Study Of Fishing Gear Selectivity Of Trap For White-spotted Conger In The Yellow Sea

The gear selectivity, which is the result of the interaction between the gear and the fish, means that the given gear catches the fish with the given biological features only. As the gear selectivity can be widely used in fisheries resources management, fishery stock assessment and the separation of by-catch, the fishery administrative authorities pay great attention on it. Fishery management measures must be made, because the fisheries resources have been fully exploited on the whole in China. To protect the fishery resource, the first step is to study the selectivity of fishing gear. Because there will be no fisheries without fishing gear. Only by using the fishing gear with high selectivity, environmentally friendly and ecology, can the purpose of protecting fisheries resources be made. As the gear selectivity is of great importance, a series of selective researches have been made in the whole world. These researches provide a lot of scientific data and material for protecting the fisheries resources. Meanwhile, we should notice that the selective researches in our country are not as good as other developed countries'. Our researches just only focus on some large and traditional fishing gears, such as trawl and set-net. There is few researches about some small fishing gear, especially the fishing gear used in some small scale fishery, been reported in our country.Cage (or pot) fishery for white spotted conger (Conger myriaster) is an important fishery in the Yellow Sea of China. To establish the fishery management and protect fishery, there is an increasingly need to study the selectivity of cage in the Yellow Sea. Surrounding the object of protecting the resource of white-spotted conger, this paper studied the biology of white-spotted conger, the methods of analyzing the gear selectivity, the relative and similar researches in the world, and the mesh size selectivity of white-spotted conger pot and the size selectivity of escape hole of the tube trap, respectively.This paper is consisted of six chapters according to the theories and experiments. The first and second chapters mainly focus on the biology of white-spotted conger and the theories of selective researches. In the third chapter, the status of selective researches is described. The fourth and fifth chapters are about the experiments of selectivity. The last chapter draws the conclusion and suggestion.In the fourth chapter, simulating the cover-net method in a laboratory flume, the biological features of white-spotted conger was obtained. Mesh size selectivity curves of white-spotted conger were drawn by processing its biological features with a standard and using Logistic curves as a selective model. Based on the maximum likelihood method, parameters of selectivity curves were obtained. The purpose of this study is to decide the size of the eel cages and to probe the optimal mesh size for catching the white-spotted conger. The result of the experiment indicates that: the length selectivity of the white-spotted conger is nearly 0 as the mesh size is 33 mm, 40 mm, and 55 mm for the white-spotted conger whose length range is: 0-39 cm, 0-42 cm, and 0-48 cm, respectively; and the length selectivity gets to 100% as mesh size is 26 mm, 33 mm, 40 mm for the white-spotted conger's length range is: l≥48 cm, l≥60 cm, and l≥66 cm respectively; Besides the maximal length selectivity rate for the white-spotted conger is 50% when the mesh size is 55 mm. When the mesh size for the white-spotted conger is 26 mm,33 mm,40 mm and 55 mm, its L50 is 40.083,48.212,55.874 and 66.500 cm respectively, the SR is 5.419,11.578,14.354,18.000 cm and the SF is 1.542,1.461,1.397 and 1.209 respectively. By comparison, as the mesh size of cage increases the L50 and SR will enlarge while the SF reduces.In the fifth chapter, as the fisheries resources in the Yellow Sea of China have being decreased, selectivity of fishing gears was analyzed to improve the options available for fishery management. Comparative fishing experiments were carried out in the Yellow Sea in 2010, using tube traps with five hole diameters (8, 15, 18, 20, and 22 mm), to obtain the size-selectivity of the escape-hole size for white-spotted conger. Selectivity parameters and split parameters of the SELECT model were calculated using the estimated-split model and the equal-spilt model, respectively. From the likelihood ratio tests and the values of AIC (Akaike's Information Criterion), the estimated-split model was selected as a better-fit model. Size selectivity of the escape holes in the tube traps was expressed as logistic curve like mesh selectivity. The 50% selection length of white-spotted conger in the estimated-split model was 28.26, 33.35, 39.31 and 47.30 cm for the hole diameter at 15, 18, 20, 22 mm, respectively. The optimum escape hole size is discussed for white-spotted conger fisheries management. The result also indicates that the tube traps with escape holes of 18 mm diameter have a good effect in protecting the fisheries resources.In the last chapter, the author draws the conclusion of the paper and gives some suggestions to future researches.