Sun et al.: Reproductive biology of Makaira nigricans 
427 
TabBe 2 
Spawning fraction and spawning frequency during the spawning season determined using the postovulatory follicle (POF) and 
hydrated (HY) oocyte methods for female blue marlin (Makaira nigricans) in the western Pacific Ocean. EFL is the eye-to-fork 
length measurement . 
Month 
n 
POF method 
HY method 
Ovary 
with POFs 
Spawning 
fraction 
Spawning 
frequency 
Ovary with 
HY oocytes 
Spawning 
fraction 
Spawning 
frequency 
May 
20 
5 
0.25 
4.00 
8 
0.40 
2.50 
Jun 
26 
12 
0.46 
2.17 
6 
0.23 
4.33 
Jul 
39 
17 
0.44 
2.29 
17 
0.44 
2.29 
Aug 
46 
25 
0.54 
1.84 
16 
0.35 
2.88 
Sep 
33 
9 
0.27 
3.67 
9 
0.27 
3.67 
Size 
<200 cm EFL 
86 
39 
0.45 
2.21 
28 
0.33 
3.07 
>200 cm EFL 
78 
29 
0.37 
2.69 
28 
0.36 
2.79 
Total 
164 
68 
0.41 
2.41 
56 
0.34 
2.93 
o 
o 
1.0 
0.8 
0.6 
0.4 
0.2 
- 100 
-so a 
w 
co 
CD 
" 60 
- 40 
- 0 
Jan Mar May Jul 
Month 
Sep 
Nov 
o 
20 2j 
Figure S 
Monthly variation in the mean diameter of the most advanced 
group of oocytes (MOD; boxplots, left axis) and the percentage of 
vitellogenic ovaries (excluding the samples with atretic oocytes) 
(triangles, right axis) for blue marlin (Makaira nigricans ) col- 
lected at the Tungkang fish market between September 2000 
and December 2001 (monthly data combined over years). The 
numbers indicate how many ovaries were examined. 
may imply that mature females migrate to the 
spawning grounds during the spawning season. 
The exact spawning area of blue marlin in the 
western Pacific Ocean needs to be identified 
in the future in order to fully evaluate this 
hypothesis. 
Maturity classification and gonad maturation 
Misclassification of whether a female is mature 
will contribute error to the estimation of the 
maturity ogive, spawning season, batch fecun- 
dity and spawning frequency, and hence to 
uncertainty when estimating size-at-maturity 
and egg production. In this study, the use of 
histological techniques to study gonad matu- 
ration provided a more precise outcome than 
have traditional macroscopic techniques. The 
spawning season for females was estimated 
by using GSI and histology and by identifying 
oogenesis as well as the temporal variation of 
the mean diameter of the most advanced group 
of oocytes. The size-frequency distributions of 
whole oocytes (cross calibrated with histologi- 
cal characteristics) for the different maturation 
stages provided a fuller understanding of the 
dynamics of ovarian maturation and the spawn- 
ing pattern of a fish that produces multiple 
batches. The size-frequency distributions of whole 
oocytes, which can be constructed by using a dissecting 
microscope, can serve as a quick way to determine the 
ovarian development stage when histological data are 
not available. The condition factor is usually related to 
fish health, and a roughly inverse pattern between the 
temporal variation of the GSI and condition factor was 
found in male blue marlin, which may imply a lower 
feeding activity during the spawning season as has 
been argued for some other pelagic migratory fishes 
(i.e., school mackerel, Begg and Hopper, 1997). 
Size-at-maturity 
The EFL 50 for female blue marlin was estimated to be 
179.76 cm, and the smallest size at which any female 
was mature was 157.8 cm EFL. For males, there was 
considerable uncertainty regarding EFL 50 owing to a 
