100 



Fishery Bulletin 98(1) 



E 



s 

 Z 



1,500-1 



1,000- 



500- 



IZI Midwater 

  Bottom 



18 23 28 33 38 



Midpoint length (cm) 



— t- 



43 



48 



— 1 

 53 



Figure 3 



Length-frequency distribution for all roclifish species and all platforms combined. 

 Midpoint length is the midpoint of .5-cm length bins. 



(SE)=141.4 fish/lOOm'- (49.0), n=6 platforms; mid- 

 water mean density (SE)= 115.8 fish/lOOm^ (32.2), 

 «=7 platforms; Mest, /=0.44, P=0.66). On three plat- 

 forms, density was higher on the bottom than in mid- 

 water and on three other platforms the reverse was 

 true. However, there was a much larger and consis- 

 tent difference in biomass between bottom and mid- 

 water transects. For most families and all platforms, 

 total biomass was higher on bottom than midwater 

 transects (Table 2). Average biomass per platform 

 (SE) for all species combined was 19.06 kg/m^' (2.5) 

 on the bottom and 6.47 kg/m- (2.3) in the midwater 

 (^test, ;=3.75, P=0.003). This consistent difference 

 was due to the lack of adult fishes in the midwater. 

 Fewer species lived on the midwater structures 

 than on the bottom. Species richness for all rigs 

 combined was 24 in the midwater versus 40 on 

 the bottom. Average species richness per platform 

 was significantly higher on the bottom than in the 

 midwater (bottom mean richness (SE)=14.7 species 

 ( 1.5); midwater mean richness (SE)=8.2 species ( 1.4); 

 ^-test, ^=3.26, P=0.008). Average species diversity 

 (H') across platforms was identical between bottoms 

 and midwaters (bottom mean H' (SE)=:1.2 (0.2); mid- 

 water mean H' (SE)=1.2 (0.2); ^test, ^=0.09. P=0.99). 

 We present the remaining results for bottom and 

 midwater habitats separately. 



Bottom habitat 



All platforms We identified at least 40 fish species 

 around the platform bottoms (Table 2). Twenty-seven 



species were rockfishes; they were by far the most 

 speciose group. Rockfishes made up 92. 7*^ of all 

 fishes on the bottom (Table 3) and represented 96.7% 

 of the biomass (Table 2). 



Halfbanded, greenspotted, copper, vermilion, widow, 

 and fiag rockfishes, and bocaccio were among the 

 most commonly observed rockfishes (Table 3). Our 

 observations indicated that vermilion I'ockfish, flag 

 rockfish, and bocaccio of all sizes were always closely 

 associated with the platform structure (Fig. 4A). 

 Larger copper and greenspotted rockfishes also were 

 more likely to be very close to the platform. In par- 

 ticular, flag rockfish were most often seen tucked 

 well into the space formed by the bottom of the 

 lowest crossbeam and the bottom (Fig. 4B). Flag and 

 greenspotted rockfishes were almost always seen on 

 or very close to the bottom. Halfbanded rockfish, as 

 well as smaller greenspotted and copper rockfishes, 

 were less bound to the platform and were often seen 

 well away from the structure. Juvenile greenspotted 

 and copper rockfishes were usually nestled within 

 or just above the mussel, shell-covered substrata. 

 Vermilion rockfish, and to a certain extent copper 

 rockfish and bocaccio, would occasionally ascend up 

 platform legs as much as 5 m. 



Flag rockfish, as well as larger bocaccio and ver- 

 milion rockfish, often were solitary or found in small 

 groups. The exception occurred at platform Gail, 

 where one school of bocaccio comprised at least 100 

 individuals. Smaller adult or subadult vermilion and 

 copper rockfishes tended to aggi'egate, often in mixed 

 groups containing 50 or more individuals. The few 



