Influence Of Physical Environment To Anchovy Population Dynamics In The Yellow Sea

Anchovy (Engraulis japonicus) is a kind of small pelagic fish live in temperate sea, widely distributed in the Bohai Sea, Yellow Sea and East China Sea. Before the initiation of a large-scale directed fishery in early 1990s, the anchovy stock in the Yellow Sea was fluctuating around 3 million tons; and was the most abundant fish species in the Yellow Sea. Anchovy is important both as a prey species and as a major plankton feeder, plays a key role in the Yellow Sea ecosystem. It is not only of commercial importance, but also has high ecological value. Since the 1990s, as resources of major commercial fishes declined, anchovy is no longer a subordination object of fishery but become a main object. The output increased obviously, but at the same time, anchovy resources have declined rapidly. The catches become smaller and younger, and CPUE (Catch Per Unit Effort, CPUE) declines. In this process, over fishing is a possible reason, but, at the same time, the physical environment such as temperature field and current field also has obvious variation. What role environment viaration plays in this process is still unknown. To study the influence of physical environment to anchovy population is not only helpful to the recovery of anchovy resources, but also to the understanding of anchovy recruitment's interannual variation.Individual-Based Model (IBM) has been widely used in the study of marine ecosystems and population dynamics. Unlike the traditional mass-balance model and population model, in IBM model, the individual difference and the spatial heterogeneity are emphasized. This is helpful to the discussion of how the complex hydrological environment of Yellow Sea influences the life history of anchovy. At the same time, hydrological environment impacts marine ecosystem through more than one ways. On one hand, it directly influence the growth of planktons and anchovy; on the other hand, it changes the distribution of planktons and the migration of anchovy, and thus change the match-mismatch relationship between anchovy and its prey, influence high trophic level through food web. Take these two impacts into account, an IBM model coupled with hydrodynamic model and low trophic level model is used in this study.There are 3 modules in the model. The hydrodynamic module provides mix layer depth and the mean temperature and current in mix layer. Where current filed will advect planktons and nutrients and influence the swim of anchovy; temperature field will influence the growth of anchovy, many physiological activities of plankton and decomposition of organisms, it will also change the swim of anchovy indirectly though fitness; mix layer depth will influence the settling and supply of nutrients. For the low trophic level module, a modified NEMURO (North Pacific Ecosystem Model Used for Regional Oceanography) model is used, some modification are made to the original NEMURO model in order to use it in Yellow Sea. Firstly, phosphorus cycle is added. Secondly, nutrient supply from river and atmospheric deposition is taken into consider, and the model of nutrient supply form deep water is improved. Lastly, to reflect the influence of SPM (Suspended Particle Material), light intensity is calculated by transparency instead of phytoplankton density, and the method of surface light intensity calculation is also changed, in order to reflect the spatial distribution of light. The low trophic level module provides 3 kinds of zooplanktons as preys to anchovy module. The anchovy module simulates the growth, swim, spawn and death of anchovy, and drive the migration of anchovy though fitness.Most existing fish IBM models do not consider migration or use fixed migrant routes. In this model, the swim of anchovy is totally decided by external environment such as prey density, temperature and current. There is a probability for the anchovy to swim randomly as the reflection of individual difference. The life history and migration of an individual are closely associated to each other. Growth, spawn and death of an individual are all on the way of migration. Though there are still same anchovy residual in the coastal zone in winter, due to insufficient study of migration mechanism and imprecise physical field, the general migration distribution is simulated by the model.The impacts of Yellow Sea Warm Current (YSWC) on anchovy population are studied by a set of sensitive experiments trough changing temperature. The result shows, to age-1 anchovy, when YSWC is weaker than climate state, there is no obvious difference in anchovy population; when YSWC is stronger than climate state, anchovy grows much quicker. One possible reason is when YSWC is stronger than climate state, the overwinter ground of anchovy will move northward, and the distance between overwinter ground and spawning ground decreases. When spring comes, anchovy reach the spawning ground which have better prey and temperature conditions earlier and have a higher growth rate. To age-2 anchovy, when YSWC is weaker than climate state, anchovy grows quicker, and when YSWC is stronger than climate state, anchovy grows slower; this is mainly controlled by prey density. Age-3 and older anchovy shows no markedly difference when YSWC changes. This is due to their better swim ability, thus the influences of YSWC on them are not so obvious. At last, an attempt is made to simulate the interannual variation of anchovy population between 1984 and 2002. Although as a result of inaccurate hydrodynamic field and absence of fishing, there are some difference between model result and observation, some rules of the relationship between recruitment and biomass of anchovy can be observed. Continuously high recruitment makes high biomass, while excessively high biomass makes low recruitment. There are also some interannual variation of the relationship between recruitment and biomass. The simulation result shows when the relationship between recruitment and biomass is abnormal, the migration of anchovy are also different form climate state, which provide a possible reason for the interannual variation of anchovy resourceAn individual-based anchovy model is successfully established in Yellow Sea, which provide a tool to study the effect of physical environment to the life history of anchovy. It is also helpful to our future work on copepods and jellyfish.