Study On The Impact Of Gillnet Selectivity On Phenotypic Traits Of Fish Population

Owing to the continual hydraulic engineering, environmental pollution and overfishing, the global fish stocks has been heavily damaged. All kinds of fishes are now struggling in the severe environmental conditions and have change a lot in phenotypic traits, especially the miniaturization of fish resources. Despite there have been some research on the miniaturization of natural economical fish resources, the causes for this phenomenon is still controversial. It is the severe environmental or the perennial fishing pressure that cause the changes of fish genetic traits, or both, which should be studied in depth. As for the condition of serious recession of the fishery resources and need to be protected, it is urgent and necessary to conduct some studies on the selectivity of gillnet. That will be good for the protection and sustainable development of the marine fishery resources.Fishing selection is focus on economic interests, the target of fishing is the fish with good growth and big physical type. Fishing selection is adverse selection, it means the one which is selected won't stay but weeded. It is opposite to the effect of natural selection. The gene will die away with the fish which is caught. Consequently, long-term fishing will make a profound effects on phenotypic traits of fish population.The simulated model described in this paper is based on a lot of real fishery data and matured view in the world. By estimate the fishing phenotypic differentials, preliminary study on the trawl fishing induced phenotypic traits selection differential of fish. This study makes up the absence of the study on the fish evolution induced by fishing, also provide some useful advice to the fishery government to make fishery rules. The numeric simulation model described in this paper is focus on the basic research on whether constant long-period use of Gillnet selectivity may affect the phenotypic traits (such as body length)of fish population. By consulting several models of Fishery Biology, Quantitative Genetics, as well as the applicable figure simulative method, a numeric simulation model was conducted. A fish population model with definite body length was built, upon which a Gillnet selectivity formula with determinate selectivity parameters(σ,l0,R0) was applied for long-period fishing, then research the stability of the effect without fishing.The simulation result indicated that:The long-term fishing pressure of gillnet will make a profound effects on phenotypic traits of fish population. Gillnet is a fishing gear with high selectivity, and the basic selectivity curve is unimodal probability distribution, thus the fish which is caught is the same length with the the optimal length of fishing(L0), ones which are shorter or longer than L0 will not easily be caught. If L0 is coincident with the preponderant length of fish group, the number of fish in the preponderant length will decrease and the smaller or longer ones will increase due to persistent fishing pressure. If L0 is lager than the preponderant length of fish group, under the pressure of long-term fishing, the number of lager ones will reduce and the smaller ones will increase accordingly, then the miniaturization of fish resources take place. And when L0 is smaller than the preponderant length of fish group, the fish with small length will reduce and lager ones increase owing to the fishing pressure. Furthermore, this impact is probably irreversible, for the continuous fishing pressure may bring genetic changes to the population unfortunately. The lager exploitation rate (E) is, the more distinct the effect is.Compare with the study on trawl mesh selectivity, although the influence of the two mesh is irreversible, the effect of fishing is not the same. The curve of trawl mesh selectivity is parallel to the shape of "S". It means trawl mesh will catch the lager fish rather than the small ones and that will cause the miniaturization of fish resources. But gillnet selectivity is target on the ones with the same length with L0, thus, if the L0 is smaller than the preponderant length of fish group, the fish with small length will reduce and lager ones increase owing to the fishing pressure, so the gillnet selectivity will cause Upsizing rather than miniaturization of fish resources. Through the analysis above we can see that under the same rate of E, the effect of perennial gillnet fishing depends to a great extent on the selection of L0. Perhaps there is some kind of enlightenment to the protection and sustainable development of the marine fishery resources.