The Selectivity About Separating Device For Fishes And Shrimps Of TanZi Net In ShanDong

This paper has discussed the selectivity of the marine fishing in the whole world, especial for the fishing gear selectivity, and considering the fact that in the process of catching shrimps TanZi net catches a lot of young fishes simultaneously and the selectivity is bad, an experiment applies the selectivity of split net and designs a separating device for fishes and shrimps. This will do good to protect and regain conventional fishery resource and promote fisheries sustainable development.In ShanDong province of China the fishery for shrimps is rich, and the fishery for shrimps has an important station of the offshore fishery. Most of shrimps are very small like Trachypenaeus, Acetes. To improve the selectivity of fishing gears, the mesh selectivity is often applied. But when the size of target species is smaller than the size of young fishes which we should protect, we can not protect young fishes by means of cod' s mesh selectivity. So the fishing of small shrimps badly destroys the fisheries resource.Considering the problem an experiment on TanZi net of ShanDong was carried out to research activity difference between young fishes and small shrimps, and size selectivity of separating device for fishes and shrimps. The sloping angle and mesh size of split net was also discussed. This will supply a scientific base for the research about the separating device.In 2000 many data about protecting fishery were consulted, especially the separating devices for by-catch. Based on the different separating function about separating devices there are four types: the separating device for fishes and shrimps, the separating device for size, TurtleExclusion Devices (TEDs), the selective device for viable and fresh fishes. And TanZi net catches' species, length distribution and activity characteristic were beginning to investigate.Two investigations about main catches of TanZi net were carried out in June and October 2000 in the offshore of the city RiZhao. And a statistical analysis of the composition about catches was done. The results showed that the selectivity of TanZi net was poor and TanZi net caught a great number of small young fishes while catching small crustaceans. For reducing the by-catch applying split net to TanZi net was put forward, including the rigging position and sloping angle of split net.In June 2002 the experiment on sea about TanZi net with split net whose sloping angle was 45, was carried out in RiZhao, ShaoDong. Catches of six cruises were measured. The logistic function was chosen to describe the selectivity for shrimps. The data was analyzed with the maximum likelihood method. The result showed that TanZi net with split net really reduced by-catch. On the other hand because the sloping angle of split net was too small, the ocean currents greatly affected the separating efficiency. So TanZi net with split net substantially reduced the shrimp catches. So further work is needed and modifications, including the sloping angle and the position of split net were provided.