Modeling the habitat availability (opportunity) and prey availability (capacity) helps scientists understand the potential increases in Chinook salmon populations as a result of the delta restoration. By modeling the actual physical and biological changes of salmon, such as presence, diet samples, and growth rates, scientists can understand the actual responses of salmon to restoration, or realized function. This points to on-the-ground changes in salmon populations and is vital information to have in the context of salmon recovery or when planning another large scale restoration project.
The otolith is a small bone-like structure inside the inner ear of a fish. Similar to trees, the otolith forms rings that can tell scientists valuable information like how fast a fish grew (growth rate) or the amount of time spent in a particular habitat (also known as residency time.) Between 2010 and 2011, the Tribe and USGS Western Fisheries Research Center collected Chinook otoliths to establish an estuary growth rate, average residency time, and life history diversity. These results were compared to pre-restoration otolith analysis and is useful in determining if large scale estuary restoration influences Chinook growth rates and residency time.
The otoliths were analyzed to determine how long each fish had spent in different habitat zones: freshwater (FW), tidal delta (TDCK) and delta flats/nearshore (DF/NS). The fork lengths (above) show transitions between habitat types, also known as life histories. Furthermore, the number of increments shows the number of days a fish stayed within a habitat zone, while the mean increment width shows the growth rate of the fish.
»Life history: Post-restoration, all Chinook observed has one life history–“Delta Users.” This means the juvenile Chinook leave the freshwater, and migrate to the tidal delta, where they spend time before moving into the delta flats or nearshore environments. This is compared to “fry migrants,” or those fish who leave the freshwater and immediately migrate into the delta flats habitat, avoiding the tidal delta all together. Prior to the restoration, both life histories were observed.
»Growth rates: Fish growth rates increase as Chinook, both natural and hatchery, transition between the freshwater and tidal delta habitats, and as they transition from the tidal delta to the delta flats/nearshore. Pre-restoration growth rates were greater than post-restoration. However, the fish migrating in post restoration years have a greater advantage: they enter the tidal delta at a bigger size and better prepared for ocean migration.
»Residency time: Both natural and hatchery Chinook spent more time in the delta post-restoration than they did before, by as much as 1.5 weeks. Natural Chinook spend about 1/2 a week longer than hatchery Chinook, for an average of 3 to 5 weeks residency time.
To understand fish diets, the stomach content of juvenile Chinook salmon were analyzed. The prey found inside of the stomachs were compared to the invertebrate populations found in the surrounding habitats. This comparison (or prey similarity index, PSI) allowed scientists to understand if the fish were able to find enough prey within the restored area. If invertebrate communities closely matched stomach content, it implied that the surrounding habitat had the capacity to support fish. If the stomach contents varied greatly from nearby invertebrate communities, it suggested that the habitat lacked the prey capacity necessary for fish needs.
The researchers found that juvenile Chinook ate a variety of prey, ranging from crustaceans to flies. Diets also varied between channels, reflecting differing rates of recovery of invertebrates among the channels. (Learn more about why on the ‘Vegetation’ and ‘Fish: Prey Capacity’ pages).When monitoring ended in 2012, the fish diets were most similar to invertebrate communities in the reference areas. In contrast, the stomachs of fish captured in restored areas contained species different than the environments in which they were found. This suggests that restored sites still lack prey capacity needed to act as a fully functioning ecosystem. That trend decreased over the course of the study, however.
- A.T. David, C. Ellings, I. Woo, C. Simenstad, J. Takekawa, K. Turner, A. Smith, and J. Takekawa. 2014. Foraging and Growth Potential of Juvenile Salmon after Tidal Restoration of a Large River Delta. Transactions of the American Fisheries Society, 143:6, 1515-1529.