VIABILITY OF SACRAMENTO-RIVER WINTER-RUN CHINOOK-SALMON

The winter run of chinook salmon (Oncorhynchus tshawytscha) on the Sac ramento River in California (U.S.A.) was the first Pacific salmon stoc k to be listed under the U.S. Endangered Species Act. We describe some of the characteristics of Pacific salmon populations that require spe cial 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 e arly life and is reflected in the ''cohort replacement rate,'' the num ber 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 distribu tion 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 approx imating an indeterminate semelparous life history using a determinate semelparous life history leads to inaccurate estimates of extinction r ate. We derive delisting criteria that directly assess the effects of habitat improvement by explicitly including population growth rate (ge ometric mean cohort replacement rate greater than or equal to 1.0) in addition to abundance ( greater than or equal to 10,000 female spawner s). These delisting criteria allow for the uncertainty due to limited accuracy in measuring spawner abundance and the finite number of sampl es used to estimate population growth rate (estimates must be based on at least 13 years of data, assuming spawner abundance is measured wit h less than 25% error). Because the probability of extention will gene rally be very sensitive to the uncertainty involved in meeting delisti ng criteria, we recommend that similar uncertainty be accounted for in future recovery criteria for all endangered species.