Mean Free-Path Length Model Applied to Sacramento Valley Juvenile Chinook Salmon

Chinook salmon populations in the Sacramento River watershed in northern California are at low levels and have low juvenile survival rates. Based on a model derived in Anderson et al. (2005), we use telemetry data of juvenile migrations to estimate the mean length of an individual’s unconstrained movement path from release to its first predator encounter. The model considers both directional and random elements of movement to predict path lengths, and estimates the relative importance of migration distance and time in determining population-level survival rates. The original application of this model to Columbia River salmon revealed large differences between the random movements of predators and prey, indicating that the distance traveled by smolts was a strong indicator of survival. Here we apply the model to a dataset of outmigrating smolts in the Sacramento River. Our model predicts a smaller difference between the random velocities of predators and prey, suggesting that travel time is more important for determining survival rates in this river. We also see differences between tidal/non-tidal reaches as well as seasonal predation patterns. A clearer understanding of the threats facing juveniles will be vital as managers improve the Sacramento River to increase populations of Chinook salmon.