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Fishery Bulletin 94(1), 1996 



■S 

 a 



1411 



120 



10(1 



Sll 



1,11 



411 



20 



  



d 





D — -Q Madagascar 

 V V Maldives 



• • Seychelles 



Age estimation 



200 



400 



i.iiii 



800 



10(111 



1200 1400 



Age (d) 



Figure 1 



Comparisons between the ages estimated from otoliths and from 

 different annual growth rates obtained by 1) small fish length-fre- 

 quency analyses (Madagascar), and 2) tagging experiments 

 (Maldives and Seychelles), for western Indian Ocean yellowfin tuna, 

 Thunnus albacares. 



observed for each age except for 1-yr-old and 2.5-yr- 

 old individuals (Fig. 11C). That these studies assume 

 there is only one period of reproduction that occurs 

 annually during a short duration of time (November 

 to March) and that individuals are sexually resting 

 during the remainder of the year may explain the 

 discrepancy. Recent hormone analyses (unpubl. data, 

 senior author) and estimated dates of hatching show 

 that yellowfin tuna of the western Indian Ocean spawn 

 throughout the year. It is clear that the major repro- 

 ductive period is between November and March, how- 

 ever some of the population spawns from July to Sep- 

 tember. A similar observation has been noted for the 

 eastern Pacific populations of yellowfin tuna. 10 For the 

 Indian Ocean population, there is agreement between 

 the spawning period estimated from the otolith read- 

 ings and the condition of the female gonad index dur- 

 ing the year. This observation supports the assump- 

 tion of daily deposition of microincrements. 



The growth rates estimated in the present study 

 are high at the beginning of the life cycle until 1 yr 

 and regularly decrease with time to reach 1 mm-day -1 , 

 which is consistent with growth estimates for most 



10 Schaefer, K. 1993. Inter-American Tropical Tuna Commis- 

 sion. Scripps Institution of Oceanography, 8604 La Jolla Shores 

 Drive, La Jolla, CA 92037-1508. Personal commun. 



species offish and other stocks of yellowfin tuna. This 

 contradicts the growth model for the same stock 

 landed by the French purse seiners presented by 

 Marsac 1 : the young fish have linear growth, until 

 they reach 60 cm, and then grow faster in accord with 

 the von Bertalanffy growth model. A uniform growth 

 model is also reasonable because yellowfin tuna from 

 the Atlantic and Pacific oceans show no significant 

 genetic differences (Scoles and Graves, 1993). 



The present study introduces some new informa- 

 tion about the growth of yellowfin tuna in the west- 

 ern Indian Ocean; that is to say, growth is regular, 

 following a classical von Bertalanffy model without 

 stanzas. Nevertheless, some problems remain un- 

 solved. For example, we collected no fish smaller than 

 28 cm (from larvae until 28 cm size, i.e. for the first 5- 

 6 months of life) and very few large individuals (>140 

 cm). The problem is more complex in collecting small 

 fish: analysis of the stomach contents of more than 1,000 

 adult tunas or other predators did not yield any small 

 individuals as has been possible for adult tunas in the 

 Pacific Ocean ( Uchiyama and Struhsaker, 1981 ). In fact, 

 we do not know where the small Indian Ocean yellow- 

 fin tuna live or how to capture them in order to collect 

 their otoliths. Once these otoliths are obtained, an im- 

 portant first study would concern otolith growth itself 

 to determine the correspondence between life cycle 



