92 



Fishery Bulletin 91(1). 1993 



Figure 2 



Photograph from the submersible of adult Pacific ocean perch 

 Sebastes alutus spaced about 1 m apart and facing into the 

 current. Current direction is indicated by the bend in the sea 

 pens. 



observation is supported by results of previous studies 

 that used echosounding equipment to locate off- 

 bottom rockfish, and midwater and bottom trawls to 

 capture fish. In Queen Charlotte Sound, British Co- 



lumbia in 1976, Pacific ocean perch were the domi- 

 nant species in bottom-trawl catches, but were not a 

 significant component of midwater-trawl catches tar- 

 geting off-bottom fish echosignals (Gunderson & Nelson 

 1977). In the Gulf of Alaska in 1987 and 1988, Pacific 

 ocean perch were not captured in midwater-trawl hauls 

 that targeted fish echosignals 10-30 m off bottom, but 

 Pacific ocean perch were abundant in bottom-trawl 

 hauls (NMFS Auke Bay Lab., Juneau, unpubl. cruise 

 reps. JC 87-04 and JC 88-03). 



The highest densities of large rockfish observed from 

 the submersible were at trawlable sites. Densities av- 

 eraged 21.1 rockfish/lOOOm 2 of seafloor area at the 13 

 trawlable sites compared with 6.9 rockfish/1000 m 2 at 

 the 3 marginally-trawlable sites and 3.2 rockfish/ 

 1000 m 2 at the 4 untrawlable sites (Table 3). About 

 90% (3565) of the large rockfish were associated with 

 pebble substrate on flat or low-relief bottom. The re- 

 maining 455 large rockfish were among a variety of 

 rugged habitats: 226 (6%) over cobble at trawlable 

 sites, 138 (47%) over cobble and boulders at margin- 

 ally-trawlable sites, and 91 (62%) among ledges, coral, 

 etc., at untrawlable sites (Table 4). The preference of 

 trawlable substrate by Pacific ocean perch is supported 

 by two other studies: Westrheim ( 1970) mentions that 

 best trawl catches of Pacific ocean perch were on "good 

 bottom"; and Matthews et al. (1989) used sunken gill 

 nets and caught 231 Pacific ocean perch (38% of the 

 rockfish catch) on trawlable bottom, but only 25 (2% of 

 the rockfish catch) on untrawlable bottom. 



Only trawlable sites contained high densities of large 

 rockfish, but densities varied considerably among 

 trawlable sites. For example, sites 6 and 7 were 55.2 km 

 apart and had a sixfold difference in large-rockfish 

 densities; adjacent sites 16 and 17, only 2.8 km apart, 

 had a sevenfold difference in large-rockfish densities 

 (Table 3). These variations in abundance may have 

 been related to the distribution of cobble habitat: All 

 groups of more than 30 large rockfish were within 

 20 m of cobble habitat, although cobble averaged only 

 10% of the habitat at trawlable sites. The cobble was 

 in patches <30m 2 and the cobble size was <0.5m in 

 diameter. 



Distribution of small rockfish 



Of the small rockfish, 68% (781) were in groups of 2- 

 60 individuals and 32% (360) were solitary (Table 2). 

 Distribution by group size of 2-10, 11-50, and 51-200 

 rockfish was 41%, 52%, and 7%, respectively. Individu- 

 als within a group were usually separated by <lm; 

 small rockfish, in contrast to large, did not seem to 

 orient with the current. 



The highest densities of small rockfish were at 

 untrawlable sites. Their densities averaged 10.4 rock- 



