POPULATION BIOLOGY OF PACIFIC OCEAN PERCH, SEBASTES 



ALUTUS, STOCKS IN THE WASHINGTON-QUEEN CHARLOTTE 



SOUND REGION, AND THEIR RESPONSE TO FISHING 1 



Donald R. Gunderson 2 



ABSTRACT 



Production and catch per unit effort of Pacific ocean perch, Sebastes alutus, stocks in the Washington- 

 Queen Charlotte Sound region have declined drastically in recent years, largely as a result of Soviet 

 and Japanese exploitation during 1966-69. In the region off Washington and southern Vancouver 

 Island, production declined from 39,000 metric tons in 1967 to 6,000 metric tons in 1969, and catch per 

 hour declined 45% during the same period. Pacific ocean perch are ovoviviparous, and so their 

 populations lack the resilience of highly fecund, oviparous groups such as the gadoids. Their ability to 

 maintain even current levels of abundance is uncertain. 



Age composition, growth rates, and mortality rates were estimated for two separate stocks occupying 

 this region: one in Queen Charlotte Sound, B.C., and one occupying the area off northern Washington 

 and southern Vancouver Island. Instantaneous rate of natural mortality was estimated to lie between 

 0.1 and 0.2. Recruitment to the fishing grounds is not complete until age 16 and the proportion of each 

 age group vulnerable to fishing was estimated by stock for age groups 10 (0.31-0.35) through 15 

 (0.87-0.94). 



Age at sexual maturity «o.5o' differed between stocks, ranging from 9 to 11 yr for females and 6 to 7 

 yr for males. Fecundity was determined for several females, and the fecundity-length and fecundity- 

 age relationships discussed. For a variety of reasons, all fecundity estimates were regarded as tenta- 

 tive, bearing a rather uncertain relationship to the number of larvae released. 



The effects of fishing on stocks of Pacific ocean perch were examined through an approach similar to 

 the yield per recruit analysis that is commonly used in stock assessment, although the computer 

 program developed for this study enabled estimation of exploitable biomass and. population fecundity 

 as well as yield per recruit. 



Compensatory mechanisms that would tend to restore population fecundity and recruitment to 

 preexploitation levels were discussed, and the limits of some of these mechanisms (density dependent 

 growth and earlier sexual maturation) were explored with the computer program mentioned previ- 

 ously. The results of this analysis suggested that past levels of exploitation went far beyond those levels 

 that could be sustained by Pacific ocean perch stocks on a long-term basis. It was coucluded that future 

 rates of exploitation should be regulated so that the annual catch never exceeds 10% of the mean stock 

 biomass on hand during the year. 



Pacific ocean perch, Sebastes alutus (Gilbert), are 

 found throughout the northern Pacific, from 

 California to the Bering Sea, and as far southwest 

 as the Kurile Islands. Murphy (1968) has shown 

 that species with several reproductive age-groups 

 are well adapted to unpredictable levels of larval 

 mortality, and Pacific ocean perch seem to be a 

 prime example of this line of evolution. Twenty- 

 year-olds are common in this species, and there 

 are 10 or more reproductive age-groups of sig- 

 nificance. In the unexploited state, large standing 



^ased on a dissertation submitted in partial fulfillment of the 

 requirements for the Ph.D. degree, University of Washington. 



2 Washington State Department of Fisheries, Fisheries Center, 

 University of Washington, Seattle; present address: Northwest 

 Fisheries Center, National Marine Fisheries Service, NOAA, 

 2725 Montlake Blvd. East, Seattle, WA 98112. 



stocks of S. alutus accumulated, furnishing a sub- 

 stantial hedge against uncertain larval survival. 

 Quast (1972) estimated the original catchable 

 biomass of S. alutus off western North America to 

 be roughly 1,750,000 metric tons. 



Commercial fishing for S. alutus was initiated 

 in 1946 by U.S. trawlers operating off central Ore- 

 gon (Alverson and Westrheim 1961). Develop- 

 ment proceeded slowly, but by 1955, United States 

 and Canadian vessels were harvesting S. alutus 

 from as far north as Queen Charlotte Sound, B.C. 

 Westrheim et al. (1972) have characterized the 

 North American trawl fishery for Pacific ocean 

 perch as undergoing a short development period 

 (1946-51) with low production, a longer period 

 (1953-60) of moderate production, and a short 

 period (1961-66) of increasing production. Since 



Manuscript accepted November 1976. 

 FISHERY BULLETIN: VOL. 75, NO. 2, 1977. 



369 



