By-catch of rays in the trawl fishery for Atlantic seabob shrimp Xiphopenaeus kroyeri in Suriname: how effective are TEDs and BRDs?
Willems, T.; Depestele, J.; De Backer, A.; Hostens, K. (2013). By-catch of rays in the trawl fishery for Atlantic seabob shrimp Xiphopenaeus kroyeri in Suriname: how effective are TEDs and BRDs? ILVO Mededeling, 139. Instituut voor Landbouw- en Visserijonderzoek (ILVO): Oostende. 19 pp.
Part of: ILVO Mededeling. Instituut voor Landbouw- en Visserijonderzoek (ILVO): Merelbeke. ISSN 1784-3197, more
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| Abstract |
Tropical shrimp trawling fisheries are generally known to capture a large amount of unwanted organisms along with the targeted shrimp. To reduce this by-catch, the fishery for Atlantic seabob shrimp Xiphopenaeus kroyeri in Suriname uses nets fitted Turtle Excluder Devices (TEDs) and By-catch Reduction Devices (BRDs). It is unclear, however, to what extend these selectivity measures, designed to reduce capture of marine turtles and small roundfish respectively, are reducing by-catch of rays. Due to their life-history characteristics, rays (Batoidea; Chondrichthyes: Elasmobranchii) are generally vulnerable to overexploitation and several endangered species are known to occur in Surinamese waters. The objective of this study therefore is to assess the effect of the selectivity devices currently in place (TEDs and BRDs) on ray by-catch in the X. kroyeri trawling fishery. Hereto, sixty-five simultaneous catch-comparison hauls were conducted, comparing ray by-catch in trawls fitted with (test-net) and without (control-net) TEDs and BRDs.Five different ray species occurred in the by-catch, Gymnura micrura and Dasyatis guttata being the dominant species. Overall, catch rate of rays was reduced by 36% in the test-net. Moreover, rays that did end up in the test-net codend were on average 21% smaller than those in the control-net. This confirms the presumption that rays escape through TEDs rather than BRDs, smaller individuals being able to pass through the TED, but larger ones being guided to the escape opening at the bottom of the net. TEDs were most efficient in excluding Dasyatis geijskesi, the largest ray species. By-catch of D. guttata was reduced as well, but exclusion was highly dependent on size. A similar, but less pronounced relationship between size and exclusion rate was observed for G. micrura. Nevertheless, large individuals of both species were relatively rare, the bulk of the ray by-catch being made up by small sized (< 40 cm body width) individuals of G. micrura and D. guttata, complemented with Urotrygon microphthalmum, a small-sized species. Although TEDs and BRDs seem efficient in reducing by-catch of large rays, they seem inappropriate to protect small-sized individuals, which are more abundant in the population. We therefore suggest that further by-catch related efforts in this fishery are concentrated on reducing the incidental capture of small-sized rays. |
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