128 



Fishery Bulletin 94(1), 1996 



for both male and female adjusted relationships of 

 fork length to age indicated no significant differences 

 in the slope (F (1 ,,„ =1.270, P>0.05) or intercept 



120>" 



(F 121 =0.359, P>0. 05). 



The mean growth rate of yellowfin tuna in the In- 

 dian Ocean, based on this study, is approximately 2 

 mm/day for fish measuring 30 cm (FL), 1.3 mm-day" 1 

 for length between 60 and 80 cm, and 1 ram-day" 1 

 for fish larger than 110 cm. 



1500 



S 1200 



900 



600 



o 



— 



300 



300 600 900 1200 1500 



Number of increments on oblique section 



Figure 5 



Comparison between the number of increments read on 

 transverse and on oblique sections of yellowfin tuna, 

 Thunnus albacares, otoliths. 



Date of hatching 



Spawnings occurred throughout the year, but mainly 

 from October through March. If we compare the 

 monthly development of the number of births (back- 

 calculated from age estimations) and the variation 

 of the gonad index (GI) established by Hassani and 

 Stequert, 7 there is a similarity in development for 

 reproduction ( Fig. 9 ). Analysis of correlation yields a 

 significant correlation (r=0.997, P<0.05, n=12) be- 

 tween these two data sets. The months of hatching 

 established from otolith estimations correspond to 

 the months of the maximum reproductive activity 

 determined from GFs. 



Discussion 



Choice of otolith preparation method 



Age readings of yellowfin tuna previously have been 

 made either directly on the external surface of the 

 otolith or on the acetate replica of this surface (Wild 

 and Foreman, 1980; Wild, 1986). Using the external 

 surface of the otolith etched with 0.5N HC1, Wild and 

 Foreman ( 1980) showed that the age reading should 

 be done on the primordium-postrostrum axis (EP, 

 Fig. 1) for interpreting the increments. They con- 

 cluded that age was underestimated when the pri- 



7 Hassani, S., and B. Stequert. 1990. Sexual maturity, spawn- 

 ing and fecundity of the yellowfin tuna (Thunnus albacares) of 

 the western Indian Ocean. Document presented at the FAO 

 expert consultation on stock assessment of tuna in the Indian 

 Ocean; Bangkok. Thailand, 2-6 July 1990. FAO, Indo-Pacif. Tuna 

 Prog., Coll. Vol. of Working Documents TWS/90/68, p. 91-104. 



Table 1 



Results of the microincrement readings on acetate replicas of the external otolith face of yellowfin tuna. Thunnus albacares, 

 otoliths. In = number of increments for the nth reading. I = mean of the increment numbers. SE = standard error 



