Abstract.- The equilibrium con- 

 tribution of hatchery-released juve- 

 niles to a rockfish fishery is evalu- 

 ated by using a yield-per-recruit 

 model. Hatchery-released juveniles 

 may be worth up to an estimated 

 US$0.16 per juvenile to the fishery. 

 The use of hatchery releases to re- 

 store a depleted population of Pacific 

 ocean perch Sebastes alutus is exam- 

 ined with the Deriso-Schnute model. 

 This model indicates that hatchery 

 releases have the potential to sub- 

 stantially increase a stock's yield and 

 rate of recovery during the recovery 

 period. 



Evaluation of Hatchery Releases of 

 Juveniles to Enhance Rockfish Stocks, 

 with Application to Pacific 

 Ocean Perch Sebastes alutus * 



Jeffrey J. Polovina 



Honolulu Laboratory, Southwest Fisheries Science Center 



National Marine Fisheries Service. NOAA 



2570 Dole Street. Honolulu. Hawaii 96822-2396 



There is a long history of attempts 

 worldwide to enhance marine fisher- 

 ies by releasing hatchery -reared juve- 

 niles. However, few attempts have 

 had long-term success (Yatsuyanagi 

 1982, Botsford and Hobbs 1984, Isi- 

 basi 1984, Ulltang 1984). In theory, 

 hatchery releases can enhance fish- 

 eries in two ways. First, juvenile re- 

 leases can be added to the natural 

 stock on a long-term basis to support 

 a higher level of fishery harvest than 

 that achieved from the natural stock 

 alone. Secondly, juvenile releases can 

 be used on a short-term basis to in- 

 crease a depleted natural stock more 

 rapidly, then discontinued once the 

 natural stock has recovered. 



Pilot releases of the rockfish Sebas- 

 tes schlegeli indicate that large-scale 

 rearing and releases of rockfish may 

 be biologically and technically possi- 

 ble (Sakai et al. 1985, Kusakari In 

 press). The merit of hatchery releases 

 of juveniles for fishery enhancement 

 is examined in the present paper with 

 mathematical models. Specifically, 

 the Beverton and Holt (1966) yield- 

 per-recruit model will be used to eval- 

 uate the sustainable increase in fish- 

 ery catches from a long-term release 

 of hatchery-reared juveniles. The De- 

 riso-Schnute delay-difference age- 

 structure model (Zheng and Walters 

 1988) will be used to evaluate the 

 fishery benefits from short-term ju- 



venile releases to increase the recov- 

 ery of depleted rockfish stocks. 



Models and methods 



To evaluate the equilibrium contribu- 

 tion of hatchery-released juveniles to 

 a rockfish fishery, the Beverton and 

 Holt (1966) equation was used to ex- 

 press the equilibrium yield-per-re- 

 cruit as a function of the following 

 ratios: instantaneous natural mortal- 

 ity to von Bertalanffy growth (M/K), 

 length at recruitment to asymptotic 

 length, and fishing mortality to 

 natural mortality (F/M) (Beverton 

 and Holt 1966). If a recruit in the 

 yield-per-recruit model is taken to 

 represent a hatchery-released juve- 

 nile rather than a recruit from the 

 natural population, then the equilib- 

 rium yield per hatchery-released ju- 

 venile can be computed from yield- 

 per-recruit tables (Beverton and Holt 

 1966). Specifically, let Y/R denote 

 the yield per recruit where the re- 

 cruits are of some reference age, say 

 the age at which they have just be- 

 come demersal. Then the yield per 

 released fish at age t is just 



»Mt 



Rn 



Manuscript accepted 7 August 1990. 

 Fishery Bulletin, U.S. 89:129-136 (1991). 



'Presented at U.S. -Japan Symposium on Re- 

 production and Early Life History in the Genus 

 Sebastes, Honolulu, Hawaii, June 1989. 



where M is the natural mortality rate 

 from the reference age until release 

 age t. The values of Y/R are tabled 

 as a function of M/K and F/M (Bever- 

 ton and Holt 1966). The equilibrium 

 yield per hatchery-released juvenile 



129 



