FISHERY BULLETIN: VOL. 75, NO. 2 



tain even current levels of abundance is uncertain. 



The biology and population dynamics of Pacific 

 ocean perch in the Washington-Queen Charlotte 

 Sound region were examined in detail, to gain 

 some insight into the effects of different fishing 

 strategies on this species. Two stocks were de- 

 lineated: one in Queen Charlotte Sound (QCS 

 stock) and one inhabiting the waters off northern 

 Washington and southern Vancouver Island ( WVI 

 stock). 



Production in the region occupied by the WVI 

 stock plummeted from 39,000 metric tons in 1967 

 to 6,000 metric tons in 1969 (an 85% decline), and 

 catch per hour by North American trawlers de- 

 clined 45% during the same period. The QCS stock 

 was affected less drastically by fishing, since 

 biomass estimates and CPUE data indicated that 

 S. alutus were initially more abundant in the 

 former area and did not undergo such intensive 

 exploitation. During 1966-68, production declined 

 50%, while CPUE of Washington trawlers de- 

 clined 36%. 



Changes in size and age composition of Pacific 

 ocean perch in the commercial landings were ex- 

 amined for the years 1967-73. Substantial quan- 

 tities of large S. alutus were present in Queen 

 Charlotte Sound during 1956-58 and subsequent 

 changes in size and age composition reflected the 

 changes caused by commercial fishing and re- 

 cruitment of two strong series of year classes. The 

 first series was centered around the 1952 year 

 class and included the 1951-53 brood years, while 

 the second series centered around the 1961 and 

 1962 brood years. Size composition data for the 

 WVI stock were too limited to be useful prior to 

 1961, but data for subsequent years suggested 

 that the same year classes that predominated in 

 Queen Charlotte Sound were also predominant in 

 landings from the WVI stock. 



Fisheries exploitation has resulted in drastic 

 reductions in the abundance of the 1951-53 year 

 class series in both the QCS and WVI stocks and 

 the 1973 Washington trawl catches from these 

 stocks were dominated by 10- to 13-yr-old fish. 



Growth rates were estimated from commercial 

 fisheries and research cruise data, taking perti- 

 nent features of the life history such as seasonal 

 and bathymetric variability in the age-length re- 

 lation into consideration. Parameters of the von 

 Bertalanffy growth model were estimated by sex 

 for both the QCS and WVI stocks. 



Although fish older than age 10 are large 

 enough that almost all can be caught by conven- 



tional trawling gear and retained for market 

 sales, age composition data from commercial 

 catches and research surveys showed that re- 

 cruitment to the fishing grounds is not complete 

 until much later than age 10. The proportion of 

 each age group vulnerable to fishing (V,) was es- 

 timated by employing a model that assumed that 

 natural mortality (M) and V, were constant 

 throughout the year. The results suggested that 

 recruitment to the fishing grounds differed some- 

 what between stocks, but that V, ranged from 

 0.31-0.35 during age 10 to 0.87-0.94 during age 15. 

 Estimation of V, could not be made for fish less 

 than 10 yr old, since these age groups may be 

 subject to substantial rates of discard by fisher- 

 men. 



Any yield per recruit analysis of Pacific ocean 

 perch stocks must take these recruitment patterns 

 into consideration to be meaningful. Recruitment 

 to the fishing grounds is quite gradual, and many 

 age groups that could potentially be retained by 

 conventional mesh sizes are poorly represented on 

 the fishing grounds. Evaluation of the effects of 

 different size or age restrictions would be quite 

 misleading if this were not considered. 



Age composition data (number caught per hour 

 by age-group) were used to estimate the survival 

 of 14- to 18-yr-old Pacific ocean perch in year n to 

 ages 15-19 in year n + 1, or to ages 16-20 in year n 

 + 2. These survival estimates were then conver- 

 ted to total instantaneous mortality rates (Z) and 

 plotted against total international effort (/*) on the 

 assumption that they conform to the model: Z = qf 

 + M, where M = instantaneous natural mortality 

 rate. The data seemed to fit this model in a general 

 way but there was a relatively low correlation 

 between Z and f (r = 0.3-0.5), due principally to 

 wide variability in the availability of Pacific ocean 

 perch to on-bottom trawls (totally unrelated to 

 variations in actual abundance) and to the low 

 quality of the data on international fishing effort. 

 Despite this, there was good agreement between 

 the estimates of M derived from this study (0.07 

 for the QCS stock and 0.23 for the WVI stock) and 

 results obtained in previous studies. It was 

 concluded that between-stock differences in natu- 

 ral mortality probably should not be overempha- 

 sized, and that the results of the mortality studies 

 served mainly to show that M in the Washington- 

 Queen Charlotte Sound region lies in the range 

 between 0.1 and 0.2. 



Data on the proportion of sexually mature indi- 

 viduals in each age-length group were sum- 



400 



