| Yellowfin tuna（Thimnus albacares） is the highly migratory pelagic species and one of the most important high value species. In this paper, the age, growth and the living environment of different life history stages of yellowfin tuna were studied by analyzing the morphological microstructure and trace elements of its otolith. The objective of this study was to provide a reference for yellowfin tuna resource management, assessment and sustainable use in the Central Pacific Ocean.Based on the otolith samples of yellowfin tuna collected by Chinese tuna longliner "Feng Hui 17" operating in the Central Pacific Ocean from April to September of 2015, the morphology, age, growth and trace elements of the yellowfin tuna were studied. There were 97 otoliths,85 otoliths were analyzed. The results were as follows:（1） The range of fork length was from 51cm to 166 cm. The relationship between fork length （cm） and body weight （kg） was WG= 5.5509 X 10-5x27237 （R2= 0.9225）, and there was no significant difference on the relationship between female and male.（2） The otoliths were cut transversely to ensure that the primordium of the otoliths were aligned. The section consisted of long arm and short arm, and the long arm provided the most reliable age estimate. Sections were observed under transmitted white light microscope. Regular rings could be observed in the long arm.（3） There were significant linear relationships between otolith cross-sectional length, otolith length and fork length. Unremarkable variation on fork length and weight can be found for individuals with same annual rings. Those provided the evidence that otolith could be used to age yellowfin tuna efficiently in the Central Pacific Ocean.（4） The von Bertalanffy growth function （VBGF） could be selected to describe the relationship between fork length and age that identified by the otolith. von Bertalanffy growth function （VBGF） was:L= 181.806* (1-exp-0.2511（t+0.4759）).（5） There was positive linear relationship between otolith weight and age. There was no significant difference between the back calculated age based on the otolith weight and the measured age by the χ2 test （P> 0.05）.（6） Age estimations indicated that age 3+ and 4+ were the dominant age classes, accounting for 41.23% of all the age classes. Individuals of 4+ were the most, and the individuals of male were more than that of the female. The proportion of age 1+,2+ and over 6+ individuals was small, in addition, only one 7+individual.（7） The age estimated in the present study ranged from 1 to 7 years. Otolith section could be used to determine the age of the young fish although some difficulties in age determination were occurred below the 2 years of yellowfin tuna.（8） Yellowfin tuna otolith contained Ca and other 47 trace elements, including Ca elements existenced in the form of CaCO3. The eight high content elements were:Ca, Na, Sr, Mg, Zn, Ba, Mn and Li. The elemental variation analysis showed that:the content of Ca was very stable, the content of Li and Mn was low and highly volatile, the content of Ba and Na was stable and their coefficient of variation was about 30%.（9） Analysis of variance showed that there were no significant difference on the otolith trace element content of Ca （P= 0.873）, Sr （P= 0.318）, Mg （P= 0.204）, Ba （P= 0.105）, and Zn （P= 0.384） between male and female, but there were significant differences on the otolith trace element content of Na （P= 0.047）, Mn （P= 0.013） and Li （P= 0.025） between male and female.（10） There were no significant difference on the otolith trace element content of Na in different parts of the otolith between juvenile and adult individual. There were significant difference on the otolith trace element content of seven other trace elements in different parts of the otolith between juvenile and adult individual. The individuals of juvenile were different from the adult. There were relationships between uptake of otolith trace elements and the individual development, thermoregulation and external environment.（11） The content ratio of Sr/Ca and Ba/Ca displayed variation with the age increases, wherein the content ratio of Sr/Ca decreased first, then increased and further decreased with age. However, the variation of Ba/Ca content ratio was contrary with Sr/Ca content ratio in general, the variation of Sr/Ca over the entire range was significantly less than that of the Ba/Ca. The Sr and Ba could be regarded as the indicator elements of environment for yellowfin tuna migratory behavior. |
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