The Age And Growth Of Bigeye Tuna (Thunnus Obesus) In The South-central Indian Ocean

Bigeye tuna (Thunnus obesus) is one of pelagic species with highly economicvalue in the Indian Ocean. As age and growth parameters are important for couductingstock assessment, the selection of methodology for studying age and growth canprovide the important prerequisite and basis for requiring age and growth data. Basedon the analysis on growth rings of sectioned otolith for bigeye tuna in the South-centralIndian Ocean, the present study estimate the relationship between age and growth andanalyze the precision of aging estimation. Comparison on the age estimations derivedfrom the caudal vertebrae and the first dorsal spine, the present study also determinethe reasonable method for aging bigeye tuna. The results derived from the presentstudy can provide basic data and information for reasonable exploitation and scientificevaluation and management of bigeye tuna stock in the South-central Indian Ocean.Based on the otolith, caudal vertebrae and spine samples of bigeye tuna collectedby Chinese tuna longliner “Longxing602” operating in the Southern-central IndianOcean during September2008to April2009,79otoliths were analyzed and only68otoliths were finally confirmed. In addition, a total of68vertebras and28spines fromthe correspondent individuals were analyzed according to the otolith information. Theadvantages and disadvantages of three clarified materials for aging bigeye tuna werecompared. The main results can be found as follow,(1) The range of fork length was from57cm to152cm, the relationship between forklength (mm) and body weight (kg) can be described as WG=8.0417×10-6FL3.1753（R2=0.9172），and no significant difference for the relationship can be found betweenfemales and males. The relationship between the fork length and total length (TL) wasdescribed as TL=1.0578FL+2.2071(R2=0.9957). (2) The otoliths were cut transversely to ensure that the primordiums of the otolithswere aligned. The section consisted of long arm and short arm, and the long armprovided the most reliable age estimate. Sections were observed under transmittedwhite light microscope using the same magnification for all otoliths, regular rings canbe observed.(3) Unremarkable variation on fork length can be found for individuals with sameannual rings. The length of otolith (LOT) and the external part on transverse section (D)were proportional significantly to the fork length of the fish, providing the evidencethat otolith can be used to age efficiently bigeye tuna in the South-central IndianOcean.(4) The high precision of age interpretation can be observed for aging estimation basedon sectioned otolith. The total percent of agreement (TC), coefficient of variation (CV)and the average percent error (APE) are60.87%,6.35%and4.98%respectively(5) The von Bertalanffy growth function (VBGF) can be selected to describe therelationship of fork length and age that identified by the otolith, vertebra and spine, andthe L∞are estimated as207.13cm,197.71cm and175.32cm respectively.(6) Otolith’s weight had significantly positive linear relationship with age (P<0.05).Although inconsistent results can be found for the age estimations derived fromotolith’s weight and annual counts of sectioned otoliths, Kolmogrov-Smirnov testdemonstrates that there is no significant difference between the above estimationmehtods (P>0.05).(7) Age estimations indicated that age3+and4+were the dominant age classes,accounting for61.76%of all the age classes. The proportion of age1+and over6+individuals was small.(8) The relationship of vertebrae’s radius and fork length for bigeye tuna can beexpressed as FL=4.9867Rv+28.862(P<0.05, R2=0.9335). And the relationshipbetween dorsal spine and fork length is described as FL=29.616Rd+11.578(P<0.05,R2=0.9351)(9) Two-sample K-S test showed that there was no significant difference between ageestimations based on otolith, vertebrae and spine.(10) The age estimated in the present study ranged from1to8years. Otolith sectionscan be used for the whole age range although some difficulties in age interpretationoccur in the first five years of bigeye tuna. The vertebra may underestimate the age of older fish since it’s difficult to distinguish the marginal increment. Resorption of thenucleus occurred in the spines for the individuals above age3, so it can only be used toidentify the individuals aged of below3years accurately.