The Study Of Trawl Selectivity Affect On Phenotypic Traits Of Fish

Large changes were taking place in the phenotypic traits of commercially fish due to the strong selection generated by fishing gear in heavily exploited fish stocks. There was a wealth of information demonstrating changes in growth and maturation in major commercial fish stocks over time. The cause of these phenotypic changes was often not understood, and genetic change arising from the selective effects of fishing may be a contributory factor. The selection differential on a phenotypic trait was a necessary condition for fish to cause evolution, and also the evolutionary change caused by fishing were likely to be hard to reverse. It was very important to study the body length of phenotypic traits induced by the fish selection.Firstly, in view of the difficulty of surveying in practice, this dissertation was mainly focused on the basic research on whether the long period selective fishing (fishing gear selectivity actually) were the main cause for the phenotypic traits (such as body length, sex maturity) in the fishing industry. In the process, the theory models of Fishery Biology, Fishery Resource Analytics, Quantitative Genetics, as well as the applicable figure simulative method were referred.Secondly, the simulation methodology was used in the model which long period selective fishing (fishing gear selectivity actually) were the main cause for the phenotypic traits (such as body length, sex maturity) in the fishing industry by different parameters. At the same time, the simulation chose the software of matlab, results were given by different E,Ms,range of initial length and so on..Thirdly,In this dissertation, the stock with special body length which was created by random function. At the same time, Cov was used to correct the body length in this stock, then divided the stock into different teams by the demand of the range of the body length.,then exploited the stock by the the selection curve of trawl, in terms of selection curve eliminated the every team. Remnant stock was taken out the male and female by chance, the quantity of male was as same as the female, and they mated by chance. The body length of the next year was gotten by their parents. The H coefficient was used to control the quantity of the sock, at last got the body length data of the next year stock. According to this method, simulation was used for many years and the result of the simulative stock was easy to observe.Fourthly, the result shown that different quantity of the stock can get the change of the body length steadily, the initial quantity of the stock was 5000; the result of simulation was given with different E,Ms and the range of initial body length. The result shown that the fish's advantage body length was shortening of the simulative group by different E, the quantity of shorten body length team was increased, the quantity of the long body length was reduced; along with the change of the Ms, the size of the Ms was from 4cm to 6cm, L50% (body length of the selective rate was 50% )from 12cm to 18cm; under the fixed condition , along with the range of the body length increased, the character of the stock's body length was more prominent, the reason was the increased quantity of stock can increased the body length team, at the same time it induced the quantity of the every team was reduced, moreover the quantity of the next year's fish was reduced. The body length of the stock was close to the average length year by year without fishing, and concentrated in the shorten body length team, that was shown the body length of the stock was reduced due to the continuous fishing pressure for so many years . Furthermore, this change can be accompanied with the both the genetic changes for the whole group and un-convertible character.