Zudaire et al. : Reproductive potential of Thunnus albacares in the western Indian Ocean 
253 
Figure t 
Map of locations where female Yellowfin Tuna (Thunnus albacares ) were sam- 
pled in the first survey (22 January-23 March 2009), second survey (6 May- 
25 July 2009), and third survey (12 January-13 April 2010). The surveys were 
conducted aboard a commercial purse seiner in the western Indian Ocean to 
study the reproductive potential of Yellowfin Tuna in this region. 
tion to sea-surface temperature (>24°C), 
which seems to regulate spawning activ- 
ity (Schaefer, 2001). In the Indian Ocean, 
spawning mainly occurs in the equatorial 
area (0— 10°S) from December to March, 
and the main spawning grounds have 
been observed west of 75°E (IOTC 1 ). 
However, other authors have reported 
variations in the spawning season of 
this species, describing periods between 
January and June (Zhu et al., 2008). 
Stequert et al. (2001) related reproduc- 
tive activity of Yellowfin Tuna with the 
north monsoon (main spawning period) 
from November to March and with the 
south monsoon (less reproductive activ- 
ity) from June to August. Similarly, dif- 
ferent estimates of maturity have been 
published for this species in several stud- 
ies that used the same method to iden- 
tify maturation stages. For example, the 
length at which 50% of females mature 
(L50) was estimated at around 100 cm in 
fork length (FL) for Yellowfin Tuna in the 
Indian Ocean by the Indian Ocean Tuna 
Commission (IOTC 1 ), at 108 cm FL in 
the western Pacific Ocean by McPherson 
(1991), at 92 cm FL in the eastern Pacif- 
ic Ocean by Schaefer (1998), and at 104 
cm FL for the equatorial western Pacific 
Ocean by Itano. 2 
In spite of the ecological importance 
of Yellowfin Tuna and the significant 
decrease of its catches in various re- 
gions during recent years, there have 
been few studies that have focused on 
the reproductive biology of the Indian Ocean Yellowfin 
Tuna. This study aims to contribute to the knowledge 
of various reproductive traits essential for examination 
of the reproductive potential and population dynamics 
of Yellowfin Tuna in the western Indian Ocean, with an 
estimation of the following qualities: 1) length at 50% 
maturity; 2) spawning season; 3) fecundity; and 4) fish 
condition during reproduction. 
Materials and methods 
Field sampling 
Sampling was done by scientific observers during 3 sur- 
veys conducted on board a commercial purse seiner in 
2 Itano, D. G. 2000. The reproductive biology of yellowfin 
tuna (Thunnus albacares ) in Hawaiian waters and the west- 
ern tropical Pacific Ocean: project summary. Pelagic Fisher- 
ies Research Program (PRFP), Joint Inst. Mar. Atmospheric 
Research (JIMAR), Univ. Hawaii (UH), HI. JIMAR contri- 
bution 00-328, p. 69. JIMAR, MSB 312, 1000 Pope Road, 
Honolulu, HI 96822. 
the western Indian Ocean during 2009 and 2010 (Fig. 
1). At sea, during these surveys, 320 female Yellowfin 
Tuna were sampled. In addition, 692 ovaries from fe- 
male Yellowfin Tuna that were captured by the purse- 
seine fleet that operated in the western Indian Ocean 
during the same period were collected at the Seychelles 
cannery (Table 1). Yellowfin Tuna were identified at sea 
and in the cannery through the use of characters given 
in Collette and Nauen (1983). Fork length (in centime- 
ters), total weight (in kilograms), sex, maturation stage 
and gonad weight (in grams) were recorded for each in- 
dividual. Liver weight (in grams) was measured only in 
females that were sampled at sea. Their ovaries were 
removed and weighed to the nearest gram. A cross sec- 
tion of the gonad of 4-5 cm was cut between the middle 
and end part of the right or left lobe and preserved in 
4% buffered formaldehyde immediately after collection 
until it could be processed in the laboratory. 
In the laboratory, cross sections (~1 cm) from the 
preserved portions of 819 ovaries were embedded in 
resin, sectioned at 3-5 pm, and stained with hematoxi- 
lyn and eosin (Korta, 2010). The histological classifi- 
cation of Yellowfin Tuna ovaries followed the criteria 
