| Shrimp beam trawl is a kind of bottom fishing gears, which is one of the main fishing practices. This kind of trawl has low selectivity. The problem of bycatch and discards is serious. It could cause the wasting of ocean resources, especially the juvenilefishes and shrimps. Then it could lead to the decline of the fishery resources. To solve the problem, this paper studied the selectivity of the shrimp beam trawl in Erjiegou sea area, Panjin, Liaoning on May, 2014 and May, 2015. The study contained 3 parts. The first part was study on the cod-end mesh selectivity of beam shrimp trawl. The second part was optimal design of the beam shrimp trawl rope matching. The last part was study on shrimp and conch separating device for the beam shrimp trawl.The selectivity tests of the beam trawl with different mesh-sizes of cod-end to Palaemon gravieri and Palaemon carinicauda were studied by the covered net method. The results showed that the 50% escapable body length(L0.5) increased with the mesh size of cod end. When the mesh sizes of cod end were 20, 25, and 33 mm, 50% escapable body lengths(L0.5) were 30.7, 36.0 and 40.0mm respectively for Palaemon gravieri, and 38.3, 40.2 and 45.3 mm for Palaemon carinicauda. The Selection Ranges(SR) were 12.2, 10.4, 13.8 mm for Palaemon gravieri, and 9.3, 14.4, 12.9 mm for Palaemon carinicauda. And the Selection Factor(SF) were 1.5370, 1.4394, 1.3310 for Palaemon gravieri and 1.9131, 1.6082, 1.3727 for Palaemon carinicauda. The relations between 50% escapable body length and mesh size of the two species mentioned above can be described as follows: Palaemon gravieri: L0.5=0.6869m+17.6918; Palaemon carinicauda: L0.5=0.5500m+26.9557. Combined with study results and relevant documents it showed that 20 mm-cod-end-mesh beam trawl could catch a lot of juvenile shrimp and cause a huge destruction of the fishery resources in Panjin sea area.We changed the rope matching coefficient of the trawl, and did a contrast experiment on the trawl before changed and the trawl after changed to compare their parameters of selectivity. The result showed that the percentage of Palaemon gravieri with body length less than 40 mm in the trawl before changed was 50.75%, while the percentage of which in the trawl after changed was 39.74%. And the percentage of Palaemon carinicaudawith body length less than 40 mm in the trawl before changed was 14.64%, while the percentage of which in the trawl after changed was 12.30%. The number of juvenile shrimps in the trawl after changed are smaller than the number in the trawl before changed. The 50% escapable body length of Palaemon gravieri was 32.6 mm in the trawl before changed and 36.6 mm in the trawl after changed. The 50% escapable body length of Palaemon carinicauda was 38.0 mm in the trawl before changed and 41.4 mm in the trawl after changed. The lengths of both two kinds of shrimps in the trawl after changed were bigger than the lengths of these in the trawl before changed. We concluded that the optimal design could release more juvenile shrimps and protect the genetic resources of shrimp.We design a new kind of separating device for beam trawl in Panjin sea area. The device uses a horizontal net to divide the beam trawl into the upper half and the bottom half. The upper half is for shrimps and the bottom half is for conchs. To prove if the device could work, we did an experiment on it, meanwhile compared selectivity between the productive trawl and the trial trawl in Panjin sea area. The result shows that the device could separate parts of the shrimp from the catches, and the separating rates are 45.24% for Palaemon gravieri and 40.10% for Palaemon carinicauda. The selection parameters show that the trial beam trawl could release more young shrimp. That means it could improve the quality of the shrimp and it benefit the continuable development of fishery. |
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