The catch structure characteristics of Siniperca chuatsi captured by five fishing gears and its impact on Siniperca chuatsi resources in Xiaosihai Lake

Mandarin fish Siniperca chuatsi is one of the most important piscivorous fish in freshwater ecosystem, and their natural resources in the Yangtze lakes have declined dramatically due to overfishing and eutrophication. The unreasonable use of fishing gears is often considered to be an important factor leading to overfishing. In this study, the catch structure characteristics of S. chuatsi captured by five fishing gears in a shallow lake(Xiaosihai Lake)along the middle reaches of the Yangtze River in 2007 were investigated and analyzed to explore the potential impact of the five fishing gears on the resource of S. chuatsi, and also provide basic data for fisheries management. The results indicated that a total of 359 samples of S. chuatsi were collected and the total length ranged from 92 to 600 mm. The dominant length ranged from 251 to 350 mm, accounting for 61. 0% of the total. The age structure of our collected samples contained 5 age groups ranging from 1 to 5, and most individuals were 2-3 years old accounting for 74. 6% of the total. Gill net, longline fishing and weir had strong fishing selectivity on S. chuatsi. The mature individuals with 2-year-old or above in S. chuatsi catches captured by gill net with 80 and 100 mm meshes accounted for 93. 3%. The average body weight of S. chuatsi captured by gill net was 466 g, and among them 57% individuals met the demand of "the standard S. chuatsi"(0. 4-0. 75 kg/ind. , having the best market price per kilogram). The mature individuals with 2-year-old or above in S. chuatsi catches captured by longline fishing accounted for 86. 9%, and the individuals of "the standard S. chuatsi" accounted for 43. 5% of the total. 90% of the S. chuatsi catches captured by weir were immature individuals under 1-year-old, which meant that weir was harmful to the recruitment population of S. chuatsi. The ranges of the total length of catches captured by electronic trawl-net and electric catching instrument were clearly higher, which indicated that the two fishing gears had little fishing selectivity on S. chuatsi. Their catches were dominated by 1- and 2-year old individuals, which may impact the population structural stability of S. chuatsi. Comparative analysis showed that gill net and longline fishing were good for sustainable utilization of S. chuatsi resources, and their catches could gain better economic benefit. So gill net(mesh should not be less than 100 mm.)was suitable to be the main fishing gear to fish S. chuatsi, and longline fishing could be an auxiliary fishing gear to fish S. chuatsi due to its low fishing efficiency. Weir was harmful to the recruitment population of S. chuatsi, so it was not suitable to fish S. chuatsi. The electronic trawl-net and electric catching instruments were both illegal fishing gears, which were extremely harmful to fisheries resources. Therefore, supervision work should be intensified so as to forbid the illegal use of these two gears.