Fish: Realized Function


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.

Otolith Analysis

This magnified cross section of a Nisqually unmarked juvenile Chinook otolith is representative of the results observed during monitoring. To “read” the otolith, move from right (starting at E) to the left (ending at N). In this otolith, increments (or rings) were thin and narrowly spaced until the fish transitioned from the freshwater to the tidal delta (marked as TDCK). Beyond then, the increments were thicker and more widely spaced, which indicates increased growth.  Standard measurements between habitats such as otolith radial distances, mean increment widths (MIW) and increment counts can be taken to provide growth rates, residence times, and related information. Abbreviations: E=emergence, FF=first feed, FW=freshwater residence, TDCK=tidal delta check, and TD=tidal delta residence.

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.

Diet Analysis

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).

Chinook Salmon Foraging and Growth Potential. Mean proportional diet composition (by prey mass) for juvenile Chinook Salmon at each fyke-net sampling event in restored (Rest) channels (restored in 2006 and 2009) and reference (Ref) channels (Nisqually [Nisq Ref] and Red Salmon Slough [RSS Ref]). The numbers above the bars are sample sizes (number of fish analyzed for diet composition). Source: David et al.

Chinook Salmon Foraging and Growth Potential. Mean proportional diet composition (by prey mass) for juvenile Chinook Salmon at each fyke-net sampling event in restored (Rest) channels (restored in 2006 and 2009) and reference (Ref) channels (Nisqually [Nisq Ref] and Red Salmon Slough [RSS Ref]). The numbers above the bars are sample sizes (number of fish analyzed for diet composition). Source: David et al.

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.

Return to Fish.

Read more:

  • 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.