254 
Fishery Bulletin 111(3) 
Table 1 
Period of sampling, ranges of fork lengths of tuna sampled, number of females sampled in each survey, and number of fe- 
males used in different analyses for our study of the reproductive potential of Yellowfin Tuna ( Thunnus albacares ) in the 
western Indian Ocean. Analyses included the histological analysis of ovaries (Hist.), analysis of fecundity (Fee.), and analysis 
of the condition indices of gonadosomatic index (GSI), hepatosomatic index (HSI), and condition factor (K). 
Survey 
Sampling period 
Length (cm) 
Females 
sampled 
Hist. 
Fee. 
GSI 
HSI 
K 
1 
22/01/2009-23/03/2009 
37-158 
114 
110 
6 
106 
105 
106 
2 
5/06/2009-25/07/2009 
30-161 
95 
95 
5 
95 
77 
95 
3 
03/04/2010-21/05/2010 
31-157 
114 
114 
- 
95 
94 
95 
Cannery 
12/01/2010-13/04/2010 
61-147 
692 
500 
31 
511 
- 
511 
described in Zudaire et al. (2013). On the basis of ter- 
minology used in Brown-Petersen et al. (2011) and ap- 
plied to Yellowfin Tuna (Zudaire et al., 2013), the ova- 
ries were classified according to the most advanced 
oocyte stage present in the ovary: immature phase 
(including the primary growth stage [PG]), developing 
phase (including the cortical alveolar [CA], primary 
vitellogenesis [Vtgl], and secondary vitellogenesis 
[Vtg2] stages), spawning-capable phase (including the 
tertiary vitellogenesis [Vtg3], germinal vesicle migra- 
tion [GVM], and hydration stages), and regenerating 
phase. The different stages of alpha-atresia were clas- 
sified according to Zudaire et al. (2013). The ovaries 
collected at the cannery were not included in the 
analyses of atresia because they had been exposed to 
brine preservation used on board purse seiners, mak- 
ing it impossible to quantify alpha-atresia precisely. 
The ovaries collected at sea and at the cannery were 
analyzed for the identification of postovulatory fol- 
licles; however, no postovulatory follicles were found 
in these ovaries. 
Length at 50% maturity 
All females included in this analysis were staged histo- 
logically. L 50 was calculated by fitting the proportion of 
mature females by 5-cm size classes to a logistic equa- 
tion (Ashton, 1972; Saborido-Rey and Junquera, 1998): 
^mature = e^ L /l+e a ^ L , 
where P ma ture = the predicted proportion of mature 
females; 
L = the FL in centimeters; and 
a and p are the coefficients of the logistic 
equation. 
The L 50 was estimated as the ratio of the coefficients 
(-a*p -1 ). A nonlinear regression (the Marquardt meth- 
od without restrictions; Marquardt, 1963) was used to 
fit the logistic equation to the data. The curve of L 50 
was estimated on the basis of the assumption that fe- 
males with ovaries at the cortical alveolar stage on- 
ward were mature (Brown-Peterson et al., 2011). A 
second criterion was used to enable comparisons of our 
results with results reported in previous publications 
(Schaefer, 1998; Itano 2 ; Zhu et al., 2008): females with 
cortical alveolar oocytes at the most advanced develop- 
mental stage were considered immature and females 
with advanced vitellogenic oocytes were considered 
mature. 
Condition indices 
Three condition indices were measured to estimate the 
condition of females: the gonadosomatic index (GSI), 
hepatosomatic index (HSI), and condition factor (K). 
These 3 indices were defined in this manner: 
GSI = (W g /W) x 10 2 ; 
HSI=(W[/W) x 10 2 ; 
K=(W / L 3 ) x 10 2 ; 
where Wg = gonad weight; 
W[ = liver weight; 
W = fish gonad-free weight in grams; and 
L - FL in centimeters. 
The seasonal variations in those indices and their ef- 
fect on the reproductive cycle were analyzed by month. 
To identify possible physiological changes during the 
ontogeny of Yellowfin Tuna, the seasonal develop- 
ment of the 3 condition indices was analyzed in 2 size 
groups: fish <100 cm FL and fish >100 cm FL — the 
Lso’s adopted by the Indian Ocean Tuna Commission 
[IOTC 1 ]). Tuna sampled at the cannery were not in- 
cluded in the HSI analysis because the livers of these 
specimens were not weighed. 
Fecundity estimation 
Batch fecundity (BF), the total number of oocytes re- 
leased per batch, was estimated for 40 ovaries with the 
gravimetric method (Hunter et al., 1989) by counting 
the oocytes in the germinal vesicle migration or hy- 
dration stages. Homogeneity in oocyte density among 
