Rooper: An ecological analysis of rockfish assemblages in the North Pacific Ocean 
7 
1.0 
Central Gulf 
of Alaska- 
shelf 
Southeastern 
Alaska- 
break 
r 
Aleutian 
Islands-shelf 
Aleutian J 
Islands-break | 
Aleutian 
Islands-slope 
Juvenile silvergray rockfish 
Harlequin rockfish — 
Yelloweye rockfish — 
Redbanded rockfish — 
Rosethorn rockfish 
Adult silvergray rockfish 
Redstripe rockfish 
Sharpchin rockfish 
Juvenile northern rockfish 
Adult northern rockfish 
Dusky rockfish 
Juvenile Pacific ocean perch 
Adult Pacific ocean perch 
Juvenile rougheye rockfish 
Shortspine thornyhead 
Adult rougheye rockfish 
Shortraker rockfish — 
0.8 
Relative similarity 
0.6 0.4 
0.2 
Figure 3 
The results of cluster analysis of rockfish showing relative similarity amongst species-subgroups. The x-axis 
shows the relative similarity amongst species derived from the multinomial overlap indices among species-group 
pairs along the three environmental gradients (depth, position, and temperature). The dashed line (0.73) is 
where rockfish species assemblages were defined based on a similarity of 0.9 across the three environmental 
gradients. 
fourth assemblage (Aleutian Islands-break) included 
juvenile rougheye rockfish and adult POP. These spe- 
cies were distributed at the shelf break at depths of 244 
and 212 m, in the north Gulf of Alaska and Aleutian 
Islands (mean weighted distances 1205 and 685 km). 
A final assemblage (Aleutian Islands-slope) was com- 
posed of adult rougheye rockfish, shortraker rockfish 
(S. borealis ), and shortspine thornyhead ( Sebastolobus 
alascanus). These species were generally distributed 
in the north Gulf of Alaska and Aleutian Islands, at 
depths from 318 to 355 m. Bivariate plots of the three 
environmental variables indicate that the assemblages 
had very different distributions across the depth and 
position variables, and less distinct separation around 
the temperature variable (Fig. 4). There were clear lines 
of division along the depth variable between shelf and 
shelf break assemblages at 180 m, and between shelf 
break and slope assemblages at 275 m. There were also 
divisions along the position variable between southeast- 
ern Alaska species, central Gulf of Alaska species, and 
the Aleutian Island species at approximately Icy Strait 
(-350 km) and Seward, Alaska (250 km). 
According to the trawl survey CPUE, the most abun- 
dant 10 species subgroups captured were adult POP, 
shortspine thornyhead, juvenile POP, juvenile northern 
rockfish, juvenile rougheye rockfish, adult northern 
