Study On Changes Of Ecological Characteristics Of Four Demersal Fishes In The Taiwan Strait And Its Adjacent Sea

Marine ichthyecology studies the behavior of fish, the rules of fish population fluctuation, and controlling and predicting the variation trends of fishes, etc. The results could provide the scientific basis for conservation of fishery resources and fishery ecosystem management. Changes in ecological characteristics of fish, especially in fish populations and ecological parameters, play a guiding role in fish ecology and provide a practical approach to utilize fishery resources appropriately. This article studied on changes of ecological characteristics of four demersal fishes in the Taiwan Strait and its adjacent sea area, so as to provide scientific data for conservation of fishery resources and ecosystem management.The Taiwan Strait, which is located in the southeast of China, is an important channel connecting the East China Sea and South China Sea. There are famous fishing grounds in the Taiwan Strait and its adjacent area, including the Mindong Fishing Ground, the Minzhong Fishing Ground and the Minnan-Taiwan Bank Fishing Ground. Hydrological condition in Taiwan Strait and its adjacent area was complex, and plankton and fishery resources were abundant. But under the influences of climate changes and human activities, the ecological characteristics of demersal fishes have been changed significantly in this area.Based on methods and theories in Fishery Resources and Fish Ecology, this study analyzed the fluctuation trends of population structures and ecological characteristics of four demersal fishes in Taiwan Strait and its adjacent area, including the silver croaker (Pennahia argentata), red bigeye(Priacanthus macracanthus), yellowfin seabream (Acanthopagrus latus) and Jordan’s damsel (Teixeirichthys jordani). The impact factors causing the variations were discussed as well. Furthermore, related management strategies were suggested according to the current situation of demersal fishery resources in this area. The major results of this study were presented as follows:(1) Compared the population structure and ecological parameters in the year of 2006with that of1970s,1980s and1990s, the maximum and mean body lengths, the dominant lengths and weights, the maximum and mean body weights, the mean ages, the minimum sizes at first sexual maturity, the asymptotic body lengths (L∞) and the asymptotic body weights (W∞) of the four demersal species decreased gradually in recent decades, indicating that the populations were younger in age, smaller in size and earlier in sexual maturity. At the same time, the growth coefficients (k) increased and ages at the inflexion point of weight (tr) were younger than before. The total mortality coefficient (Z) and the fishing mortality (F) of four demersal species were at high levels, the exploitation ratio (E) had been reached or exceeded0.5, which indicated that the four demersal species population had been fully exploited or overexploited.(2) Fishing and climate changes were the most important factors leading to the fluctuations of population and ecological parameters of fishes. The fishing effects on the fishery resources were as follows:firstly, the amount of resources decreased; secondly, in population level, individuals were smaller in size and younger in age, and population structure became simplistic, reflecting in ecological characteristics, individuals manifested as the growth rate increased, age at the inflexion point of weight being younger and sexual maturity being earlier, etc.; thirdly, in ecosystem level, the long-lived and high trophic level fishes had been replaced by the short-lived and low trophic level invertebrates and fishes gradually; finally, some irrational fishing practices such as trawling greatly changed the marine habitats, and then threatened the growth and reproduction of marine organisms, especially demersal fishes. In addition, overfishing had made the fish populations to a fragile status; thereby strengthened the threats caused by climate changes, and made the ecosystem under high exploitation pressure more sensitive to environmental changes. Climate changes could affect fish population and their ecological characteristics in both direct and indirect ways. The increasing sea surface temperatures caused by climate changes would affect fish growth and reproduction from organization, individual, population and ecosystem perspectives. Climate extreme events such as ENSO had leaded to marine environmental changing of Taiwan Strait and its adjacent area. Thus the primary productivity (diatoms) and secondary productivity (zooplankton) were affected, and then affected the growth, reproduction, migration and distribution of fishes through the food chain.(3) According to the current situation of fishery resource, relevant management strategies were proposed. The fishing efforts should be reduced and fishing industry structure should be adjusted; The minimum catchable size should be implemented and the closed season should be modified; Establish regulations to protect the juvenile fishes, spawning groups and their spawning grounds.In order to obtain the sustainable development of fishery resources, Ecosystem-based Fishery Management (EBFM) should be taken into account from the perspective of the whole ecosystem.