22 
Fishery Bulletin 1 10(1 ) 
170°0'0"W 160°0'0 n W 150WW 140°0'0"W 130 o 0'0"W 
160°0'0"W 150WW 140°0'0"W 
Figure 1 
Map of the northern Gulf of Alaska showing stratum boundaries (gray lines) for the 
Gulf of Alaska bottom trawl survey. The survey is conducted at stratified random 
stations in the area bounded by lines at the Islands of Four Mountains (170°W longi- 
tude, 53°N latitude) and Dixon Entrance (133°W longitude, 55°N latitude). The inset 
shows the station pattern for each survey year since 1996 for the “Snakehead,” an 
area historically important for the commercial rockfish fishery. 
ranged from a low of four in 1996 to 12 in 2009 (Fig. 1). 
Thus, habitats are not sampled in proportion to their 
importance to rockfish in some years. The specific pat- 
Table 1 
Number of Gulf of Alaska bottom trawl survey hauls 
included in the modeling analysis of rockfish abundance 
for each year that a survey was conducted. Also reported 
are the minimum and maximum depths of bottom trawl 
survey tows in each year of the survey. 
Year 
No. of hauls 
Minimum 
depth ( m) 
Maximum 
depth (m) 
1996 
605 
31 
479 
1999 
665 
23 
946 
200H 
376 
24 
448 
2003 
668 
31 
667 
2005 
690 
23 
882 
2007 
660 
31 
903 
2009 
811 
21 
984 
Total 
4475 
1 Only the western Gulf of Alaska was surveyed during this year. 
terns in rockfish habitat use can have a significant 
effect on the accuracy and precision of bottom trawl 
survey biomass estimates (Cordue, 2007). In addition, 
survey designs may change from year to year, such as 
has occurred with the Alaska Fisheries Science Center’s 
Gulf of Alaska bottom trawl survey, where interannual 
variability in agency funding has resulted in dissimilar 
spatial and depth coverage during some survey years 
(Table 1). Habitat and survey-design effects are often 
observed as large coefficients of variation around bio- 
mass estimates for rockfish species and as biologically 
unlikely changes in biomass estimates from year to 
year. Thus, there is a serious need for the inclusion of 
habitat information into methods for estimating annual 
indices of abundance for rockfish species. 
The objective of this project was to develop a generic 
framework to create a habitat-based annual abun- 
dance index for rockfish species in Alaska. First, a 
model of catch per unit of effort in relation to habitat 
variables was developed that also contained an index 
of the year effect on abundance. The residuals from 
this model were then examined to determine whether 
spatial structure was present. We compared the mod- 
el-based annual abundance index to the swept-area 
biomass estimates, where both were derived from the 
