A State-Space Approach to the Estimation of Juvenile Out-Migrant Survival of Chinook Salmon (Oncorhynchus tshawytscha) in the Lower American River, California

Over the last century, anadromous salmon and steelhead (Oncorhynchus sp.) populations have declined significantly across the West coast of North America.  This is especially evident in the waters of California’s Central Valley.  Since the passage of the Central Valley Improvement Act in 1992, there have been efforts to increase the number of naturally reproducing salmon and steelhead in these waters.  Of particular concern is the estimation of the survival rates of out-migrant juveniles (fry, parr, and smolts), specifically the transition from an out-migrant juvenile to a spawning adult.  Previous modeling has documented that these transition rates are perhaps the most influential factors in determining population dynamics of salmonids in the Central Valley.  Here, we compare two methods for estimating juvenile out-migrant survival of Chinook salmon (O. tshawytscha) in the American River, CA.  The first method uses a life history model and a heuristic optimization routine while the second method is a Bayesian state-space approach.  The state-space approach provided much more consistent and more ecologically sound estimates of out-migrant survival rates.  In addition, the state-space approach was also much less data intensive and thus is more likely to applicable to other populations where extensive data is not available.