A Simulation Study On Impacts Of Trawl Selectivity On Traits Of Fish Life History

Over the past few decades, extreme changes have occurred in the characters ofexploited fish population. The majority of these changes have affected the growthtraits, which is one of fish life history’s traits, such as a smaller size-at-age and anearlier age-at-maturation. Currently, the causes of these life history traits changes stillrequire systematic analysis and empirical study. The explanations that have been citedare merely expressed in terms of fish phenotypic adaptation, and it is claimed that theoriginal traits of fish can be recovered if the intensity of exploitation of the fish iscontrolled. Sustained environmental and fishing pressure will change the traits ofmost fish life history, so the fish individual’s traits are still in small size-at-age andearlier age-at-maturation in exploited fish population, the important environmentalpressure is climate change including: shifts in temperature, acidification, and oxygendepletion.These environmental stressors affect the distribution, biomass andphenotypic characters of fish population, and induce the population vulnerability,change of distribution range, the change of population’s structure and evolution ofbiological characters. The changes of traits of fish population are not only from theeffects of various environmental factors but also from long-term selective fishing.Fishing gear selectivity is directional, and the directional selectivity character is theimportant reasons that induce the changes of phenotypic traits of fish population. Thechanges of fish population can not reserve.Trawl is a main fishing gear in fishery. Long-term use of trawl would result inphenotypic traits changes of the exploited fish population and the change of fish lifehistory. In this study, we simulated two fish populations’ individual life history withsize characteristics of trawl fishing. The first population produced one generation ofoffspring and lives for one year. The second was perennial fish population. Herein,we presented the simulated fish population and provided the model that simulatesfishing cycle. We considered the heredity of every generation. the model based on thephenotypic genetic traits in quantitative genetics, and the model of individual level ofquantitative genetic trait in breeding science. We simulated long-term trawl fishing selectivity affecting the changes of life history in perennial fish population and annualyear fish population. The changes are different if we choose fishing pressure and livefor a long period without fishing pressure. We investigated whether trawl selectivitywould induce the evolution of individuals’ phenotypic traits. In the present study, wefocused on the age structure of fish population, the change of length at age inperennial fish population and the change of body length in annual year populationunder trawl fishing and without trawl fishing.The results show that the changes in the body length of simulated annual fishpopulation are very obvious after several decades of trawl fishing; in particular, themean body length is reduced, and the body length can not recover after long-termtrawl fishing; the age structure is mainly young fishes and the length at age presentsminiaturization in perennial population by long-term trawl fishing; the length at agecan not recover if we stop trawl fishing and the population is still presentminiaturization. The age structure of perennial fish population presents that the youngfishes increase after long-term trawl fishing and can not recover without trawl fishing.According to the two numerical experiments, the results show that long-termtrawl fishing pressure can induce the changes of biological characters in simulatedfish populations. All these changes can get along with the changes of trawl fishingpressure. We give the explanation that these changes are phenotypic plasticity, andthe biological characters can be recovered if we stop trawl fishing. The results alsoshow the body length of simulated fish population can not be recovered without trawlfishing pressure. Maybe the genetic of the fish population changed with the changesof the biological characters. Probably there is the evolution of individual phenotypictraits of fish population, and at last induce the evolution of fish life history. Accordingto the results of exploited fish individual’s life history traits, significant effects ofminiaturization and young age structure, we give suggestions on this subject in thefuture and provide better methods of fishery management for protection of the fisheryresources.