Age And Growth Of Bigeye Tuna (Thunnus Obesus) In Waters Near Marshall Islands

Bigeye tuna (Thunnus obesus) is a fish species of great commercial importance in the tropical and subtropical waters of the Atlantic, Indian, and Pacific Oceans. Effective management of any fishery requires reliable information regarding population parameters such as length and weight, age and growth, mortality and recruitment pattern of exploited stock. In this study, we used four hard parts of otolith, vertebra , dorsal spine and scale to obtain estimates of size-at-age and von Bertalanffy growth parameters, and hoped to draw the most reasonable way for estimates of size-at-age of bigeye tuna. The objective of this study is to provide information on the age and growth of bigeye tuna in the waters near Marshall Islands. Such information can give a reference for the tuna fisheries management in the eastern and central tropical Pacific Ocean.From November, 2009 to January 2010, a total of 1203 bigeye tuna were randomly sampled which caught by the Chinese longliner operating in the waters near Marshall Islands. About 436 samples have been collected including otolith, scale, vertebra and dorsal spine which were used to identify the age of bigeye tuna. The results of this work are as follows. (1) The sacles treated by 5% KOH were observed using a dissecting microscope on the slides. We found that the size of scales from the same fish were different. There were a lot of irregular black spots on the surface of scales and no core, no rings were found. So the scales can't be used to identify the age.(2) Dorsal spines were rinsed in boiling water and 8–10% KOH to remove connective tissue, washed in running water for 24 h and air dried. We measured the length (L) and width (C) of fin spines. The fracture rate of fin spines was over 50% and need to be cautious observation when we measured. The regression analysis showed that the both relationships between fork length and the length (L) and width (C) of fin spines were the power function and they were increased with fork length. In the laboratory, the cross sections of the spines were taken along the length of each spine above the condyle base with an Isomet Low Speed Saw. Sections ranging from 400-500μm thick were examined with a dissecting microscope with transmitted light. Images of the sections were captured and analysed by the software Image-Pro plus version3.1. In this study, no counting of the rings was made and we chose to measure, for each spine: the total surface area of the section (S, mm²), the surface area of vascularized core S₀ (mm²), and the surface area included into the translucent rings (S₁, S₂,…S_n, mm²). The relationships between S and fork length, S0 and fork length were the power function and the ratio of S0 to S was about 0.4. Form the frequency analysis of the surface area of the translucent rings, the fork length of bigeye tuna from 1 to 3 years was 48 cm, 71 cm, and 90cm, respectively.(3) We measured the length, radius of vertebras and counted the number of rings (based on translucent rings) after vertebras were stained by Alizarin. The relationships between length of verebra and fork length, radius and fork length were all described by power functions. The radius of the same ring was gradually diminish while fork length increased. The growth equation of the von Bertalanfy was L_t = 2 43.17 (1-e^{-0 .118(t+0.782)}) and the Index of average percentage error was 3.55%.(4) There were many ravines on otolith of bigeye tuna which increased the difficulty on the experiment of transverse sections. We can't use optical microscope Physalospora to discriminate rings because they were too densely. So, this study analyzed the morphology of otolith of bigeye tuna. There were three channels on the otolith which divide the otolith four sub-areas (Dorsum, Wing, Rostrum, lateral). It was found that the overall morphological characteristics and the various parts of the left statolish were the same as the right (P=0.999). The result of principal component analysis on 12 morphologic indices showed that the dorsal length (DL) and Maximum statolith thickness (MST) can be used to represent the length features of the otolith, the dorsal angle (DA) may reflect its angle features. The relationship between DL and MST were best described by linearity, while the power functions between fork length (FL) and DL, fork length (FL) and weight of otolith (OW). There was fluctuant of growth of DA when the length of DL was from 500μm to 700μm by One-way random block design. The DL (about 600μm) was a sign of bigeye tuna which were mature for the first time.By the comparison among four hard parts of scales, fin spines, vertebras and otoliths, the results showed that there were no clear structure of rings in the scales. The nucleus was also not clear, so we can't use scales to identify the age. The daily periodicity of increment deposition on the sagittal otolith was demonstrated, but the daily rings were too densely to observe by the optical microscope Physalospora. We didn't obtain the date of age from otolith because of the adverse experimental conditions. If we want to identify the age of bigeye tuna using otolith accurately, we must use the otolith marginal increment method on raster microscope. Because of the vascularized core, we only identified age of 1-3 years of bigeye tuna, although there were clear rings on the section of fin spine. There were subsidiary rings and marginal rings on vertebras. Especially between the second translucent rings and nucleus, there were lots of subsidiary rings and we can't estimate accurately where was the translucent ring. The age of bigeye tuna should be identified using the fin spine and vertebra combined. The final growth equation of von Bertalanfy was L_t = 203. 77 (1-e^{-0. 18(t+0.204)}), which was fitted using the data of fork length of fin spine (1-3years) and vertebra (4-7years).