Ramon and Bailey: Spawning seasonality of Thunnus alalunga 



731 



Figure 6 



Mean monthly gonadosomatic index values for female albacore, 

 Thunnus alalunga, by month, sampled from New Caledonia and Tonga 

 from 1990 to 1992 (±2 standard errors; n = 140). Numbers above error 

 bars indicate sample size for each month. 



the difference in females was less pronounced in New 

 Caledonia (55.8%) than in Tonga (95.1%). 



The left and right ovaries showed no significant 

 difference in oocyte diameters between the left and 

 right ovaries in the paired /-test (T 005(2) - 83 =1.902). 

 Only one of the 150 ovary pairs examined revealed a 

 difference in maturity stages, with the left and 

 smaller ovary being classified as developing and the 

 right and larger as late developing. 



Discussion 



Our data indicate that albacore caught near New 

 Caledonia and Tonga are seasonal spawners, spawn- 

 ing mainly during the austral summer months, No- 

 vember-February. This supports the work on albacore 

 reproduction in the South Pacific by Otsu and Hansen 

 (1962). Nishikawa et al. (1985) and Ueyanagi (1969) 

 analyzed larval data and determined that spawning 

 was greatest in spring and early summer (October- 

 December). Few larval surveys, however, were under- 

 taken later in the season, January-March in the spawn- 

 ing area ( see Fig. 37 in Nishikawa et al. , 1985 ). It should 

 be noted that albacore larvae are reported to occur south 

 of 10°S in the South Pacific for all months except July- 

 September (Nishikawa et al., 1985), indicating that 

 spawning may be protracted. Leis et al. (1991) found 

 high concentrations of albacore larvae near the islands 

 of French Polynesia ( 14°-17°S) in January and Febru- 



ary, corresponding to the period of peak reproduction 

 period observed in this study. 



From the distribution of larval and juvenile alba- 

 core, various authors have shown that the spawning 

 area of South Pacific albacore lies between 10° and 

 25°S (Otsu and Hansen, 1962; Nishikawa et al., 1985; 

 Ishii and Inoue, 1956; Ueyanagi, 1969). In this study, 

 most maturing albacore (i.e. with GSI's above 1.7) 

 were taken between 20° and 23°S. 



Both the New Caledonia and the Tonga GSI data 

 support the view that female albacore reach matu- 

 rity at a minimum length of around 82 cm (Fig. 5). 

 Although, GSI values alone may not present a full 



