Chen et ah: A modeling approach to identify optimal habitat and suitable fishing grounds for Ommastrephes bartramn 
9 
to the AMM and GMM models, respectively (Fig. 7A), 
and produced 16.02% and 41.39% of the total fishing 
effort according to the AMM and GMM models accord- 
ingly (Fig. 7B). 
Moreover, the monthly CPUEs from 1999 to 2004 
were compiled and calculated according to the grouping 
of AMM-based and GMM-based HSI values estimated 
from the three environmental variables (SST, SSHA, 
and chi a). The CPUE values were found to increase 
with the AMM-based HSI value, but the CPUE was not 
the same for the GMM-based HSI value (Fig. 7C). When 
the AMM-based HSI values for an area ranged from 0 
to 0.2, the average CPUE was only equal to 1.44 ±0.34 
t/d (mean ±standard deviation). For the areas with HSI 
values ranging between 0.6 and 0.8 and higher than 
0.8, the average CPUEs were 2.50 ±0.26 t/d and 3.01 
±0.59 t/d, respectively (Fig. 7C). All the results from 
the fishery data from 1999 to 2004 indicated that the 
AMM model was more suitable than the GMM model for 
estimating the HSI for O. bartramii, as we assumed. 
HSI model validation 
With the HSI value estimated from the AMM model in 
2005, we mapped the spatial distribution of monthly 
HSI values, fishing locations, and CPUEs (Fig. 8). The 
HSI values >0.6 were mainly found in the areas of 
152°30'-156°30'E longitude and 42°30'-44°00'N latitude, 
and 156-159°E longitude and 40°30'-42°30'N latitude 
(Fig. 8A), in which the catch and fishing effort occupied 
78.17% and 65.17% of the total catch and total effort, 
respectively (Fig. 9, A and B) and the average CPUE 
was 3.51 t/d in August (Fig. 9C). In September, the HSI 
values >0.6 were widely distributed in the waters of 
150°30'-151°30'E longitude and 40°30'-41°30'N latitude, 
and 152-165°E longitude and 40°30’-43°30'N latitude 
(Fig. 8B), in which the catch and fishing effort were 
96.36% and 93.19% of the total catch and effort, respec- 
tively (Fig. 9, A and B) and the average CPUE was 3.77 
t/d (Fig. 9C). However, there was no fishing activity in 
the area between 156-160°E longitude and 40°30'-42°N 
latitude, and 160-165°E longitude and 40°30'-43°30'N 
latitude (Fig. 8B). In October, the HSI values >0.6 were 
located in the areas of 152°30'-156°E longitude and 
42°30'-44°30'N latitude, 156-157°45E longitude and 
42°-43°N latitude, 158°-160°E longitude and 41°-43°N 
latitude, and 162°-163°30'E longitude and 43-44°30'N 
latitude (Fig. 8C), where the catch and fishing effort 
were 68.90% and 68.16% of the total catch and efforts 
(Fig. 9, A and B), respectively, and the average CPUE 
was 3.10 t/d (Fig. 9C). These results indicate that AMM 
can yield a reliable prediction of the potential fishing 
ground for O. bartramii. 
Discussion 
The biophysical environments in the transition zone 
region of North Pacific Ocean have been hypothesized 
to influence the migration, distribution, and abundance 
HSI 
HSI 
Figure 7 
The relationship between the habitat suitability index 
(HSI) values estimated from the arithmetic mean model 
(AMM) and the geometric mean model (GMM), and (A) 
actual percentage of catch, (B) actual percentage of 
fishing effort, and (C) catch per unit of effort (CPUE) 
for Ommastrephes bartramii from 1999 to 2004 in the 
Northwest Pacific Ocean. 
of O. bartramii (Tian, 2006; Ichii et al., 2009). Within 
more specific ranges, the highest density of O. bartramii 
was found in waters with favorable ranges of SST, SSS, 
SSHA, and chi a (Table 2). These ranges can be consid- 
ered indicators of areas with the highest probability of 
finding O. bartramii. The highest squid abundance or 
optimum habitat were concentrated around the 19-20°C 
SST isotherm, the 33.3-33.4 psu SSS isohaline, and the 
0.3mg/m 3 chl-a isopleth in August; 16-17°C SST, 33.3- 
33.4 psu SSS, and 0.4-0. 5 mg/m 3 chi a in September; 
