108 
Fishery Bulletin 11 7(1-2) 
al., 1990). Additionally, growth pa¬ 
rameters of sharks can vary as a con¬ 
sequence of population density (e.g., 
Sminkey and Musick, 1995; Carlson 
and Baremore, 2003; Cassoff et al., 
2007). Therefore, estimation of growth 
parameters requires that the effects of 
these sampling and technical biases be 
taken into consideration. 
It has been considered that the 
growth parameters described by Na- 
kano (1994), for blue sharks of the 
North Pacific Ocean, are representa¬ 
tive of the life history of this species 
and for this population (ISC, 2017), 
given that a relatively large number 
of samples were collected over a wide 
area throughout the North Pacific 
Ocean in 1982 and 1983. However, 
stock biomass of blue sharks in the 
North Pacific Ocean decreased in the 
1980s, reached their lowest level in 
the early 1990s, and increased from 
the mid-1990s to 2005 (Hiraoka et al., 
2016; Ohshimo et al., 2016; ISC, 2017). 
Therefore, it is possible that life his¬ 
tory parameters, such as growth rate 
and age at maturity, of this population 
had changed over these 3 decades be¬ 
cause stock biomass fluctuated widely. 
Accordingly, our objectives were to 
determine 1) present-day sex-specific 
growth parameters of blue sharks in 
the western North Pacific Ocean, on 
the basis of analysis of vertebrae of a 
wide size range collected from a large 
geographic area throughout the year, 
and 2) if any change in growth rate 
had occurred over the last 3 decades. 
Materials and methods 
Sample collection 
Blue sharks were captured between 
2010 and 2016 by Japanese research 
vessels (longline, driftnet, and trawl) 
and by commercial longliners operat¬ 
ing in the western North Pacific Ocean 
(Fig. 1A). Sex was determined by pres¬ 
ence or absence of the male copulatory 
organs (claspers). Precaudal length 
(PCL), the distance from the tip of the 
snout to the precaudal pit, and dorsal 
length (DL), the distance from the ori¬ 
gin of the first dorsal fin to the origin 
of the second dorsal fin, in a natural 
position were measured to the nearest 
centimeter for specimens collected by 
50°N - 
; 
- . ;; fiffi 
*• * ‘ 
40°N 
CD 
T3 
3 
30°N 
20°N 
10°N 
Pacific Ocean 
• Male (n=659) 
o Female {n- 620) 
120°E 130°E 
140°E 
- 1 - 1 
150°E 160“E 
Longitude 
i i r 
170°E 180° 170°W 
B 
200 
150 
b Male (n=659) 
a Female (n- 620) 
o 
c 
CD 
3 
cr 
CD 
100 
50 - 
50 100 150 200 
Precaudal length (cm) 
250 
Figure t 
(A) Map of sampling locations and (B) length-frequency distribution for 
blue sharks {Prionace glauca ) captured between 2010 and 2016 in the 
western North Pacific Ocean. The letter n refers to the number of samples 
used for growth analysis. 
