The Otolith Morphology And Trace Elements Of Bigeye Tuna(Thunnus Obesus) In The Central Pacific Ocean

Bigeye tuna(Thunnus obesus) is one of the dominating catch species of the longline fishery in the Central Pacific Ocean.It will be beneficial to understanding the population structure, age and growth and life history to study the otolith morphology and trace elements of bigeye tuna in the Central Pacific Ocean.The otolith morphology, age and growth and trace elements of bigeye tuna in the Central Pacific Oceanwere studied by using the samples collected in the Central Pacific Ocean from April 11, 2015 to September 4,2015 on board the longliner "Fenghui 17" of Zhejiang Fenghui Distant-water Fishing Corporation.The sampling area was 3oS—11oS and 155oW—143oW.The biological data were collected from 202 bigeye tuna,where in the female were 83, male were119, including fork length(cm) and weight(kg).Otoliths were collected from 70 bigeye tuna, including 30 females, 40 males.(1) The results were as follows: The relationship between fork length and weight was (2 = 4.8390 * 10-52.7602, 2= 0.9609, where, FL was the fork length(cm), W was the weight(kg).There was no significant difference between the male and female by analysis of covariance.(2) By analysis of otolith morphology structure, the otolith was curved shape,the surface was not smooth and had many crystal.The otolith was divided into four sub-areas(Dorsum, Wing, Rostrum, Lateral),and the boundary between rostrum and lateral was not obvious.There were three obvious ravines on the otolith surface.This three ravines intersected at one point.This point was called the core.The core was the most important regional to study the age and trace elements.The result of component analysis onotolith external morphological parameters showed that the dorsal length(DL) couldbe used to represent the length, breadth and thickness features of the otolith, the dorsal angle(DA) and wing angle(WA) might reflect its angle features.(3) The otoliths were cut transversely to ensure that the primordiums of the otoliths were aligned.The sections consisted of long arm and short arm,and the long arm provided the most reliable age estimate, the short armwas used to verify the age estimate.The length of otolith(LOT) and the length of long arm were proportional significantly to the fork length of the fish, and the weight of otolith was proportional significantly to the weight. The length of long arm couldbe reflected the number of rings.When the length of long arm wasthe same, the age of bigeye tuna was basically the same, and the variation range of the fork lengthwas very small. This illustrated that otolith could be used to determine the age of bigeye tuna efficiently in the Central Pacific Ocean.(4) Age estimations indicated that age 2+、3+ and 4+ were the dominant age classes,accounting for 80% for all the age classes.The numbers of age 1+ and 7+ individuals was separately 3 and 1.There was significant positive linear relationship between otolith weight and age.There is no significant difference for the age estimations derived from otolith weight and annual counts of sectioned otoliths, so otolith weight could be used as a method to age estimations.At the same time, it couldbe used to test the age accuracy determined by vertebrae and spine. The high precision of age interpretation could be observed for aging estimation based on sectioned otolith. The total percent of agreement(TC), coefficient of variation(CV) and the average percent error(APE) were 68.03%, 6.53% and 5.31%, respectively.(5) The growth parameters, L∞, k, t0 of the von Bertalanffy growth equation were 165.24, 0.29, and-0.31, respectively.The oretical fork length(cm) from ageone to seven was 52.23, 80.68, 101.96, 117.89, 129.81, 138.73 and 145.40, respectively.(6) There were 54 trace elementsin the bigeye tunaotolith. The highest content was Ca, and that accounted for 41.2% ± 1% in total.Ca was mainly in the form of Ca CO3.There were eight dominated trace elements: Ca, Sr, Ba, Li, Mg, Zn, Mn and Na.(7) There was no significant differences on the content of dominated otolith trace elements(Ca, Sr, Ba, Li, Mg, Zn, Mn, Na) between male and female bigeye tuna.(8) There were significant differences on the content of Na,Sr,Ba,Li,Zn,Mg among different parts of bigeye tuna otolith. There was no significant difference on the content of Mn and Ca.(9) There were significant differences on the content of Sr, Mg and Mn of the bigeye tuna otolith trace elements between Kiribati waters and high seas.There was no significant difference on the content of Ca,Na,Ba,Li and Zn.The Sr/Ca ratio of bigeye tuna otolith trace elementsin high seas was less than that in Kiribati waters.(10) The content of Sr and Mg trace elements could be used to study the life cycle and migration of bigeye tuna.