Sun et al.: Reproductive biology of Makaira nigricans 
425 
is a valid index for determining reproductive activity. 
Few ovaries containing vitellogenic oocytes (exclud- 
ing the samples with atretic oocytes) were observed 
in January, February, and November. However, there 
was evidence for yolk accumulation between March 
and September based on the monthly variation of the 
percentage of vitellogenic ovaries and the MOD (Fig. 5). 
Mature ovaries were first seen in March, and females 
with spawning stage ovaries were observed from May 
to September (Fig. 6A). Postspawning females (i.e., the 
recovery stage) were observed from May to December. 
For males, mature testes were observed throughout 
the sampling period, and males with spawned testes 
were observed from March to December (Fig. 6B). The 
information in Figures 4, 5, and 6 together imply that 
the major spawning season for blue marlin in western 
Pacific is from May to September. 
Spawning pattern 
That oocytes of various developmental stages were pres- 
ent at the same time in an ovary was based on histologi- 
cal examination and is also evident from Figure 7, which 
shows that there were several modes in the distribu- 
tions of oocyte diameter. There was a single distribution 
mode for diameters of early-stage oocytes: 25-35 pm 
for chromatin nuclear oocytes and 45-165 pm for peri- 
nuclear oocytes of immature fish. Two modes (cor- 
responding to CN and PN ) were evident in the oocyte 
distribution for resting fish (Fig. 7B). The number of 
previtellogenic oocytes (PV; 190-330 pm) began increas- 
ing with the development of the ovary for maturing 
fish (Fig. 70. Vitellogenic oocytes (VT; 250-800 pm) 
and hydrated oocytes (800-1200 pm) appeared in the 
oocyte distribution when fish matured, and ocytes of 
several stages were present in the oocyte diameter 
distribution for these fish (Fig. 7D). However, there 
was a gap at an oocyte diameter at roughly 1000 pm. 
This gap indicates that oocytes larger than this size 
may be spawned soon (Fig. 7D). Modes corresponding 
to vitellogenic and unovulated hydrated oocytes were 
observed with the appearance of postovulatory follicles 
after spawning (Fig. 7E). PNs were most abundant for 
fish in the recovering stage, although there were also 
some PVs and VTs in their ovaries (Fig. 7F). Overall, 
Figure 7 indicates that oocytes grew in an asynchro- 
nous manner and that individual female blue marlin 
spawn multiple times during the spawning season. 
Size-at-maturity 
Maturity ogives were estimated for females and males 
caught during the spawning season (May to September). 
The relationship between the fraction mature and size 
can be described by a logistic curve with lengths at 50% 
and 95% maturity ( EFL 50 and EFL 95 ) of 179.76 ±1.01 
cm EFL (estimate ±standard error, SE) and 194.2 ±1.01 
cm EFL ( /? = 394 ), respectively. For males, EFL 50 and 
EFL 95 were 130 ±1 and 130.13 ±46.56 cm EFL (n = 442, 
Fig. 8). 
Figure 3 
Developmental stages and spermatogenic cells observed 
in blue marlin ( Makaira nigricans ) testes. (A) SG, 
spermatogonia; IB) SCI, primary spermatocytes; SC2, 
secondary spermatocytes; (C) ST, spermatids; SZ, sper- 
matozoa. Scale bars: 100 pm. 
Batch fecundity 
There was a clear gap in the oocyte distribution for 
mature ovaries at 1000 pm (Fig. 7D), and the oocytes 
of most advanced mode (composed of the most advanced 
yolked, migratory nucleus, and nonovulated hydrated 
oocytes) were considered to be those of the spawning 
batch. Batch fecundity, estimated for the 26 mature 
ovaries with no early postovulatory follicles, ranged 
from 2.11 to 13.50 million eggs (6.94 ±0.54; mean ±SE). 
