income. In some fisheries, this practice extends to landing not only large fish for human consumption but also small fish for the production of fishmeal in fish and shrimp farming. This part of the incidental catch may also include juvenile fish, including species of commercial importance. Although minimum landing size (MLS) regulations and fishing gear restrictions may be in place to protect the catches of these fish, inadequate fishery enforcement capability means that fishers can operate with few controls on the landed catch, other than the demands of the marketplace. The practice of catching small juvenile fish can therefore continue unabated without fear of prosecution.
The shrimp-trawl fishery in the Southwestern Sea of Vietnam fishery is such a fishery. It operates in shallow waters, 5–25 m deep, and shrimp catches are dominated by giant tiger shrimp (Penaeus monodon), greasy-back shrimp (Metapenaeus ensis), and pink shrimp (M. affinis). In all, 271 vessels are registered to trawl for shrimp in these waters, but several hundred unregistered vessels operate illegally. Total landings of wild-caught shrimp, therefore, are unknown, although Huy (1998) reported a 50% reduction between 1992 and 1996 in the average shrimp catch per registered vessel to 7 t annually. The vessels registered to operate in the fishery are categorized in four groups, based on their engine power: 
17% of the vessels have less than 11 kW, 15% have 12–22 kW, 62% have 23–33 kW, and the remainder have 34–44 kW (Keingiang Fishery Department, 2002).
A suite of management regulations exists in the fishery to protect juvenile shrimp and fish stocks, and to minimize illegal fishing and the physical impacts of fishing on the environment. These include a minimum codend mesh size of 20 mm and 30 mm for vessel groups with engine power less than 23 kW and 23–44 kW, respectively; banning of trawl activity in less than 5 m of water; and prohibition of explosive, electric, or chemical fishing (Fisheries Ministry of Vietnam, 1996). According to Huy (1998), however, fishers usually ignore these regulations with impunity. There is also a MLS of 150 mm (total length) for valuable fish and 12 mm [carapace length (CL)] for shrimp, but these are ignored; fish as small as 130 mm are sold at local markets, and all shrimp, irrespective of length, are either sold for human consumption or broodstock. Fish smaller than 130 mm are also landed and usually converted into fishmeal. These small fish are the so-called trash fish.
Although the recording of catch data and fishing effort is almost non-existent in this fishery, there is evidence of the effect of these landings on stock biomass and health. According to Linh (2002), most inshore fish stocks are overexploited, and Huy (1998) reported that dwindling catches had caused many larger vessels to leave the fishery to operate in other waters. Anecdotally, the catch of large, valuable fish is increasingly rare, and the length of fish and shrimp available for market has diminished with time. Other fisheries, some of which target the same species, have also reported similar problems, and increasingly target smaller fish to remain profitable. Despite these problems, there is little incentive for fishers to alter their practices, given the high poverty levels in the region, a need for food security, and a lack of resources to enforce fishery regulations.
In this study, we assessed the performance of a juvenile and trash excluder device (JTED) and its potential for adoption by shrimp fishers in the Southwestern Sea of Vietnam. This device was originally developed by the Southeast Asian Fisheries Development Centre (SEAFDEC) and tested in several Southeast Asian countries (see Chokesanguan et al., 2002, for details).
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Material and methods |
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A JTED was constructed and fitted to the codend of a shrimp trawl. The JTED consisted of three rectangular panels joined with hinges (Figure 1). The first two panels consisted of a framework of parallel vertical bars designed to allow fish to escape from the codend. The third panel consisted of a rectangular sheet of small-mesh netting to prevent escaped fish from re-entering the codend. The outer frame of each panel was constructed of 8 mm diameter mild steel rod. The parallel bars of the first two panels were welded to the outer frame and made of 6 mm diameter mild steel rod. The distance between the bars was 20 mm. The mesh size of the netting panel was 5 mm. The outer frames of the first and third panels measured 0.5 m x 0.2 m, and the second panel measured 0.5 m x 0.4 m. The total weight of the JTED in air was 5 kg. Three 120 mm diameter plastic floats were attached to the upper leading edge of the JTED, and two floats were attached to the upper trailing edge. The JTED was inserted into a full-size codend constructed of 15 mm polyethylene diamond mesh netting, measuring 280 meshes in circumference and 220 meshes in length (despite being illegal, 15 mm mesh is commonly used in the fishery). The codend was then tested in a flume tank to check the orientation of the device and the influence of that orientation on codend geometry. A total of 30 filled ballons, each weighing 2 kg, was inserted into the codend to simulate catch, a typical weight in the fishery. An electromagnetic water-speed probe was inserted into selected locations (stations) in and around the codend to measure the influence of the JTED on water flow. A cover net constructed from 15 mm polyethylene diamond mesh netting was attached to the upper panel of the codend, adjacent to the escape openings of the JTED. This net was designed to retain fish that escaped from the JTED. Two 120 mm floats and five small gillnet floats were attached laterally across the cover net to ensure adequate clearance for fish to escape between the cover net and the JTED. Another 15 water-filled balloons, each weighing 2 kg, were inserted into the cover net to simulate the estimated weight of escaped fish. The orientation and geometry of the cover net and its influence on water flow around the JTED was also assessed, and the electromagnetic water-speed probe was inserted into the cover net at station C (Figure 2) to measure the influence of the cover on water flow. The JTED, codend, and cover net were then delivered to Vietnam for at-sea testing on the commercial fishing grounds. A simple questionnaire was conducted as part of face-to-face interviews with Vietnamese shrimp fishers, seeking information about the proportion and relative value of trash fish compared with the total catch, and their preparedness to use a bycatch reduction device.
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Sea trials
The sea trials were conducted aboard a chartered 33 kW shrimp trawler over a 5-d period near Phu Quoc Island in the Southwestern Sea of Vietnam. In all, 15 3-h trawl hauls were carried out at a towing speed of
1.0 ms–1. Three trawl hauls were carried out each day as per normal commercial practice, and the precise location of each haul was determined by the skipper of the trawler. Fishing depth was 12–15 m. The shrimp trawl had a headrope length of 20.8 m and was constructed from polyethylene netting, with mesh ranging in size from 35 mm in the wings of the trawl to 15 mm in the codend.
Catch and data analysis
Three species of valuable fish—snakefish (Trachinocephalus myops), bartail flathead (Platycephalus indicus), and Japanese threadfin bream (Nemipterus japonicus)—and one species of shrimp, giant tiger shrimp (Penaeus monodon), were selected for detailed analysis. The fish species were chosen because, typically, they dominate the fish catch. P. monodon was chosen because it is the most valuable shrimp species—the price of large P. monodon (CL 
2 cm) is about seven times higher than the price of other shrimp species (Lam, 2002)—and females are particularly sought after to supply broodstock for shrimp culture.
After each haul, the catch from the cover and the codend was emptied into separate sorting areas aboard the vessel. The catch of shrimp, trash fish, and valuable fish from each net was placed in separate bins and weighed. Penaeus monodon were removed from the shrimp catch, counted, measured (CL), and weighed. Adult T. myops, P. indicus, and N. japonicus were separated from the valuable fish catch, measured (total length), and weighed in species groups. A subsample of trash fish was collected for onshore assessment. Each subsample was weighed, and juvenile T. myops, P. indicus, and N. japonicus were sorted by species group, measured, and weighed. The number of each species in the subsample was then adjusted, based on the total subsample weight. The value of shrimp, trash fish, and valuable fish from the cover net and codend was then calculated, based on the beach price paid to fishers at the time of this study.
Yield-per-recruit (YPR) analysis of the three valuable fish species and P. monodon was also conducted to assess the potential long-term benefits to fishers using a JTED to delaying length-at-first-capture. We used a length-based YPR model from Sparre and Venema (1998), and von Bertalanffy growth and natural mortality parameter values from Froese and Pauly (2006) and Motoh (1981), respectively.
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Results |
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JTED–hydrodynamic performance
The average water velocity entering the codend in the vertical plane was 1.062 m s–1 ± 0.02. At this speed, the vertical height of the codend at the leading edge of the JTED was 390 mm, 300 mm where the first and second panels are hinged together and 230 mm where panels two and three are hinged together. In all stations tested, the relative water flow was lower than that entering the codend (Figure 2). At each station, water flow was greatest near the bottom of the codend or above the top panel of the codend. Water flow was least behind the JTED, near the top panel of the codend. Immediately behind the leading floats of the JTED, the water flow was reversed. At station F, water flow in the codend appeared to be slow and was less than that at the same height in the preceding station.
The addition of the cover net with simulated catch did not alter the orientation or geometry of the JTED and codend. Careful design of the cover net and placement of floats ensured a clearance between the JTED and cover net of up to 410 mm. There was also no contact between the cover net and the codend posterior of the JTED. Relative water flow above the JTED at station C was significantly less (p < 0.01) at all heights, when the cover net was in place.
Catch results and economic impact
In all, 519 kg of shrimp, 151 kg of valuable fish, and 339 kg of trash fish were caught in the codend and cover net. The JTED excluded almost 8% of the shrimp catch, 73% of the trash fish catch, and 16% of the valuable fish by weight. A total of 4995 P. monodon was caught in the codend and cover net, with 81% of individuals retained in the codend. A total of 1095 small P. monodon less than the minimum landing size was caught, and 60% of these were retained in the cover net. Of the larger shrimp, 93% were retained in the codend. A two-sample Kolmogorov–Smirnov test detected a significant difference (p <0.01) in the length compositions of P. monodon caught in the codend and cover net. A selectivity curve indicated that shrimp length at 50% retention (L50) was 9.5 mm (Figure 3). By delaying length-at-first-capture until male and female P. monodon reach length-at-maturity (Lm) values of 37 mm and 47 mm, respectively (provided by Motoh, 1981), an increase in YPR of 31% and 50% could be realized.
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For each of the three valuable fish species, the catch of small fish was greatest in the cover net (Table 1). In contrast, at least 82% of large fish were retained in the codend. For each species, a two-sample Kolmogorov–Smirnov test detected a significant difference (p < 0.01) in the length compositions of fish caught in the codend and cover net. Selectivity curves of pooled data for each species indicated that fish length at 50% retention (L50) differs little from the MCS (130 mm), and smaller than MLS (150 mm; Figure 4). For P. indicus and T. myops, the recorded L50 values were substantially smaller than Lm values of 27.5 cm and 21.1 cm, respectively (from Froese and Pauly, 2006). There was little difference between L50 and Lm values for N. japonicus.
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YPR analysis for both P. indicus and T. myops indicated that the yield of each species was not being optimized with respect to MLS or Lm. For P. indicus, delaying length-at-first-capture until individuals reach the MLS would increase YPR by 4%, whereas a delay until individuals reach Lm would increase yield by 26%. A delay in length-at-first-capture for T. myops until individuals reach MLS or Lm would increase YPR by 8% and 21%, respectively. For N. japonicus, there was little difference in YPR at any length-at-first-capture.
During this 5-d study, the JTED was responsible for an average overall reduction in catch revenue of almost 7.3 million VND (Table 2). This is equivalent to almost 8% of the combined catch value from the codend and cover net. There was only a 2% loss of revenue from large P. monodon, but there was a 42% loss of revenue from the trash fish.
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Preparedness to use a bycatch reduction device
Face-to-face interviews were conducted with the 65 owners or skippers of a shrimp trawler, accounting for
24% of the total number of registered boats in the fishery (Table 3). Fishers from the smallest boat groups were more reliant on trash fish as a source of income and, therefore, were least prepared to use a bycatch reduction device (BRD) to exclude trash fish from the catch. Many fishers commented that, to adopt a BRD, they would need assurance that they would not suffer a loss of shrimp or valuable fish. Other criteria included low construction and maintenance costs, and ease of handling, both on board and when fishing.
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Discussion |
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Although the JTED has been demonstrated previously in at least two other locations in Vietnamese waters (Chokesanguan, 2002), this study is the most detailed and comprehensive test of this device to date, and the reported catches are considered to be consistent with those from normal commercial operations. The location and depth fished, the trawl gear used, and tow duration were all consistent with normal practice and determined by the skipper. Only the addition of the JTED and cover net were inconsistent with normal practice, and the influence of these additions to the trawl is debatable. In the flume tank, the orientation of the codend with the JTED fitted was expected and remained unchanged when the cover net was attached. The cover net, however, did reduce water flow adjacent to the escape openings of the JTED, but because this device is designed primarily to filter small fish from the catch mechanically, its was arguably less influenced than a device that relies heavily on fish behaviour or swimming performance to facilitate escape, such as a fisheye or radial escape section. At sea, testing of the effect of the cover net on JTED performance was beyond the scope of this study and remains a possibility for future work.
An effective BRD not only reduces catches of non-target animals but also meets the needs of commercial fishers. Clearly, the loss of valuable catch recorded in this study is detrimental to the ready acceptance and use of the JTED in this fishery. In addition to the loss of P. monodon and other shrimp, fishers are unlikely to be satisfied with a loss in revenue of around 42% and 14% for trash fish and valuable fish, respectively. Although most of these fish were smaller than the MLS, there are no incentives for the fishers to change their practices and, until this situation changes, it is unlikely that the JTED will be adopted in this fishery. Further, hampering the adoption of the JTED is the cost of materials to construct the device. Although the JTED is constructed from cheap, readily available materials, its cost was approximately equivalent to half of the revenue generated by a 23–33 kW trawler in one 5-d fishing trip. From the fishers’ perspective, a positive outcome of this study was that the JTED posed no additional difficulties handling the trawl, and no problems were encountered during trawl deployment and retrieval.
From an ecological perspective, the application of the JTED seems to be a step towards reduced fishing impact. For each of the three fish species studied, at least 70% of individuals less than the MLS were excluded by the JTED. For each species, the mean length-at-first-capture (L50) closely approximated the MCS, and an increase in bar spacing would be required to delay capture until fish had reached MLS. This would have a positive effect on the yield of P. indicus and T. myops (but not N. japonicus), although the length-at-first-capture is still considerably smaller than the respective Lms reported in Froese and Pauly (2006).
In addition to an increase in bar spacing, the catch of trash fish and small valuable fish could be reduced further by installing a square-mesh window or other BRD behind the JTED. Flume tank results indicated that, immediately behind the JTED, the relative water flow was substantially less than the flow in the remainder of the codend. This is most likely the result of a "shadowing" influence on water flow, caused by the presence of the device. Water may also have been displaced forward inside the codend, similar to the situation reported by Broadhurst et al. (1999). At sea, both phenomena may increase the exclusion of small fish because reduced effort is required to swim towards this region of the codend and through the escape openings of a BRD.
Greater effort is clearly required to convince Vietnamese shrimp fishers to alter their fishing practices and reduce the catch of small shrimp, trash fish, and small valuable fish. The JTED has demonstrated that the catch of these animals can be reduced, but unless additional steps are taken to control the behaviour of fishers, there is little chance that existing fishing practices will change.
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References |
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Broadhurst M. K., Kennelly S. J., Eayrs S. Flow-related effects in prawn-trawl codends: potential for increasing the escape of unwanted fish through square-mesh panels. Fishery Bulletin US (1999) 97:1–8.
Chokesanguan B., Suppachai A., Somboon S., Worawit W., Nguyen L. Studies on juvenile and trash fish excluder devices (JTEDs) in Vietnam. Report, Training Department, Southeast Asian Fisheries Development Centre. (2002) 20.
Froese R., Pauly D. FishBase. (2006) www.fishbase.org; version (04/2006).
Fisheries Ministry of Vietnam. National Regulations for Management of Living Marine Resources. (1996) Hanoi. (in Vietnamese).
Huy T. P. Study on the current status of prawn trawl fishery in Southwestern Sea of Vietnam. Fishery Department of Ca Mau (1998) (in Vietnamese).
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Lam N. Vietnamese Cultures. Educational Publishers, Hanoi. (2002) (in Vietnamese).
Linh H. Current status and some strategies for the development of Vietnamese Fisheries. Journal of Fisheries, Hanoi, 3/2002: 23–25. (2002) (in Vietnamese).
Motoh H. Studies on the fisheries biology of the giant tiger prawn, Penaeus monodon, in The Philippines. SEAFDEC Aquaculture Department, Technical Report 7. (1981) 128.
Sparre P., Venema S. C. Introduction to tropical fish stock assessment. Part 1: Manual. FAO Fisheries Technical Paper 306/1, Rev. 2. (1998) 407.
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