448 



Fishery Bulletin 93(3), 1995 



TTH 





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Figure 1 



Study site off central California marked for 200-m, 400-m, and 600-m depth comparisons of population estimates from 

 video transects conducted from an ROV survey and from swept-area trawl surveys. 



(slowly moving into the frame); 3) no response (no 

 movement); 4) weakly avoided (slowly moving out of 

 the frame); and 5) strongly avoided (rapidly moving 

 out of the frame). A time line, at which the fish were 

 counted, was chosen in the center of the viewing area. 

 To determine transect width at the time line, the ROV 

 was transected over a 5-m square grid, and known 

 lengths from the grid were measured on the moni- 

 tor. From these lengths and standard photometric 

 equations (Wakefield and Genin, 1987), the transect 

 width was calculated to be 1.8 m (see Fig. 2). The 

 vertical perspective of the video ranged from a height 

 of 0.7 m at the camera to a visual horizon of 2.4 m in 

 front of the ROV. The number of fish per transect 

 was converted to fish per hectare by dividing the 

 number offish observed by the area covered (transect 

 distance multiplied by transect width). 



Trawl operations 



Trawling was conducted on separate cruises on the 

 RV David Starr Jordan. Three trawl surveys were 

 conducted and a sample size of at least three trawls 

 per station was taken per cruise (for dates, see Table 

 1). During the April 1991 cruise, all three replicates 

 were not completed owing to inclement weather. Par- 

 allel tows were made along the same isobath as the 

 ROV transect at a speed of approximately 3.7 km/hr 

 (2 knots) for 30 minutes, although control of the trawl 

 was not as exact as that of the ROV. The net, an Ab- 

 erdeen high-rise trawl net with a 29-m (96-ft) head- 

 rope, was equipped with 1.5 x 2.1 m steel doors. Trawl 

 openings were not measured; this type of trawl 

 has an average horizontal opening of 13.5 m (see 

 Fig. 2) and an average vertical opening of 5.5 m 



