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Fishery Bulletin 107(4) 
EFL 50 and EFL 95 were estimated by maximiz- 
ing a log-likelihood function, and by assuming a 
binomial error distribution with AD Model Builder 
(Fournier, 2000). 
Batch fecundity 
Annual fecundity was estimated from the number 
of oocytes released per spawning (batch fecun- 
dity), the percentage of females spawning per 
day (spawning fraction), and the duration of 
the spawning season (Hunter and Macewicz, 
2003; Murua et al., 2003) because blue marlin 
spawn multiple times during the season and have 
indeterminant fecundity (see Results section). 
There are two methods for estimating the batch 
fecundity: 1) the hydrated oocyte method; and 
2) the oocyte size-frequency method (see Hunter 
et al., 1985). The oocyte size-frequency method 
was employed because the sample size for the 
batch fecundity estimates from the hydrated 
oocyte method is insufficient for later analy- 
sis of the relationship between batch fecundity 
and EFL or RW (fewer samples have migratory 
nucleus and hydrated oocytes been observed). 
The most advanced yolked, migratory nucleus, 
and hydrated oocytes that were destined to be 
spawned were identified for individuals on the 
basis of size-frequency distributions of whole oocytes 
(larger than 1000 microns). Batch fecundity was 
then back-calculated gravimetrically by the product 
of gonad weight and oocyte density, where oocyte 
density was the mean number of oocytes per gram of 
three ovarian tissues with no early postovulatory fol- 
licles (Hunter et al., 1985). Relative batch fecundity 
was expressed as batch fecundity divided by the round 
weight of the fish. 
Spawning frequency 
The spawning frequency of blue marlin was estimated 
indirectly by the inverse of the spawning fraction 
because direct monitoring of the spawning frequency 
of pelagic fish during the spawning season is dif- 
ficult. The spawning fraction was calculated as the 
proportion of fish that spawned each day during the 
spawning season. There are two approaches to deter- 
mine this average (Hunter and Macewicz, 1985): 1) 
the postovulatory follicle method; and 2) the hydrated 
oocyte method. Indirect methods for estimating 
spawning frequency are based on four assumptions: 
1) females are spawning asynchronously throughout 
the spawning season; 2) fishes do not immigrate to or 
emigrate from the spawning ground; 3) the POFs of 
blue marlin are histologically detectable for no more 
than 24 hours or all hydrated oocytes are spawned 
in less than 24 hours (as observed for yellowfin tuna; 
Schaefer, 1996); and 4) the POFs do not degenerate 
continuously if a fish is caught and put into the refrig- 
erator immediately. 
Results 
Gonad samples, size distribution, and sex ratios 
No significant differences in MOD were found between 
each lobe or layer within each individual blue marlin 
(split-plot ANOVA; F=0.41, df=l and 52, P>0.05; 
F=1.30, df=2 and 51; P>0.05), although there were 
differences in MOD between the ovaries (F=49.97, 
df=2 and 52, P<0.05). Similar results were found in 
the analysis of oocyte number. Thus the gonad samples 
were collected from the random location of ovaries 
throughout this study (most from the posterior por- 
tion). Of the 1001 sampled fish, 406 were female (size 
range: 124.1-275 cm EFL), 463 were male (size range: 
121-232 cm EFL), and sex could not be determined for 
the remaining 132 fish. Most of the sampled fish were 
between 140 and 220 cm EFL (Fig. 1). Blue marlin 
exhibit sexual dimorphism in growth. Specifically, 
almost all sampled blue marlin larger than 180 cm 
EFL were female (Fig. 1). The relationship between 
the female proportion ( P and EFL can be described 
by the logistic function (Fig. 1): 
p _ ^^ + e -ln(19)[(£FL-175.16)/23.59]j 
The sex ratio for the entire study deviated from the ex- 
pected 1:1 ratio (% 2 = 4.93, df=l, P<0.01) and varied 
among months (more males in the September 2000 
[0.32], October 2000 [0.23], and May 2001 [0.31] col- 
lections, and more females in the November 2001 col- 
lection [0.67]). 
