The reproductive biology
of blue marlin (Makaira nigricans)
was assessed from 1001 fish (ranging
from 121 to 275 cm in eye-to-fork
length; EFL) caught by Taiwanese
offshore longliners in the western
Pacific Ocean from September 2000
to December 2001 and from 843 gonad
samples from these fish, The overall
sex ratio of the catch was approximately
1:1 dur ing the sampling
period, but blue marlin are sexually
dimorphic; females are larger than
males. Reproductive activity (assessed
by histology), a gonadosomatic index,
and the distribution of oocyte diameters,
indicated that spawning occurred
predominantly from May to September.
The estimated sizes-at-maturity
(EFL50) were 179.76 ±1.01 cm (mean
±standard error) for females and 130
±1 cm EFL for males. Blue marlin are
multiple spawners and oocytes develop
asynchronously. The proportion of
mature females with ovaries containing
postovulatory follicles (0.41) and
hydrated oocytes (0.34) indicated that
the blue marlin spawned once every
2–3 days on average. Batch fecundity
(BF) for 26 females with the
most advanced oocytes (≥1000 μm),
but without postovulatory follicles,
ranged from 2.11 to 13.50 million
eggs (6.94 ± 0.54 million eggs). The
relationships between batch fecundity
(BF, in millions of eggs) and
EFL and round weight (RW, kg) were
BF = 3.29 × 10 –12 EFL5.31 (r2 = 0.70)
and BF = 1.59 × 10–3 RW 1.73 (r2= 0.67),
respectively. The parameters estimated
in this study are key information
for stock assessments of blue
marlin in the western Pacific Ocean
and will contribute to the conservation
and sustainable yield o
