Kneger and Sigler: Catchability coefficient for rockfish 



283 



light was available for observations. In this study 

 catchability coefficients are estimated for rockfish 

 that inhabit deep depths where ambient light is 

 minimal and that cannot be tagged because of gas 

 bladder expansion when sampled. Catchability co- 

 efficients are estimated by comparing trawl catch 

 rates with counts from a manned submersible. 



Pacific ocean perch, Sebastes alutus, was the tar- 

 get species of this study Biomass estimates of Pa- 

 cific ocean perch in the Gulf of Alaska, based on 

 bottom-trawl surveys conducted triennially, were 

 214,827, 138,003, and 460,755 metric tons for 

 1987, 1990, and 1993 (Heifetz et al. 2 ). These ex- 

 treme changes in biomass are unlikely, given the 

 slow growth and low mortality rates of Sebastes 

 spp. (Heifetz et al. 2 ). To improve Pacific ocean 

 perch biomass estimates from bottom-trawl sur- 

 veys, manned submersibles and bottom trawls 

 have been used since 1988 to study their substrate 

 association and spatial distribution, and the effi- 

 ciency of bottom trawls for sampling them. Most 

 Pacific ocean perch observed from a submersible 

 were concentrated on smooth substrates in groups 

 of 2-200 fish (Krieger, 1993). Counts of Pacific 

 ocean perch from a manned submersible in 1989 

 were compared with catch rates from trawl hauls, 

 and a catchability coefficient of 2.1 (SE=0.4) was 

 estimated for the distance between the wingtips 

 of the net. This catchability coefficient was consid- 

 ered preliminary for three reasons: 1) only nine sub- 

 mersible-trawl comparisons were made; 2) no fish 

 were assumed captured while the trawl was re- 

 trieved; and 3) all fish were assumed counted within 

 the viewing range from the submersible (Krieger, 

 1993). The catchability coefficient estimated in this 

 study is based on an increased number of submersible 

 versus trawl comparisons and on improved density es- 

 timates from trawl catch rates and submersible counts. 



Materials and methods 



137 W 



133 W 



60 N 



■' Alaska Stody 



56 N 



Gulf of Alaska 



Figure 1 



Locations of submersible dive sites that were sampled with bot- 

 tom trawls in the eastern Gulf of Alaska, 1989 and 1991. 



transects with a bottom trawl. Each transect was 

 determined by following a compass heading for 15 

 min, and there were 5 min of travel between 

 transects. Trawl hauls were considered successful 

 only if the gear was retrieved intact (no tears in the 

 net or broken cables) and the trawl intersected at 

 least three of the four submersible transects. Sub- 

 mersible dives and bottom trawling were completed 

 during daylight at 180-283 m depths. The time be- 

 tween dives and trawls was <4 h for 13 of the 16 

 comparisons. Substrates consisted of pebble or sand, 

 interspersed with cobble <0.5 m in diameter and boul- 

 ders 0.5-5.0 m in diameter. 



Sampling of submersible counts and trawl catch rates 

 for Pacific ocean perch were conducted in August 

 1989 and 1991 on the outer continental shelf in the 

 eastern Gulf of Alaska (Fig. 1). The sampling strat- 

 egy was to count rockfish from the submersible along 

 four parallel transects, and then intersect the four 



2 Heifetz, J., D. M. Clausen, and J. N. Ianelli. 1994. Slope rock- 

 fish. In Plan Team for the Groundfish Fisheries of the Gulf of 

 Alaska (eds.), Stock assessment and fishery evaluation report 

 for the groundfish resources of the Gulf of Alaska as projected 

 for 1995, Sect. 5. North Pac. Fish. Manage. Counc, P.O. Box 

 103136, Anchorage AK 99510. 



Submersible 



We used the two-man submersible Delta chartered 

 by the National Oceanic and Atmospheric Adminis- 

 tration, National Undersea Research Center. This 

 battery-powered submersible is 4.7 m long, dives to 

 365 m, and travels 2-6 km/h for 2-4 h. It is equipped 

 with halogen lights and internal and external video 

 cameras. In addition, it has a magnetic compass, di- 

 rectional gyro compass, underwater telephone, and 

 transponder that allow tracking of the submersible 

 from a surface vessel. The pilot maintained the sub- 

 mersible within 0.5 m of the bottom, while a scien- 



