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Fishery Bulletin 107(4) 
The relationships between batch fecundity (BF) and 
EFL, and between BF and RW were BF = 3.29 x 10 -12 
EFL 5 31 (r 2 = 0.70; Fig. 9A) and BF = 1.59xlO- 3 RW 173 
(r 2 = 0.67; Fig. 9B), respectively. The batch fecundity of 
blue marlin was size related, and fecundity increased 
nonlinearly with body size. The relative fecundity of 
blue marlin ranged from 115 to 25 mature eggs per 
gram of female body weight (55.45 ±3.36). 
Spawning frequency 
The spawning fractions were estimated as the propor- 
tion of mature females with POFs or hydrated oocytes 
during the spawning season (May to September, 150 
days). There was no significant difference in the 
spawning fraction among months within the spawn- 
ing season for the postovulatory follicle and hydrated 
oocyte methods (x 2 =3.62, df=4, P>0.05; x 2 = 1.97, df=4, 
P>0.05). There was also no significant difference in 
the spawning fraction between different size groups 
Monthly variation in the gonadosomatic index (GSI; boxplots, 
left axis) and condition factor (mean= solid-line diamonds, and 
interquartile range = dashed-line diamonds, right axis) for (A) 
female and (B) male blue marlin ( Makaira nigricans ) collected 
at the Tungkang fish market between September 2000 and 
December 2001 (monthly data combined over years). The num- 
bers indicate how many individuals were examined. 
(<200 and >200 cm EFL) for these methods (x 2 = 0.29, 
df=l, P>0.05; x 2 = 0.03, df=l, P>0.05). Finally, there 
was no difference between the two methods in terms 
of the monthly spawning fraction (chi-squared inde- 
pendence tests; % 2 = 3.54, df=4, P>0.05). The spawning 
fraction of mature females based on the postovulatory 
method was 0.41 (n = 164; data combined over months), 
which indicates that each female would have spawned 
on average once every 2.4 days, or 62 times during 
the spawning season (Table 2). In contrast, 34% of 
the mature females had hydrated oocytes during the 
spawning season, which is equivalent to a mean spawn- 
ing interval of 2.9 days, or a spawning frequency of 
51 times (Table 2). Annual fecundity was estimated 
as 120-769 million eggs based on the product of batch 
fecundity and the average spawning frequency from 
the two methods (57 times). 
Discussion 
Size distribution and sex ratio 
The sizes of blue marlin caught by Taiwanese 
offshore longliners in the western Pacific Ocean 
were primarily between 140 and 220 cm EFL. 
However, blue marlin are sexually dimorphic; 
animals 170 cm and less are generally males 
and those larger than 180 cm are generally 
female. Shung (1975) obtained similar results 
for blue marlin in the South China Sea (Pratas 
Islands). Reproductively active male blue marlin 
in the eastern Pacific Ocean are often smaller 
than 220 cm EFL, but all animals 230 cm EFL 
and larger are female (Kume and Joseph, 1969). 
Several billfish species, such as sailfish (Chiang 
et al., 2006b), blue marlin (Wilson et al., 1991) 
and swordfish (DeMartini et al., 2000; Wang 
et al., 2003) have been shown to exhibit sexu- 
ally dimorphic growth. Several hypotheses have 
been proposed to explain this, including 1) sex 
change or a sex-specific mortality rate (deSylva, 
1974); 2) sex-specific growth rates (Wilson et 
al., 1991; Sun et al., 2002); and 3) sex-specific 
natural mortality rates (Skillman and Yong, 
1976; Sun et al., 2005). Histological analyses 
indicated that sex-change does not occur in blue 
marlin. Furthermore, Chen (2001) showed that 
growth of blue marlin differs between females 
and males. Thus, this dimorphism is most likely 
caused by sex-specific growth and mortality 
rates. However, there is a need to examine the 
degree to which sexual dimorphism is due to 
each of these latter two factors. Departure from 
a 1:1 sex ratio is not expected for most fish spe- 
cies even though females dominate the large size 
classes. The sample was male-biased for the non- 
spawning season. However, the sex ratio during 
the putative spawning season (May-September) 
was more balanced (x 2 =0.07, df=l, P>0.05). This 
