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Fishery Bulletin 94(4), 1996 



Gonadosomatic index 



Because albacore gonads often have a fatbody at- 

 tached, only those samples processed at the South- 

 west Fisheries Science Center (SWFSC), with the 

 fatbody removed, were used in calculating GSI. The 

 weight of the fatbody was found to compose an aver- 

 age of 4.4% of ovarian weight, from 0.0% to 31.6% in 

 the samples sent to the SWFSC for processing. Be- 

 cause of the large variability in the weight of the 

 fatbody, we decided not to attempt any corrections 

 for fatbody presence but to use only the weight of 

 ovaries measured in the laboratory. The relationship 

 between GSI and fork length of sampled female al- 

 bacore is shown in Figure 5. 



In this study, a GSI of 1.7 was used as the critical 

 GSI value, delimiting the point at which 100% of the 

 females in our sample displayed an advanced stage 

 of oocyte development (>0.3 mm)(Fig. 3). GSI values 

 of 1.6 (Kikawa, 1959, 1966), 2.0 (Shung, 1973; Koido 

 and Suzuki, 1989), and 2.1 (Kikawa, 1962) have been 

 used to indicate group maturity of different species 

 of female tunas. Female albacore appear to be ca- 

 pable of achieving this level of ovarian maturity and 

 therefore of spawning once they reach 82 cm FL (Fig. 

 5). Evidence of spawning, as indicated by the pres- 

 ence of residual hyaline oocytes, was found in an 85- 

 cm-FL albacore with a GSI of 2.8 and a mean maxi- 



mum oocyte diameter of 0.57 mm. The largest GSI 

 measured was 5.2 for a 89-cm female captured in 

 Tonga in November. The weight of its ovary was 368.3 

 g, and the mean oocyte diameter of its most advanced 

 oocytes was 0.60 mm. 



The mean monthly GSI's of females show that peak 

 reproductive development ( GSI > 1.7) occurs in the 

 austral summer months, November-February (Fig. 6). 

 The first indication of maturation was observed in Oc- 

 tober, when three of 15 albacore sampled were found 

 to have GSI's greater than 1.7. These three had par- 

 tially and fully yolked oocytes in the 0.35-0.54 mm size 

 range. The first large increase in mean monthly GSI 

 value occurred in November, when the mean GSI in- 

 creased to 3.2 (Fig. 6). The largest monthly GSI, a mean 

 of 3.3, occurred in February ( Fig. 6). In March, the mean 

 GSI declined to 1.2, although a small number of fe- 

 males still exhibited some spawning activity as indi- 

 cated by females with high GSI values and late devel- 

 oped ovaries with oocyte diameters greater than 0.3 

 mm. The mean GSI then dropped to a resting level, 

 close to 0.5, for the austral autumn and winter months. 



Gonadal asymmetry 



The right gonad was found to be heavier than the 

 left in 80.0^ of the females examined, and in 98.6% 

 of the males (Table 2). In terms of collection sites, 



