Abstract.— A manned submers- 

 ible was used in the eastern Gulf of 

 Alaska to observe spatial distribu- 

 tions of Pacific ocean perch Sebastes 

 alutus and other Sebastes spp.. and 

 count rockfish for comparison with 

 bottom-trawl catch rates. Twenty 

 submersible dives were completed in 

 1988 and 1989 at depths of 188- 

 290 m. Approximately 80<7f of the 

 5317 rockfish observed from the sub- 

 mersible were Pacific ocean perch. 

 Most adult Pacific ocean perch were 

 in groups of 2-200 over flat, pebble 

 substrate. Fish within a group were 

 1— 4m apart, usually oriented into 

 the current, and 0-7 m above bot- 

 tom. Most juvenile Pacific ocean 

 perch, and juveniles and adults of 

 other Sebastes spp., were associated 

 with rugged habitat (cobble, boul- 

 ders, pinnacles, and coral). Densi- 

 ties of Pacific ocean perch estimated 

 from bottom-trawl catches were ap- 

 proximately twice those observed 

 from the submersible, indicating 

 that the bridles and otter doors 

 herded fish into the trawl. Bottom- 

 trawl surveys may overestimate 

 Pacific ocean perch abundance be- 

 cause of this possible herding effect 

 and the preference of adult Pacific 

 ocean perch for smooth (trawlable) 

 substrate. 



Distribution and abundance 

 of rockfish determined from 

 a submersible and by 

 bottom trawling 



Kenneth J. Krieger 



Auke Bay Laboratory, Alaska Fisheries Science Center 



National Marine Fisheries Service, NOAA 



1 1305 Glacier Highway, Juneau, Alaska 99801-8626 



Manuscript accepted 29 September 1992. 

 Fishery Bulletin, U.S. 91:87-96 ( 1993). 



Pacific ocean perch Sebastes alutus 

 is a commercially important rockfish 

 found along the North American 

 coast from southern California to the 

 Bering Sea, and along the Asiatic 

 coast from Cape Navarin to the Kuril 

 Islands (Balsiger et al. 1985). Prima- 

 rily an offshore species that inhabits 

 the outer continental shelf and up- 

 per slope regions, it is caught with 

 bottom trawls at depths of 165-290 m 

 (Hart 1973). 



In the Gulf of Alaska, Pacific ocean 

 perch were heavily exploited in the 

 1960s by the Soviet and Japanese 

 trawl fleets. Foreign catches of rock- 

 fish (consisting mainly of Pacific 

 ocean- perch) peaked in the Gulf of 

 Alaska in 1965 at 350,000 metric tons 

 (t). By the late 1970s, rockfish catches 

 had declined to less than 10,000 1, 

 and catch-per-unit-effort (CPUE) had 

 decreased by 80 c i (Balsiger et al. 

 1985). Domestic trawling replaced 

 foreign trawling in the Gulf of Alaska 

 in the 1980s. Although rockfish stocks 

 remain depressed from overfishing, 

 domestic rockfish catches (mainly Pa- 

 cific ocean perch) have increased in 

 the Gulf of Alaska from 1000 1 in 1985 

 to 20,000 1 in 1990 (Heifetz & Clausen 

 1990). 



Reliable stock assessments are 

 needed for managing depressed 

 stocks of Pacific ocean perch. Cur- 

 rent stock assessments, based prima- 

 rily on catch rates from bottom-trawl 

 surveys, are questionable because (1) 



the catch efficiency of bottom trawls 

 on Pacific ocean perch is unknown, 



(2) Pacific ocean perch are not 

 sampled in areas where the habitat 

 is too rugged for bottom trawling, and 



(3) information is limited on Pacific 

 ocean perch spatial distribution, mi- 

 gration, and off-bottom movement. 

 Leaman & Nagtegaal (1986) noted 

 their dissatisfaction with rockfish bot- 

 tom-trawl surveys because of wide 

 confidence intervals associated with 

 the biomass estimates, and because 

 alternate assessment techniques in- 

 dicate that the estimates themselves 

 may be grossly in error. Balsiger et 

 al. ( 1985) reported that bottom-trawl 

 surveys probably underestimate 

 populations because Pacific ocean 

 perch occupy the water column above 

 the opening of the trawl. 



An understanding of both the be- 

 havior and habitat of Pacific ocean 

 perch and the catch efficiency of bot- 

 tom trawls is needed to improve bio- 

 mass estimates. In this study, a two- 

 man submersible was used for in situ 

 observations of rockfish and a bot- 

 tom trawl to sample rockfish at sub- 

 mersible dive sites. The main objec- 

 tives were to describe the spatial 

 distribution of rockfish, visually de- 

 termine their abundance in trawlable 

 and untrawlable habitat, and deter- 

 mine the efficiency of bottom trawls 

 for capturing rockfish. Pacific ocean 

 perch was the target species of this 

 study. 



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