GUNDERSON: POPULATION BIOLOGY OF SEBASTES ALUTUS 



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g 40- 





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FEMALES ^o ° 



MALES 



Apr-May  

 Jun-Aug - 

 Sep Dec - 



NE 

 Corner 



SE 

 Corner 



SW 

 Corner 



Ca P e Triangle Vlrl 3' n 

 Scott y Rocks 



FIGURE 11. — Fitted length at age 15 for Pacific ocean perch in 

 Queen Charlotte Sound ( 1966-72), by fishing ground, season, and 

 sex. 



fish. Both age-length and size composition data 

 indicate that aggregations of these fish are least 

 available during April and May, and that it is only 

 during September-December that they are fully 

 available on all fishing grounds. This general pat- 

 tern seemed to hold throughout Queen Charlotte 

 Sound, at least on the major fishing grounds. In 

 some instances, however, availability of large, 

 fast-growing fish was unusually high on a rela- 

 tively minor fishing ground. This seemed to be the 

 case at NE Corner during May and Cape Scott 

 during June- August (Figures 9, 11). 



Sampling problems caused by disporportionate 

 fishing intensity in extremes of the bathymetric 

 range are usually insignificant compared with the 

 problems caused by differential availability. Re- 

 search cruises have shown that mean length at 

 age decreases as depth increases (S. J. Westrheim, 

 pers. commun.), so that fitted length at age 15 

 should either remain constant (if fishery shifts in 

 response to stock location) or decrease (if fishery 

 shift is independent of stock location) as the 

 fishery shifts to deeper water during September- 

 December (Figure 5). Instead, mean length at age 

 actually increases during September-December 

 (Figure 11) because this is the season when large, 

 fast-growing fish are most available. 



Considering all sources of data on catch, CPUE, 

 and biological composition of the landings, it is 

 apparent that there is a significant increase in the 

 size of the exploitable population inhabiting 

 Queen Charlotte Sound during September- 

 December. Age-length data collected during 



January-August consequently apply to only a 

 fraction of the known population in Queen Char- 

 lotte Sound. If it is assumed that all Pacific ocean 

 perch are fully vulnerable to fishing by fall, how- 

 ever, the September-December market sampling 

 data can be taken as representative of the exploit- 

 able segment of the QCS stock. 



Consequently, 1967-71 age-length data from 

 September-December market samples were used 

 to estimate growth parameters for the QCS stock. 

 Queen Charlotte Sound was treated as a unit, 

 mean length at each age was determined by sex, 

 and the resulting data were fitted to the von Ber- 

 talanffy growth model. Both the original data and 

 fitted mean length at age are shown in Table 3. 



Washington-Vancouver Island 



Availability of Pacific ocean perch in this region 

 influences the age-length relationship, but in a 

 different manner than in Queen Charlotte Sound. 

 Results from research surveys off northern 

 Washington (Gunderson 1974) suggest that mean 

 length at age actually decreases with increasing 

 availability, rather than increasing. The highest 

 rates of catch in this region were obtained when 

 aggregations of large, old, slow-growing fish were 

 most available. 



The WVI and QCS stocks also differ substan- 

 tially in the degree to which mean length at age 

 varies with depth. In contrast to Queen Charlotte 

 Sound, mean length at age has been shown to 

 decrease sharply as depth increases off Wash- 

 ington and Vancouver Island (Westrheim 1973; 

 Gunderson 1974). The decline is so sharp, in fact, 

 that Westrheim (1973) has suggested that there 

 are separate shallow and deepwater stocks in this 

 region. 



It is clear, then, that both depth of fishing and 

 availability must be taken into consideration in 

 order to arrive at an age-length relationship that 

 characterizes the WVI stock. Research cruise data 

 obtained off the coast of northern Washington 

 (Gunderson 1974) are particularly well suited to 

 do this, since age-length relationships and avail- 

 ability were systematically observed throughout 

 the bathymetric range. Availability varied widely 

 during these cruises, and, as previously men- 

 tioned, this phenomenon masked any long-term 

 changes that occurred during 1968-72. Availabil- 

 ity was maximal during the July 1972 cruise, 

 however, and the results from that cruise were 

 used to represent growth in the WVI stock. 



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