420 
Abstract— The reproductive biology 
of blue marlin ( Makaira nigricans ) 
was assessed from 1001 fish (rang- 
ing 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 approxi- 
mately 1:1 during 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 diame- 
ters, indicated that spawning occurred 
predominantly from May to Septem- 
ber. The estimated sizes-at-maturity 
(EFL 50 ) 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 contain- 
ing postovulatory follicles (0.41) and 
hydrated oocytes (0.34) indicated that 
the blue marlin spawned once every 
2-3 days on average. Batch fecun- 
dity (BF) for 26 females with the 
most advanced oocytes (>1000 pm), 
but without postovulatory follicles, 
ranged from 2.11 to 13.50 million 
eggs (6.94 ±0.54 million eggs). The 
relationships between batch fecun- 
dity (BF, in millions of eggs) and 
EFL and round weight (RW, kg) were 
BF = 3.29 xlO- 12 EFL 5 31 (r 2 = 0.70) 
and BF = 1.59 x 10~ 3 RW 1 73 (r 2 =0.67), 
respectively. The parameters esti- 
mated in this study are key infor- 
mation for stock assessments of blue 
marlin in the western Pacific Ocean 
and will contribute to the conserva- 
tion and sustainable yield of this 
species. 
Manuscript submitted 29 September 2008. 
Manuscript accepted 3 June 2009. 
Fish. Bull. 107:420-432 (2009). 
The views and opinions expressed 
or implied in this article are those 
of the author and do not necessarily 
reflect the position of the National Marine 
Fisheries Service, NOAA. 
Reproductive biology of 
blue marlin ( Makaira nigricans) 
in the western Pacific Ocean 
Chi-Lu Sun (contact author) 
Yi-Jay Chang 
Chien-Chung Tszeng 
Su-Zan Yeh 
Nan-Jay Su 
Email address for contact author: chilu@ntu.edu. tw 
Institute of Oceanography 
National Taiwan University 
1 Sec 4 Roosevelt Rd 
Taipei, Taiwan 10617 
Blue marlin ( Makaira nigricans ) are 
widely distributed throughout the 
tropical and subtropical waters of the 
Pacific and Indian oceans (Nakamura, 
1985). In the Pacific, blue marlin are 
harvested mainly by longline fisher- 
ies targeting tunas. Genetic studies 
(Buonaccorsi et al., 1999) and fishery 
data (Kleiber et al., 2003) indicate 
that there is a single stock of blue 
marlin in the Pacific Ocean. Annual 
landings of blue marlin in the west- 
ern and central Pacific over the past 
decade have been stable at about 
13,000 metric tons. However, stock 
assessments of Pacific blue marlin 
are uncertain, with results ranging 
from the stock being close to fully 
exploited (Kleiber et al., 2003), over- 
fished (Yuen and Miyake, 1980), or in 
a healthy state (Hinton, 2001). 
Quantifying the reproductive po- 
tential of blue marlin is important 
for understanding the population dy- 
namics of this species and for stock 
assessment purposes. For example, 
estimates of the size- and age-at- 
sexual-maturity are necessary inputs 
for age- and size-structured stock as- 
sessments models (Quinn and Deriso, 
1999). Despite the size of the fisher- 
ies for the stock of blue marlin in the 
Pacific Ocean, there have been few 
published studies on the reproductive 
biology of this stock. In the eastern 
Pacific, Kume and Joseph (1969) es- 
timated the size-at-maturity of blue 
marlin from data from the Japanese 
longline fishery, and Hopper (1990) 
described spawning activity around 
the Hawaiian Islands. In the western 
Pacific, Nakamura (1944) reported 
that blue marlin spawned off Tai- 
wan. However, none of these stud- 
ies provided detailed information on 
ovarian development, even though 
knowledge of gonad development in 
individual fish is needed to establish 
the spawning season, the size- and 
age-at-maturity, and the spawning 
pattern. The reproductive biology of 
blue marlin has been studied more 
extensively in the Atlantic Ocean 
than in the Pacific. For example, La 
Monte (1958) first described the go- 
nad of blue marlin in the Atlantic 
Ocean, Erdman (1968) observed the 
reproductive cycle off Puerto Rico, 
Cyr (1987) defined the gonad devel- 
opment and spawning cycle in the 
northwestern Atlantic Ocean, and 
Arocha and Marcano (2008) esti- 
mated the size-at-maturity of this 
species in the western central Atlan- 
tic. The objectives of this study were 
to evaluate the reproductive biology 
of blue marlin in the western Pacific 
Ocean. We determine reproductive 
activity and describe ovarian devel- 
opment using histological techniques. 
Key parameters required for stock 
assessments including sex ratio, 
reproductive season, and size-at- 
maturity are also estimated. 
