Reproductive Biology Of Female Bigeye Tuna(Thunnu Obesus) In Southern Pacific Ocean

Bigeye tuna(Thunnu obesus) is one of the main target species in Pacific tuna longline fishery. However, updated stock assessments showd that catch in Pacific Ocean had approached or even exceeded the maximum sustainable yield. Under such stock status, it is necessary to conduct reproductive biological research and further to understand current stock situation of bigeye tuna. The research shall not only contribute to the conservation and management of the stock, but also benefit the commercial fishing and exploitation of Chinese tuna fleet in the high seas.Ovary samples were collected by longline fishery observers in Southern Pacific(0°40′S~11°12′S, 168°1′W~133°19′W) during September to next January in 2010~ 2013. By using technique of paraffin section to detect ovary stage, the reproductive biology research has been conducted. And the visual errors of observers have been discussed. Using confirmed catch data from observer sample, the research focus on the variations of different maturity stages corresponding to catch depth, latitude and month of famale bigeye tuna. The estimated fecundity has been applied to egg-per-recruit model to analyze the stock situation and to evaluate effect of fishing mortality. The results are as follow:(1) Female GSI ranged from 0.26 to 8.58, with a mean of 3.56, increasing with fork length. Length at 50% maturity amounted to 116.32 cm.(2) The average mature proportion from September to next January arrived at 60.29%. And the mature proportions are approximately 60% during September to November. It peaked 66.0% in November. Since then, the mature proportion declined gradually, and hit the lowest 51.9% in the next January.(3) Average batch fecundity occurred at 109×104 pcs, and average relative fecundity amounted to 20.5 pcs?g-1.(4) Checking the observer data, over 50% of the maturity stage records are accurate. One third are fluctuating by only one level, and this may contribute to the main source of the total visual error. Only less than 10% have a significant differences. The average visual error is 0.63. Observer data can be used in research directly.(5) There is an obvious relationship between maturity stage and catch depth. Catch data showed that immature individuals stay shallower water than mature ones.(6) When bigeye tuna individuals are maturing, they will swim to areas(3°S~9°S) for spawning, and when the species finished spawning, they leave there. In Southern Pacific, the areas(3°S~9°S) are the spawning ground of bigeye tuna.(7) In Southern Pacific, bigeye tuna has multiple spawning seasons. September is one of them. After September, bigeye tuna return to immature and develop again. In December and next January, it reach the next peak for spawning seasons.(8) At the current fishing mortality, the EPR value is lower than the management reference point. Using average fishing mortality of each age in EPR model would enhance the value, so it is not suit for the model.Results indicate that the reproductive biology characters of bigeye tuna in southern Pacific are unique. Comparing with other places in the Pacific, they have both similarity and differences in the same time. Reproductive biology research should keep going in southern Pacific. However, results of both reproductive biology and EPR model show negative effect on bigeye tuna stock due to overfishing. Fishing strategy adjustment should include two aspects. The catch of juvenile bigeye tuna in purse seine fishery should be reduced. Another is that in longline fishery, vessels should been suggested fishing in the particular depth, latitude and month which mature bigeye tunas prefer.