| Having advantages of simple construction, low cost, high economic benefit and fresh catches, stow net has become an important fishing method in Shandong Province. However, stow net, which is a kind of passive fitter fishing gear, has a striking feature in its various captures, including large numbers of small size species and larvae of fishes. According to a mass of literatures about the catch composition of stow net, this kind of fishing gear makes serious damage to offshore fishery resources in China. So it is necessary to conduct a research on the selectivity of stow net, to provide a reference for rational utilization of offshore fishery resources and the management of fisheries resources.The survey was conducted in the coast of the Zhaitang Island, Qingdao, during the spring and autumn 2014. In the spring, selectivity of a 30 mm diamond mesh codend (30D) and three square mesh codends with different mesh sizes including 20 mm (20S),30 mm (30S) and 40 mm (40S) was studied with the parallel haul method and Logistic curve. In the autumn, the experimental nets were two diamond mesh codends and three square mesh codends with different mesh sizes, which were 30 mm and 35 mm diamond mesh codends (shown as 30D and 35D respectively) and 20 mm, 30 mm and 35 mm square mesh codends(shown as 20S,30S and 35S respectively). A 16 mm (2a) diamond codend were used as the control net (CN) for both of the two experiments. The main results showed that:1. The composition of dominant species in stow net catches obviously varied between spring and autumn and the main catches for the control net were low economic value species. There were dominated by Ammodytes personatus, Syngnathus schlegeli, Thryssa kammalensis and Amblychaeturichthys hexanema in spring, by Pseudosciaena polyactis. Syngnathus schlegeli. Enedrias fangi, Latreutes anoplonyx and Johnius belangerii in autumn. Moreover, mostly catches were economic species larvae, which damaged the offshore fishery seriously.2. Selective experiment results in the spring shown that:(1) All the experimental codends exhibited a good effort in releasing Ammodytes personatus, Syngnathus schlegeli and larvae of some economic fish comparing with the CN. (2) Square mesh codend showed a better selectivity than diamond mesh codend, which the releasing ratios (in terms of individual numbers) of A. personatus and S. schlegeli for 30S were 4.61% and 16.80% higher than those for 30D and the 50% retention body length (L50) of Amblychaeturichthys hexanema and Loligo japonica for 30S were 38.47%and 61.29% greater than those for 30D, respectively. (3) Comparison with catches data for different square mesh codends found that the L50 of A. hexanema was 6.58 cm and 7.20 cm, and the selection rage (SR) was 3.02 cm and 0.87 cm for the 20S and 30S, respectively; the L50 of L. japonica was 2.88 cm,4.03 cm and 10.49 cm, and the SR was 0.74 cm,0.52 cm and 2.67 cm for the 20S,30S and 40S, respectively, showing that the L50 of A. hexanema and L. japonica increased with the increase of the square codends mesh size. (4) Results in the present study suggest that square mesh codend with 30 mm mesh size could not only potentially protect the fishery resources, but also does not reduce the fishery production with the comprehensive analysis of the economic value of catches, releasing ratios of low economic value captures, and labor input of sorting catches and therefore it should be implemented in the region.3. Selective experiment results in the autumn shown that:(1) Catches composition of 35D,30S and 35S had significant difference with the control net, and the others did not have significant difference. (2) All the experimental codends exhibited a good effort in releasing Syngnathus schlegeli and larvae of some economic fish, comparing with the CN net. Except the 20S, the other experimental codends had a good effort in releasing Enedrias fangi, and the square mesh codends (20S,30S and 35S) were berrer at releasing Loliolus japonica than the diamond codends. (3) Square mesh codends showed a better selectivity than diamond mesh codend, which the R test statistic for CN & 30S and CN & 35S were greater than those of CN&30D and CN&35D respectively and the 50 % retention body length (L50) of Pseudosciaena polyactis for 30S and 35S was 4.88% and 7.37% greater than those of 30D, respectively. (4) Comparison with catches data for different diamond mesh codends and square mesh codends found that the L50 of P. polyactis was 8.20 cm and 9.09 cm for 30D and 35D respecively, and the L50 of P. polyactis was 7.41 cm,8.60 cm and 9.76 cm for the 20S,30S and 35S respectively; showing that the L5o of P. polyactis increased with the increase of the diamond and square codends mesh size. Comparing with the control net. the variances of catches composition for different diamond and square mesh codends increased with the increase of mesh size. (5) Results in the present study suggest that square mesh codend with 30 mm mesh size could be used during the catching season for P. polyactis and Trichiurus lepturus, but in the catching season of Enedrias fangi, the 20 mm square mesh codend would be better. |
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