Influence of Time and Size at Release of Juvenile Coho Salmon (Oncorhynchus kisutch) on Returns at Maturity

Abstract

Juvenile coho salmon (Oncorhynchus kisutch) were raised in six ponds at Rosewall Creek, Vancouver Island, British Columbia, from which releases were made at four times: April 14, May 12, June 10 and July 8, 1975. Prior to each release a portion of the juveniles in each pond were graded into three size groups (small, medium, large) based on size distributions in each pond. The juveniles were nose-tagged according to size group, pond, and release date, and marked by adipose fin removal. A total of 57 groups were released. Returns of adults and precocious males (jacks) to the weir and to the fishery (commercial, sport) were subjected to response surface analysis. Maximum adult returns of 43.5%, to the weir and fishery, were predicted for release of 25.1-g coho juveniles on the 173rd (Julian) day from January 1 (June 22, 1975). A significant interaction was noted between release time and size: maximum returns from early (April 14) releases would be expected from release of 16- to 17-g juveniles. Returns of jacks from the six ponds ranged from 0.0 to 4.65%. Predicted returns of jacks would be maximized from early release of large juveniles (> ~20 g in April). The benefit–cost ratio at the center of the time and size of release surface for maximum adult returns was estimated as 12.2:1. Maximum benefit–cost ratios were calculated for release of smaller juveniles between June 17 and 22, 1975. Benefit–costs associated with releases on the path of joint optimality between the two response centers rose from 12.2:1 (maximum adult returns) to a calculated 16.5:1 for release of 7.5-g juveniles on June 17, 1975. Size and time at juvenile release and success of adult returns are viewed as initial and final aspects of a biological system whose central components as yet are imperfectly understood. Some parts of the system are discussed in relation to possible ways of increasing the efficiency and cost–effectiveness of salmonid hatchery technology, as well as in predicting the success of returns in relation to conditions influencing survival and growth of juvenile migrants from natural stocks.Key words: coho salmon, juvenile salmon, release time and size, response surface analysis and optimization, survival, biomass, benefit–costs