Chen et al.: A modeling approach to identify optimal habitat and suitable fishing grounds for Ommastrephes bartram/i 
5 
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Sea surface temperature (°C) 
Sea surface salinity (psu) 
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Sea surface height anomaly (cm) 
Chlorophyll-a concentrations (mg/m 3 ) 
Figure 3 
The total fishing effort measured in days (d) for Ommastrephes bartramii in the Northwest Pacific Ocean by the 
Chinese squid jigging fleets in relation to (A) sea surface temperature, ( B ) sea surface salinity, (C) sea surface height 
anomaly, and (D) chlorophyll-a concentrations during August 1999-2004. 
to 2004 were also produced according to ranges of calcu- 
lated HSI values (i.e., HSI = [0-0.2]; [0.2-0. 4]; [0.4-0. 6]; 
[0.6-0. 8]; and [0. 8-1.0]). Therefore, we assumed that a 
positive linear relationship exists between the value of 
HSI and fishing effort. The model can be written as 
Y = a+bX, 
where Y = the percentage of the fishing effort corre- 
sponding to different HSI values calculated 
for the same time; 
X = the HSI value; and 
a and b are the two parameters to be estimated. 
The performance of different HSI models with one, two, 
three, and four environmental variables were evalu- 
ated and compared to identify the most suitable HSI 
model based on the Akaike’s information criterion (AIC; 
Akaike, 1981). 
The model that yielded the minimum AIC value 
was selected as the best model. This model was then 
used for model testing and validation. The spatial 
distributions of HSI values derived from the above 
selected HSI model in 2005 were mapped with Ma- 
rine Explorer, vers. 4.0 (Environmental Simulation 
Laboratory Co, Ltd, Saitama, Japan) for forecasting 
potential fishing grounds and were compared with 
the actual fishery data from the Chinese squid jigging 
fleets in 2005. 
Results 
Squid catch in relation to environmental variables 
During August, fishing effort was highest in waters 
with SSTs ranging from 17° to 20°C (Fig. 3A), and the 
preferred SST tended to be centered at 19-20°C. High 
fishing effort (>2000 days) with respect to SSS occurred 
in areas where sea surface salinity (SSS) varied from 
33.1 to 33.5 (Fig. 3B), and where the preferred SSS was 
between 33.3 and 33.4. We also found that high fishing 
effort (>2000 days) related to SSHA and chi a occurred 
in waters with SSHA ranging from -20 cm to 5 cm 
and with chl-a values between 0.2 to 0.4 mg/m 3 . The 
optimum SSHA and SSS tended to be between -5 and 
0 cm and between 0.3 and 0.4 mg/m 3 (Fig. 3, C and D). 
Similar results were shown for September and October 
(Figs. 4 and 5). 
The spatial distribution of fishing effort for O. bar- 
tramii from August of 2004 is presented in Figure 6 
to show its relationship with environmental variables 
SST, SSS, SSHA, and chi a. The center of fishing ar- 
eas with a high aggregation of squid occurred in the 
waters of 42°-44° N latitude and 154°-157°E longitude 
(Fig. 6). The environmental maps of four variables 
indicated that squid were aggregated mostly in warm 
water near the 19°C SST isotherm (Fig. 6A) and 33.3 
psu SSS isohaline (Fig. 6B), in the edge of a cold ring 
near the -5 cm SSHA (Fig. 6C), and in relatively high 
