Differences in Survival and Physiology between Coho Salmon Reared in Seminatural and Conventional Ponds

Abstract

Concern that hatchery fish may not transition well upon release into the wild has resulted in research directed at producing fish with more wild characteristics. To explore the possibility of generating more adaptable fish with alternative rearing strategies at a production-level hatchery, we modified an existing dirt-bottomed rearing pond at the Elochoman Hatchery, Washington, into a seminatural rearing pond (SNP) by adding large woody debris and pit-run rock. The pond was stocked with Type-N lower Columbia River coho salmon Oncorhynchus kisutch from three consecutive broods (1995-1997) at densities about 57, of those that a-re typical for hatchery rearing. Fish were allowed to migrate volitionally from treatment and control ponds (CP) and were sampled weekly to determine size, condition factor ( 10(5) . weight/length(3)), and gill Na+, K+-ATPase enzyme activity. Each group was coded-wire-tagged, and smolt-to-adult survival was estimated. In 2 of the 3 years, fish from the SNP were larger at migration than control fish. The rate of migration A as faster and the midpoint of migration was reached about 10 d earlier for the CP fish than the SNP fish in 1997 and 1999 the reverse was true in 1998. Smolts from each pond were qualitatively similar in appearance. Gill Na+, K+-ATPase levels were similar among weeks and treatments. In 2 of the 3 years. the SNP fish exhibited higher smolt-to-adult survival rates than control fish. For the 1995 brood, the CP survival rate was 0.35%, compared with 0.20% for the SNP fish. For the 1996 brood, CP survival was 0.75% and SNP survival was 3.9%. Smolt-to-adult survival for the 1997 brood was 1.51% for CP fish and 2.17% for SNP fish. Survival of the CP group in all 3 years was similar to that of adjacent hatchery production fish. The increased survival of the SNP fish did not offset the overall loss in the adult yield relative to what would have been realized under standard production. Use of seminatural rearing methods may have value for rearing small lots of fish, such as those cultured in recovery programs.