4 
Fishery Bulletin 108(1 ) 
Figure 2 
Procedure for estimating the habitat suitability index (HSI) for neon flying squid (Ommastrephes bartramii) 
in the Northwest Pacific Ocean and the procedure for selecting the HSI model based on Akaike’s informa- 
tion criterion. 
effort (i.e., equal to 0) in the presence of fishing. The 
SI values between 0 and 1 were assigned to the ranges 
of the corresponding environmental variable (Table 1; 
Brown et al., 2000). 
The SI values derived from each variable were then 
combined into the empirical HSI model (Fig. 2). Two em- 
pirical HSI models, the arithmetic mean model (AMM) 
and the geometric mean model (GMM) are commonly 
used to estimate habitat availability (U.S. Fish and 
Wildlife Service, 1980a, 1980b; Hess and Bay, 2000; 
Lauver et al., 2002; Chen et al., 2009). The HSI is a 
univariate variable also having a value between 0 and 1 
Table 1 
Definitions of suitability index values for Ommastrephes 
bartramii based on the fishing effort of Chinese squid jig- 
ging fleets in one fishing unit of 0.5° latitude x 0.5° longi- 
tude in the Northwest Pacific Ocean. 
Suitability 
Description of 
index value 
habitat use 
1 
The highest fishing effort in the 
Chinese squid jigging fishery 
0.5 
Common occurrence or average fishing 
effort in the Chinese squid jigging 
fishery (between 2000 fishing days and 
the highest fishing effort) 
0.1 
Rare occurrence or low fishing effort in 
the Chinese squid jigging fishery 
(fewer than 2000 fishing days) 
0 
Fishing effort is zero in the Chinese 
squid jigging fishery (0 fishing days) 
(Brooks, 1997). The two empirical HSI models were 
described as follows: 
AMM (Hess and Bay, 2000; Chen et al., 2009): 
HSIamm = -X S7 ‘ ; ( 2 ) 
n i = 1 
and GMM (Lauver et al., 2002; Chen et al., 2009): 
n 
HSI GMM =(Y\sif n ; (3) 
i=i 
where S/ ; = the SI for i th environmental variable; 
n = the number of environmental variables used 
in the model; 
i = 1, 2, ..., and n. 
On the basis of previous studies on the relationship 
between environmental variables and squid catch, we 
considered SST as the primary variable for identifying 
habitat for O. bartramii and used different combina- 
tions of one (SST), two (SST and one other variable), 
three (SST and two other variables), and four (SST, 
SSS, SSHA, and chi a) variables as habitat data. The 
SI values derived from different combinations of habi- 
tat variables were then combined into the HSI model 
(Fig. 2). 
Selection of HSI model and validation 
The monthly HSI values from August to October of 
1999-2004 were estimated by the approach described 
above. The percentages of total fishing effort from 1999 
