Stequert et al Age and growth of Thunnus albacares 



125 



(Marcille and Stequert, 1976; Maldeniya and Jo- 

 seph 4 ; Anderson 5 ) or seasonal growth marks on cal- 

 cified structures (Yabuta et al., 1960; Huang et al., 

 1973). The second hypothesis, also based on length- 

 frequency analysis, suggests that there are two dif- 

 ferent growth periods, a slow period for young fish 

 ( 1.5 cm/month for fork length [FL] below 60 cm) and 

 a faster period for larger fish with FL >60 cm ( Marsac 

 and Lablache 6 ; Marsac 1 ). The objective of the present 

 study is to test these two hypotheses by estimation 

 of the ages (based on counting daily microincrements 

 on otoliths) of yellowfin tunas in the Indian Ocean. 



Materials and methods 



Sampling 



Sagittal otoliths were collected from fish caught in 

 the western Indian Ocean (between lat. 5°N to 15°S 

 and long. 42°E to 72°E). Otoliths were extracted 

 either directly on board French purse seiners based 

 in Mahe (Seychelles) or at the Port Louis tuna can- 

 nery ( Mauritius ) from fish caught by Mauritian purse 

 seiners. A total of 674 yellowfin between 28 and 154 

 cm fork length (measured to the nearest half centi- 

 meter from the tip of the snout to the fork of the tail) 

 were collected between May 1989 and November 1990. 

 The sagittae were removed with forceps, rinsed, dried, 

 and then stored in heat-welded, numbered plastic bags. 



Otolith preparation 



A subsample of 170 otolith pairs were chosen on the 

 basis offish-size frequency. Otoliths were cleaned in 

 sodium hypochlorite (household bleach ), rinsed with 

 distilled water, and dried. They were then prepared 



according to the methods described in Secor et al. 

 ( 1992 ). All terminology corresponded with that of the 

 otolith glossary in Kalish et al. (1994). The right 

 otolith was embedded in polyester resin (Embed 812 ), 

 and a transverse section (Fig. 1) was made with a 

 low-speed Buehler Isomet saw to obtain a slice con- 

 taining the primordium. For 33 individuals, the left 

 otolith was also embedded and sectioned in an ob- 

 lique plane (Fig. 1). In each case, the slice was at- 

 tached to a microscope slide with Crystalbond ther- 

 moplastic glue and then ground with wet sandpaper 

 (400, 600, and 1,200 grit) and polished on a polish- 

 ing plate with water and aluminium powder ( 0.3 /jm ) 

 until the primordium was reached. The microscope 

 slide was then placed on a hot plate for a few sec- 

 onds to soften the glue and to turn the section: the 

 primordium was then in direct contact with the mi- 

 croscope slide. The section was polished on the other 

 side until a thin section of 75-100 /(m maximum was 

 obtained. The surface of this section was partially 

 decalcified with 5-7% EDTA (tri-sodium-ethylene- 

 diaminetetraacetic acid, pH=7.2-7.6) to emphasize 

 the increments used to estimate the age. 



For 33 individuals ranging from 28.5 and 135 cm, 

 we counted with the aid of light microscopy the num- 

 ber of daily increments on the transverse section of 

 one otolith and the number of daily increments on 

 the oblique section of the other. The results were com- 

 pared to determine whether the interpretation of 

 transverse sections tended to underestimate age as 

 suggested by Wild and Foreman ( 1980). For 10 indi- 

 viduals ranging from 30 and 130 cm, we estimated 

 age by counting the microincrements on acetate rep- 

 licas of the external surface of the otoliths as in Wild 

 and Foreman ( 1980). The results of the acetate rep- 

 lica counts were then compared with those of the 

 transverse sections. 



4 Maldeniya, R.. and L. Joseph. 

 1985. On the distribution and biol- 

 ogy of yellowfin tuna from the west- 

 ern and southern waters of Sri Lanka. 

 FAO, Indo-Pacif. Tuna Prog., Coll. Vol. 

 of Working Documents TWS/85/21, p. 

 51-61. 



5 Anderson, R. C. 1988. Growth and 

 migration of juvenile tuna [Thunnus 

 albacares) in the central Indian 

 Ocean. FAO, Indo-Pac. Tuna Prog., 

 Coll. Vol. of Working Documents TWS/ 

 88/21, p. 28-39. 



6 Marsac, F., and G. Lablache. 1985. 

 Preliminary study of the growth of 

 yellowfin estimated from purse seiner 

 data in the western Indian Ocean. 

 FAO, Indo-Pacif. Tuna Prog., Coll. Vol. 

 of Working Documents TWS/85/22. p. 

 91-110. 



it'll 

 Ho\tro\trum / :M* 



Primordium 



Rostrum 



(anterior) 



Figure 1 



Drawing of a typical right yellowfin tuna, Thunnus albacares, sagitta. (A-B) Trans- 

 verse section axis. (C-D) oblique section axis. EP = external path. 



