FISHERY BULLETIN: VOL. 75, NO. 2 



5 





Stock 



FIGURE 24. — Relationship between parent stock and recruit- 

 ment for gadoids (a) and elasmobranchs (b). 



per year (Holden 1973). Any compensatory re- 

 sponses to increase the number of recruits must 

 act through changes in growth (with attendant 

 changes in the age at maturity) or fecundity, and 

 are relatively sluggish. Holden (1973) has 

 suggested that the stock-recruitment relation for 

 most elasmobranchs is probably of the form of 

 curve b in Figure 24, departing little from the 

 bisector on either side of the replacement point. 



By eliminating the free-living larval stage, 

 elasmobranchs have reduced the susceptibility of 

 adult stock size to environmental perturbations. 

 In the natural state, then, the compensatory 

 mechanisms that return the stock toP r do not need 

 to provide the same degree of resilience they do in 

 the gadoids. This lack of resilience makes the 

 elasmobranchs poorly adapted to harvests by man, 

 however, and they are quite susceptible to over- 

 fishing. 



Pacific ocean perch are ovoviviparous, and, like 

 the elasmobranchs, they are probably much less 

 resilient to perturbations from P, than a highly 

 fecund, oviparous species like cod. It is important, 

 therefore, that population fecundity be kept quite 

 near the levels found in the virgin stock when the 

 adult stock was presumably near P r . Any reduc- 

 tion in population fecundity from virgin stock 

 levels could easily result in reduced recruitment. 



Some increases in the number of larvae released 

 could probably come through compensatory 

 growth, since the age at sexual maturity and level 

 of individual fecundity are both correlated 

 strongly with size. There must be some limits to 

 the degree of compensation this mechanism is 

 capable of, however, and this was explored quan- 

 titatively by using the model (Table 16) and com- 

 puter program described previously. 



This analysis was begun by setting up four sets 

 of hypothetical populations ( one set for each stock ) 

 and calculating the population fecundity under 

 different levels of fishing mortality. In the first 

 population, the "standard" age-length data in 

 Table 17 were used to describe individual growth 

 in each stock. In the second and third populations, 

 the mean lengths at each age were increased 3% 

 and 5% (Figure 25) to simulate compensatory 

 growth. In the fourth population, mean length at 

 each age was again increased 59£- above standard, 

 and it was also assumed that sexual maturation 

 occurred 1 yr earlier than in the other populations. 

 The latter assumption was justified by the fact 

 that a 5% increase in growth brought 8-yr-olds 

 from the WVI stock and 10-yr-olds from the QCS 

 stock up to the size at which sexual maturity oc- 

 curred in the standard population (Figure 25). The 

 last population was presumed to embody the 

 maximum possible degree of compensation in 

 population fecundity, since the projected increases 

 in mean length at age would be quite remarkable 

 in a species growing as slowly as S. alutus. The 

 assumption that the age at sexual maturity would 

 decline because of earlier attainment of a critical 

 maturation size is also tenuous, and only time will 

 tell if this actually occurs. 



The age of recruitment was taken as age 8 for 

 the WVI stocks and age 10 for the QCS stocks, in 



50- 



40- 





30 



WVI 



5% increase 



"1- —3% increase 



'V> 



"i — i — i — i — i — i — i — i — i — i — i — r 



40 



30 



... — 5% increase 

 _ — 3% increase 



QCS 



10 



15 20 



Age (years) 



25 



FIGURE 25. — Mean length at age for female Pacific ocean perch 

 in the WVI and QCS stocks, assuming standard growth, and two 

 different levels of compensatory increase in growth. 



398 



