Abstract:

Schizothorax oconnori (Cyprinidae: Schizothoracinae) is an endemic and commercial fish in the Tibetan Plateau, and only distributes in the middle and upper reaches of the Yarlung Zangbo River and its tributaries at elevations above 3000 m. This work estimated the lifehistory pattern and exploitation status of S.oconnori in the middle Yarlung Zangbo River, also evaluated the influences of fishing intensity and the age of first capture on spawning potential of the fish. The life history pattern and exploitation status of the fish were studied by examining 1126 individuals collected from August 2008 to August 2009 in the middle Yarlung Zangbo River, Tibet. The total mortality (Z), natural mortality (M), fishing mortality (F) and exploitation ratio (E) of S.oconnori were estimated with some classical formulae. A typical Kselected species Huso dauncus and a typical rselected species Eleotris oxycephaoa were taken as references using a fuzzy cluster analysis, to evaluate the lifehistory pattern of S.oconnori by seven lifehistory parameters. The exploitation status of S.oconnori in the middle Yarlung Zangbo River was assessed using the spawning stock biomass per recruit model. Due to the uncertainty of natural mortality, we simulated the population dynamic variation under different M (female: 0053 ± 0020/a; male: 0064 ± 0020/a) in this study. The influences of fishing intensity and the age of first capture (tc) on spawning potential of the fish were also evaluated. The total mortality of S.oconnori was 0090/a for females and 0112/a for males. The natural mortality was 0053/a for females and 0064/ a for males, and fishing mortality was 0037/a for females and 0048/a for males. 〖JP2〗The exploitation level of S.oconnori was 0415/a〖JP〗 for females and 0428/a for males. The fuzzy cluster analysis indicated that the lifehistory pattern of S.oconnori belonged to Kselection. At the current F, M and tc, the spawning potential ratio was 52.80% for females, and 5164% for males, both higher than the target reference point 40%. Under three different M, the fishing mortalities of females and males were all lower than the target reference point 40%. It was concluded that the resource of S.oconnori in the middle Yarlung Zangbo River had been exploited reasonably, the population recruitment was sufficient, and this population could provide optimal yield under a stable status. However, if the fishing intensity increased continuously, once exceeded the threshold reference point F25% (0091/a for females and 0113/a for males), the population might face with overexploitation, and the recruitment pattern could not maintain the population equilibrium, and finally would be very easily depleted. High level of F beginning at late age could have the same effect on spawning stock biomass per recruit as lower F level applied at earlier age. Therefore, the catch per unit effort could be enlarged by rising tc in the long run. In addition, the smallsize fish nets (especially trap nets) should be banned to prevent fishing of the young, and the highly destructive electrofishing must be resolutely banned