Estimating Escapement of Pacific Salmon: Marking Practices to Discriminate Wild and Hatchery Fish

Abstract

Abstract Accurate discrimination between river returns of wild and hatchery anadromous Pacific salmon (Oncorhynchus spp.) is necessary if the status of the two stocks is to be monitored and if the success of hatchery mitigation programs is to be assessed. Usual hatchery management practices, including release of large numbers of unmarked fish and variable fractional marking of releases, prevent such discrimination. Many Pacific-coast salmon hatcheries presently release some fish with an adipose fin clip and a binary-coded wire tag inserted in the nasal region (AD-CWT). If a constant fraction of remaining releases carried a distinctive identifying mark, then the proportion of hatchery fish in subsequent spawning runs could be estimated. Should the majority of hatchery fish in the spawning escapement return to the hatcheries of origin, then the expected change in hatchery fish proportions between river entry and natural spawning grounds, combined with known returns to the hatcheries, may be used to estimate...