UW Aquatic & Fishery Sciences Quantitative Seminar

Rich Hinrichsen

Hinrichsen Environmental

Alternative Estimators of the Proportion of Hatchery-Origin Spawners

For salmon populations in the Columbia Basin, many of which are listed under the U.S. Endangered Species Act of 1973, reliable estimation of the proportion of hatchery-origin spawners in spawning areas, p, is needed to make inferences about status and potential for interbreeding of hatchery-origin with wild-origin adults, which may reduce the genetic fitness of subsequent generations. Despite its importance, accurate and precise techniques for estimating p are lacking, especially in the case where, in a single spawning area, there are spawner inputs from multiple source hatcheries. To help identify hatchery-origin spawners, some hatchery releases are visibly marked and some are tagged with a coded-wire-tag (CWT), and some receive both. However, different marking fractions are used at different source hatcheries and this complicates the problem of estimating p. To handle this complicated situation, we derived a new generalized least squares estimator (GLSE) of p, and a second less computationally-intensive estimator. We conducted Monte Carlo simulations using both estimators to compare precision and statistical accuracy. We then applied the estimators to 2010 carcass survey data for Hanford Reach Fall Chinook salmon (Oncorhynchus tshawytscha). An incremental change away from identical marking fractions for all source hatchery releases reduced precision, increased bias, and complicated estimation. The GLSE had greater precision than an alternative (SMME) estimator that used CWT recoveries alone. Statistical bias shrank as true spawning population size, fraction of hatchery-origin fish, or CWT fraction increased. The GLSE of p in the Hanford Reach application was 0.077 while the SMME was 0.041. To maximize accuracy and precision in the estimates of the p, we recommend that identical marking fractions (preferably 100%) be used for all hatchery releases; barring this, we recommend that CWT fractions be as large as possible. We discuss ambiguity in this estimation problem and point to possible solutions using prior information.