98 



Fishery Bulletin 98(1) 



a pair of lasers mounted on either side of the exter- 

 nal video camera. The projected reference spots were 

 20 cm apart and were visible to the observer. An 

 environmental monitoring system aboard the sub- 

 marine continuously recorded date and time, depth, 

 and altitude of the vessel above the sea floor. After 

 the dive, these data were overlaid on the original 

 videotape. 



Transect videos were reviewed either aboard the 

 research vessel or in the laboratory. For each fish, 

 we recorded 1) its species, to lowest identifiable taxa; 

 2) its estimated total length to the nearest cm; and 3) 

 the microhabitat it occupied (e.g. pipe, sand, mussel 

 shell mounds, mud). We defined young-of-year fishes 

 (YOYs) from published estimates of size at age. Sub- 

 adults are defined as juveniles in their second year 

 up to, but not including, maturity. 



During the survey at platform Gail, all greenspot- 

 ted iSebastes chlorostictus) and greenblotched rock- 

 fishes (S. rosenblatti) were inadvertently identified 

 as greenspotted rockfish. In reviewing the video- 

 tape, it was clear that some of the individuals that 

 were recorded as greenspotted rockfish were in fact 

 greenblotched rockfish. In order to correct for this 

 potential misidentification, the total number of both 

 species was adjusted by using the proportion of 

 greenblotched to greenspotted rockfishes (ratio=2.2) 

 obsei"ved at platform Gail during the following year's 

 survey (Love, unpub. data). Similar numbers of the 

 two species combined were observed during the two 

 years (1996: /z = 186,1997: «=209). 



Analyses 



We estimated length of those transects conducted 

 on the bottom by first determining the submersible 

 speed. This was done by evaluating a ten-second seg- 



ment for every one minute of transect. The video was 

 manually forwarded frame by frame and the number 

 of 20-cm segments passing the lasers in a ten-second 

 section was counted. The number of 20 cm segments 

 per 10 seconds was divided by 2 to obtain speed in 

 centimeters per second. All subsamples were then 

 averaged to obtain mean transect speed (cm/s). The 

 mean speed was then multiplied by the number of 

 seconds in the transect and divided by 100 to obtain 

 transect length in meters. The length was then mul- 

 tiplied by 2 m (the transect width) to obtain transect 

 area, allowing us to present both densities (fish/m^l 

 and biomass (kg/m^). Biomass was estimated for all 

 species by using length-weight relationships derived 

 empirically or obtained from the literature. No bio- 

 mass estimates were made for species that could not 

 be identified to the family level. 



In the midwater, we could not see the lasers pass 

 before fixed points; therefore, we could not directly 

 measure the length of the midwater transects. With- 

 out knowledge of the length of the midwater tran- 

 sects, we could not calculate density or biomass per 

 unit area as done on the bottom transects. However, 

 we were able to estimate the length of midwater 

 transects for use in estimating both fish density and 

 biomass. We did this by converting density and bio- 

 mass on the midwater transects from number and 

 kilogram per minute to number and kilogram per 

 m^, respectively. This conversion was accomplished 

 by calculating the equation for the regression of den- 

 sity in terms of number per m- on density in terms of 

 number per minute for the bottom transects where 

 both values were known (Fig. 2A). The same rela- 

 tionship was calculated for biomass (Fig. 2B). Given 

 the regression equations, density per m^ and bio- 

 mass per m- could be calculated from number per 

 minute and kilogi'ams per minute. We called these 



