Viability of Sacramento River Winter-run Chinook salmon

The winter run of chinook salmon (Oncorhynchus tshawytscha) on the Sacramento River in California (U.S.A.) was the first Pacific salmon stock to be listed under the U.S. Endangered Species Act. We describe some of the characteristics of Pacific salmon populations that require special consideration in viability analysis during development of a model specific to the Sacramento River winter run of chinook salmon. Their anadromous, semelparous life history lends to a special definition of quasi-extinction. Random variability occurs primarily in spawning or early life and is reflected in the "cohort replacement rate," the number of future spawners produced by each spawner; a measure consistent with the common practice of characterizing salmon population dynamics in terms of stock-recruitment relationships. We determine the distribution of cohort replacement rates from spawning abundance data and life-history information. We then show through simulations that replacing this distribution with a lognormal distribution with the same mean and variance has a negligible effect on extinction rates, but that approximating an indeterminate semelparous life history using a determinate semelparous life history leads to inaccurate estimates of extinction rate. We derive delisting criteria that directly assess the effects of habitat improvement by explicitly including population growth rate (geometric mean cohort replacement rate greater than or equal to 1.0) in addition to abundance ( greater than or equal to 10,000 female spawners). These delisting criteria allow for the uncertainty due to limited accuracy in measuring spawner abundance and the finite number of samples used to estimate population growth rate (estimates must be based on at least 13 years of data, assuming spawner abundance is measured with less than 25% error). Because the probability of extention will generally be very sensitive to the uncertainty involved in meeting delisting criteria, we recommend that similar uncertainty be accounted for in future recovery criteria for all endangered species.