OUNDERSON: POPULATION BIOI.OCV OK SEBASTES ALUTUS 



year class series, which would have ranged from 

 about 13 to 15 yr of age in 1966 and would have 

 been almost fully vulnerable to trawling. Re- 

 cruitment of the 1961 and 1962 year classes to the 

 fishery began to restore abundance (as indicated 

 by the number caught per fishing hour) to former 

 levels and, as of 1970, the WVI stock was on the 

 road to recovery. After 1970, however, the condi- 

 tion of the WVI stock followed an entirely different 

 course than the QCS stock. 



Exploitation rates for the QCS stock were low 

 enough to allow an increase in abundance 

 (number caught per hour) during 1970-73 (Figure 

 13), as the 1960-61 year classes became fully 

 available to the fishery. Off Washington and 

 Southwest Vancouver Island, however, exploita- 

 tion rates remained at high levels during 1970-73, 

 and the 1961-62 year classes were cropped off as 

 soon as they recruited to the fishing grounds. 

 Abundance consequently declined during 1970- 

 73, opposite to the trend in Queen Charlotte 

 Sound. The abundance offish 15 yr and older was 

 reduced below even 1970 levels, and 10 to 13 yr- 

 old fish dominated the WVI stock as of 1973. 



RECRUITMENT TO THE FISHERY 



Consideration of the length-maximum girth 

 data presented by Westrheim and Nash (1971) 

 indicates that gear selection should begin at a 

 relatively small size. The internal (between-knot) 

 measure of the cod end mesh size commonly used 

 by Washington trawlers is about 3.25 inches (8.26 

 cm) and the smallest fish retained should have a 

 girth of 2 x 3.25 = 6.5 inches. This assumes that 

 escape is not facilitated by compressability on the 

 one hand and that the rigidity of the trawl meshes 

 does not hinder escape on the other. If these as- 

 sumptions are valid, and the girth at 50% reten- 

 tion is 6.5 inches, Westrheim and Nash's results 

 show that the 50% selection length should be 24.5 

 cm. 



A 25.4-cm fish would be too small for market 

 acceptance, but previous comparisons of Pacific 

 ocean perch size composition in research catches 

 and commercial landings (Gunderson 1972) have 



indicated that 50% of all 32- to 34-cm fish on the 

 grounds are retained by Washington trawlers. 

 Virtually all fish 36 cm and larger are retained by 

 the fishermen. Reference to the age-length infor- 

 mation in Table 3 shows that the length at 50% 

 retention corresponds to an age of about 8 or 9 yr, 

 and that all fish older than 11 yr would be re- 

 tained. Slight between-stock differences in reten- 

 tion would be expected, owing to differences in 

 growth rate. 



Despite the fact that all fish older than age 10 

 are vulnerable to the fishing gear in use, and large 

 enough that almost all are retained for market 

 sales, age composition data from commercial 

 catches (Figure 13) and research surveys (Gun- 

 derson 1974) show that recruitment to the fishing 

 grounds is not complete until much later than age 

 10. On the assumption that the modal age of the 

 catch lies near the first year in which recruitment 

 is complete, these data would imply that full re- 

 cruitment could occur anywhere from age 1 1 to 14. 



The high variability in modal size is caused by 

 year to year variation in availability, year class 

 strength, and fishing mortality, and one way to 

 reduce its significance is to deal with long-term 

 averages of relative abundance. In order to do this, 

 a relative abundance index (£/,) was calculated for 

 each age group using the 1967-73 age composition 

 data for the QCS and WVI stocks. This index was 

 calculated as: 



1973 



7 n = X9&\f /" 



where U l = the relative abundance of the iih age- 

 group and ( — J = the number of fish in the iih 



age-group caught per hour. Percentage age com- 

 position during 1967-73 has been calculated from 

 these U, data and is shown by stock in Figure 14. 



The results show that although the modal age in 

 both stocks is 11 yr, recruitment to the fishing 

 grounds is quite gradual. In fact, it is not until age 

 15 that the full force of fishing mortality seems to 

 be exerted on any given year class. Estimates of 

 the exact proportion of the fish in each age-group 

 that have recruited to the fishing grounds, and are 

 vulnerable to fishing, can be derived from U, val- 

 ues, starting with the relation: 



C, = uV L N, 



where C, = catch of fish in the ith age-group 



381 



