Reproductive Biology Of Yellowfin Tuna(Thunnus Albacores) In The Pacific Ocean

Yellowfin tuna(Thunnus albacores; YFT) is one of the main target species of China’s distant water tuna fishery. Reproductive activities of YFT in the Pacific Ocean have been previously studied. However, our understanding of reproductive biology of YFT is still weak. Reproductive parameters(such as fecundity) are important information for fishery stock assessment. In this thesis, based on field sampling and histological method, reproductive information of YFT in the tropical Pacific Ocean was investigated.Yellowfin tuna samples are collected by China’s national tuna fishery observers in the Pacific Ocean from 2010 to 2013(14°2′S-11°41′N,158°43′E-128°24′W). First, based on histological method, maturity-at-size, relationship between gonadosomatic index and fork length, and seasonal changes in maturity were analyzed. Second, according to the images of tissue section, characters of different maturity stages, mature oocyte diameter, and fecundity were investigated and estimated. Third, errors in defining the maturity stage resulted from macroscopic observation and histological methods were estimated, and were used to correct the female maturity of a large number of samples with only macroscopic observations of maturity stages. Finally, egg-per-recruit(EPR) analysis was conducted using information derived from the above and other biological parameters of YFT. The results are as follows:(1) For the females, the size range of maturity stage II was the largest(80-160 cm, fork length), and the minimum size range was for stage V(115-151 cm). There was a remarkable difference in the length distribution of maturity. Length at 50% maturity was estimated to be 138.13 cm. For the males, the size range of stage III was the largest(44-177 cm), and the minimum size range was for stage VI(130-165 cm). There was also a remarkable difference in the length distribution of maturity. Length at 50% maturity was estimated to be 92.19 cm.(2) The gonadosomatic index basically increased with fork length both for female and male YFT. The trends between gonadosomatic index and month were similar for both female and male YFT, with the female’s gonadosomatic index higher than the male’s. The female mature gonadosomatic index was greater than 1, while the male’s was less than 1.(3) There was a difference in maturity between western and central Pacific Ocean(WCPO) and eastern Pacific Ocean(EPO) areas for female YFT, but not for the males. Male maturities showed seasonal variation in both WCPO and EPO, while the females in EPO only. The female mature gonadosomatic index was greater than 1 in both WCPO and EPO, and the male gonadosomatic index in the WCPO are greater than that in the EPO.(4) In southern hemisphere, the maturity frequency and gonadosomatic index were greater at the higher latitude. In the west of 160 ° W, there was an increasing tendency with latitude in both female maturity frequency and gonadosomatic index.(5) The gonadal maturation of YFT has certain continuity. The cell diameter was greater, and the range of cell is also greater with maturity stage. The average batch fecundity was estimated to be 85.5×104 pcs, and the average relative fecundity was 21.9 pcs·g-1. Batch fecundity and fork length was found to be positively correlated.(6) The errors in defining maturity stage I and stage IV resulted from macroscopic observation and histological methods are higher. These two stages were often mis-defined as stage II.(7) The value of EPR was estimated to be 151.40×104 g.ind-1, which was 90.95% of the unfished situation.