Stequert et al.: Age and growth of Thunnus albacares 



129 



mordium-ventral edge axis (Fig. 1) was used. This 

 underestimation was as high as 17% for larger indi- 

 viduals. The irregularity of microincrement structure 

 and its overlapping (Fig. 4) may explain the differ- 



c 2 



si 



lb t- 

 o 



1500 



i:on 



900 



■3 600 



E -5 



300 



300 



600 



900 



1200 1500 



Number of increments in light microscopy 



Figure 6 



Comparisons between the number of increments estimated 

 on transverse section of yellowfin tuna, Thunnus albacares, 

 otoliths with the light microscope and the SEM. 



ence observed by Wild and Foreman ( 1980) between 

 their readings on the surface of the otolith along the 

 oblique and those on the transverse axis. 



The results of the present study show that the 

 microincrements can also be counted on a transverse 

 or oblique otolith section which crosses the primor- 

 dium. We obtained similar results with all three tech- 

 niques. Because transverse otolith sections are easier 

 to prepare and have excellent statistical replicability 

 (CV <5%), they were used to count the microin- 

 crements and estimate the age of yellowfin tuna from 

 the Indian Ocean. However, our results suggest that 

 under light microscopy, a magnification of l,000x 

 must be used. 



Validation of the age estimations 



Wild and Foreman ( 1980) have shown that one daily 

 microincrement is formed each day on the otolith of 

 the yellowfin tuna from the eastern Pacific. If the 

 same phenomenon occurs in the western Indian 

 Ocean, comparable results should be found in this 

 area with other methods. In order to verify indirectly 

 our age estimations, we used the results of mark- 

 recapture studies (Cayre and Ramcharrun 8 ; Yesaki 

 and Waheed 9 ) and of length-frequency analyses of 

 young individuals for which cohorts are easily rec- 

 ognized (Marcille and Stequert, 1976). 



In a tagging experiment carried out in the south- 

 west Seychelles Islands area and in the 

 Mozambique channel (Cayre and Ram- 

 charrun 8 ), three individuals (FL=67, 73, and 

 67 cm) were recaptured after liberty at sea 

 for 252, 411, and 613 days, respectively after 

 marking. They presented a mean growth rate 

 of 27.3 cm-yr -1 . A tagging experiment in the 

 Maldives (Yesaki and Waheed 9 ) resulted in 

 a mean growth rate of 2.4 cm-mo -1 for the 37 

 recaptured fish that were at liberty for more 

 than 29 days after marking. The growth rates 

 obtained in these two experiments were simi- 

 lar to those of the present study. Following 

 the young yellowfin tuna cohorts caught by 

 Japanese pole-and-line tuna boats along the 

 northwest coast of Madagascar (Marcille and 

 Stequert, 1976), the mean growth rate of 40 



200 400 600 800 1000 1200 1400 

 Number of increments = age in days 



Figure 7 



Comparison between the number of increments on yellowfin tuna, 

 Thunnus albacares, otoliths estimated from acetate replicas and the 

 number of increments estimated from transverse sections. 



8 Cayre, P., and B., Ramcharrun. 1990. Results of the 

 tagging operations conducted within the Regional 

 Tuna Project (Indian Ocean Commission) in 1988 and 

 1989. FAO, Indo-Pacif. Tuna Prog. Coll. Vol. of Work- 

 ing Documents TRW/90/61, p. 63-85. 



9 Yesaki, M., and A. Waheed. 1992. Results of the 

 tuna tagging program conducted in the Maldives dur- 

 ing 1990. FAO, Indo-Pacif. Tuna Prog. Working Pa- 

 per 92/WP/24, 18 p. 



