Zudaire et at: Reproductive potential of Thunnus albacares in the western Indian Ocean 
259 
r 2,4 
- 2,2 
- 2 . 0 * 
• 1,8 
- 1,6 
2,8 
- 2,6 
- 2,4 
2,2 * 
2,0 
- 1,8 
- 1,6 
Figure 4 
Mean monthly variation in the gonadosomatic index (GSI), hepatosomatic index 
(HSI), and condition factor ( K ) in our study of the reproductive potential of Yel- 
lowfin Tuna (Thunnus albacares) in the western Indian Ocean for females in 
two groups: (A) >100 cm in fork length (FL) and (B) <100 cm FL 
supported by GSI values over 1.5, the value correspond- 
ing to that fish capable of reproducing (Nootmorn et 
al. 4 ), observed from November to January. The GSI val- 
ues for February and June below 1.5 could correspond 
to a period of lower spawning activity, in which the 
proportion of active females among the mature popula- 
tion decreased. The absence of samples from August to 
October affected the interpretation of the reproductive 
cycle (e.g., the commencement of the main spawning 
period). 
The analysis of the reproductive cycle by size groups 
showed asynchrony in the reproductive activity be- 
tween small and large Yellowfin Tuna. Larger and 
older individuals spawned ear- 
lier and longer in the season and 
had a higher activity than small- 
er individuals. This spawning 
behavior is in accordance with 
McPherson (1991) and Schaefer 
(1998), who described a positive 
relationship between the spawn- 
ing fraction and female size. Our 
results showed asynchrony in the 
appearance of ovaries in the re- 
generating phase between small 
and large fish. Smaller individu- 
als showed high percentages 
of ovaries in this phase earlier 
(December and January) than 
did large specimens (April and 
May). Ovaries of smaller individ- 
uals appeared in the regenerat- 
ing phase within a period (from 
November to February) in which 
females generally exhibit high 
reproductive activity. This behav- 
ior can occur more often among 
young individuals maturing for 
the first time than among older 
fish (Murua et al., 2003), and 
it may be related to the energy 
balance between somatic growth 
and reproduction, an energy bal- 
ance that is size related (Clara- 
munt et al., 2007). 
Another hypothesis that may 
explain the asynchrony in repro- 
ductive timing among sizes is 
that young females skip spawn- 
ing events. The early appearance 
of ovaries in the regenerating 
phase occurred during the main 
peak of reproductive activity 
of mature individuals, and it is 
unlikely to be related to the end 
of the spawning season of young 
fish (Murua and Saborido-Rey, 
2003). Female Yellowfin Tuna 
that skip spawning can be clas- 
sified as younger skipping females (Secor, 2007). In 
these first-maturing females, maturation involves large 
physiological and behavioral transitions. They may not 
have the required energy resources and, therefore, fore- 
go reproduction. By skipping spawning, these females 
increase their growth rate and their chances of sur- 
vival, resulting in increased lifetimes and reproductive 
outputs (Rideout and Tomkiewicz, 2011). However, the 
extended spawning season of Yellowfin Tuna, as well as 
the variable spawning period among individuals, com- 
plicates the identification of skipped spawning seasons 
in the reproductive cycle of females (Lowerre-Barbieri 
et al., 2009). Therefore, further research is needed to 
