Evaluation of separation performance on selective fishing device for fisheries resources management.

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The size selection curve of fine mesh for Anchovy larvae were estimated by using an extended SELECT model for analyzing size frequency distribution constructed from pockets catches and sampling the cod-end catches, and from covered-net experiments. Higher catch volume were decreasing escape opportunities and selectivity.Capture process of clam by dredge was modeled. First, Master curve equation of size selectivity of dredge bagnet for a clam, equilateral Venus Gomphina melanaegis was estimated by using cover-net fishing experiments with test bagnets of several mesh sizes. The size selectivity of dredge bagnet was a kind of contact selection of bagnet mesh. Second, The effect of various tooth-spacing on the contact selection and the net-mouth available selection of a dredge for the equilateral Venus clam Gomphina melanaegis was evaluated by applying a contact-probability model based on the SELECT method to the data of paired-gear tests with a control dredge of 12mm tooth spacing and fou … More r test dredges of tooth spacing 16,20,24,and 35mm. A master selection curve was estimated to be s(R)=exp(-11.23R+4.799)/[1+exp(-11.23R+4.799)], where Ris Id, the ratio of the shell length 1 to the tooth spacing d. The shell width of 50% retention was slightly smaller than the tooth spacing, which means that size selectivity of teeth was a type of contact selection which was dependent on the tooth spacing. Contact probability δ of the clams coming into contact with the teeth were 0.625,0.467,0.563,0.670,and 0.976 for tooth spacing 12,16,20,24,and 35mm, respectively. This indicates that some clams entered the dredge bagnet without contacting the teeth as the sand clogging the tooth spacing carried them into the bagnet. Available selection curve of the dredge net-mouth with teeth was obtained as δ s(R)+1-δ and examined in terms of the minimum landing size of the Venus clam.A method for assessing species-separation performance of size-selective fishing gear is presented. Species composition in catch by selective fishing gear is often discussed to assess the performance of the selective device. However, the catch number (and weight) depends on body size composition in the population when the fishing gear has size selectivity, and thus species composition varies and is not suitable as gear performance index. In fact some selective devices, e.g.mesh panel, grid, and escape-vent have been reported to be size-selective. When a selective device has retention probabilities of body size 1, r_w(l) and r_u(l) for wanted and unwanted species, respectively, separation efficiency p(l_w, 0l_u) between l_w size wanted and l_u-size unwanted species is defined as r_w(l_w)+[1-0r_u(l_u)]-1. The efficiency is 1 when wanted species is completely retained and unwanted is excluded as expected, and is -1 in the exact opposite of the expectation. The equation implied that the separation efficiency is likely to be high as there is larger difference in body size between the two species. Less