Modeling the effect of striped bass (Morone saxatilis) on the population viability of Sacramento River winter-run chinook salmon (Oncorhynchus tshawytscha)

We estimated the impact of striped bass (Morone saxatilis) pre­ dation on winter-run chinook salmon (Oncorhynchus tshawytscha) with a Bayesian population dynamics model using striped bass and winter-run chinook salmon population abundance data. Winter-run chinook salmon ex­ tinction and recovery probabilities under different future striped bass abundance levels were estimated by simulating from the posterior dis­ tribution of model parameters. The model predicts that if the striped bass population declines to 512,000 adults as expected in the absence of stocking, winter-run chinook salmon will have about a 28% chance of quasi-extinction (defined as three consecutive spawning runs of fewer than 200 adults) within 50 years. If stocking stabilizes the striped bass population at 700,000 adults, the predicted quasi-extinction probability is 30%. A more ambitious stocking program that maintains a population of 3 million adult striped bass would increase the predicted quasi-extinction probability to 55%. Extinction prob­ ability, but not recovery probability, was fairly insensitive to assumptions about density dependence. We conclude that winter-run chinook salmon face a seri­ ous extinction risk without augmenta­ tion of the striped bass population and that substantial increases in striped bass abundance could significantly increase the threat to winter-run chinook salmon if not mitigated by increas­ ing winter chinook salmon survival in some other way. Manuscript accepted 23 October 2002. Manuscript received 31 December 2003 at NMFS Scientific Publications Office. Fish. Bull. 101:321–331 (2003). Modeling the effect of striped bass (Morone saxatilis) on the population viability of Sacramento River winter-run chinook salmon (Oncorhynchus tshawytscha)