Polovina: Hatchery releases of juvenile Sebastes alutus 



131 



a Actual 

 -t- Estimated 



O 



63 64 '65 '66 67 '68 '69 70 71 72 73 74 75 76 77 



Year 



Figure 2 



Historic catches (10 3 metric tons) of Pacific ocean perch from 

 Queen Charlotte Sound and estimated catches from the fit 

 of the Deriso-Schnute model, 1963-77. 



per-recruit (expressed as a fraction of the asymptotic 

 weight of the fish, as a function of F/M, and for M/K 

 equaling 0.5, 1.0, and 2.0) are shown in Figure 1. These 

 yield curves assume that the ratio of the length-at- 

 recruitment to the asymptotic length is optimum to 

 achieve maximum yield-per-recruit. For M/K ranging 

 from 0.5 to 2.0, the optimum length at harvest will be 

 45-86% of the asymptotic length (Beverton and Holt 

 1966). Thus, in the case of Pacific ocean perch, which 

 has an asymptotic weight of about 1.4 kg, if M/K = 0.5, 

 F/M = 1.0, and M = 0.05 per year, the contribution to 

 the fishery of an individual juvenile released from the 

 hatchery at age 0.25 is calculated as the product of 1.4 

 kg x 0.17 (from Fig. 1), multiplied by exp(0.05 • 0.25) 

 or 0.24 kg. The average 1988 ex-vessel price for all 

 rockfishes was US$0.67 per kg (National Marine 

 Fisheries Service 1989); therefore, each 3-month-old, 

 hatchery-released juvenile on the average is worth 

 $0.16 to the fishery. The contribution of a juvenile to 

 the fishery is strongly inversely related to M/K (Fig. 

 1). For example, if M/K is 1.0 rather than 0.5, then the 

 contribution is only about one-half as much. Also, as 

 long as natural mortality is assumed to be low and con- 

 stant, changes in the release age have little influence 

 on the contribution of the juvenile to the fishery. How- 

 ever, it is quite possible that natural mortality of young 

 juveniles varies considerably by age and an optimum 

 release age exists, although we have no data to docu- 

 ment either case. 



Pacific ocean perch in Queen Charlotte Sound, Can- 

 ada, underwent heavy exploitation from 1963 to 1977 

 (Fig. 2) (Archibald et al. 1983). A reconstruction of the 



history of this exploitation by using a catch-at-age 

 model estimates annual exploitable biomass and 

 average fishing mortality during this period (Archibald 

 et al. 1983). The Deriso-Schnute model is fit to the catch 

 history of this fishery during 1963-77 by using esti- 

 mates of fishing mortality from the catch-at-age model 

 (Fig. 2). All but two of the parameters required for the 

 model are from published values (Table 2). Two param- 

 eters (A and B in Table 2) in the stock-recruitment rela- 

 tionship part of the model are estimated by fitting the 

 model to the 1963-77 catch series. 



Based on the Deriso-Schnute delay-difference model 

 with the parameters used to fit the catch and effort 

 series (1963-77) and to estimate the equilibrium yield 

 curve, the maximum sustainable yield (MSY) of about 

 1800 t is achieved at F MSY = 0.06 per year (Fig. 3). At 

 that level of F MSY , the corresponding equilibrium bio- 

 mass (Bmsy) is estimated at about 35,000 t. At B MSY , 

 the recruitment of 1 -year-old juveniles is estimated at 

 3.5 million fish, whereas an estimated 1.3 million 

 1-year-olds recruit if biomass is at the 1977 depleted 

 levels. 



Assuming that the goal of restoring the Pacific ocean 

 perch stock is to increase the biomass to 35,000 t so 

 it can be harvested at F = 0.06 to achieve MSY, three 

 approaches to stock recovery are examined. One ap- 

 proach has as its goal to increase the biomass to B MSY 

 as quickly as possible by setting F at until the bio- 

 mass reaches 35,000 t, then the stock will be fished at 



