Fish: Prey Capacity


The delta restoration project introduced tides to the Nisqually estuary for the first time in over a century, changing vegetation communities, channel morphology and water quality. The physical changes presented new opportunities for salmon to use restored habitats as they migrated from the freshwater to saltwater. Yet the success of this restoration cannot be determined based on the habitat opportunities alone–scientists must also examine the capacity of the new habitats to provide food for juvenile salmon. By sampling the invertebrate community structure, biodiversity, key species densities and overall biomass, scientists are able to get a broader view of the impacts of the Nisqually Delta Restoration on Chinook salmon and other fish.

In general, different salmon species prey on varying invertebrates: Chinook salmon typically eat arthropods–like flies and ants–that fall into the water from surrounding vegetation. On the other hand, chum salmon prefer planktonic invertebrates and small crustaceans found in the water. To complicate matters, salmon species migrate at different times of the year. Therefore, salmon foraging capacity is closely linked to timing of available prey, not just the variety.

Scientists from the Nisqually Indian Tribe and USGS collected three types of invertebrate samples: benthic, terrestrial, and planktonic. Benthic invertebrates are small creatures that live in the rocks and sand of waterbodies–examples include stoneflies, mayflies and dragonflies. These were monitored in August or September 2009-2012. Terrestrial invertebrates, including flies and beetles, were monitored in April-July of 2009-2012. Finally, planktonic invertebrates were monitored April-July of 2009-2012.

Additional information collected as a part of this monitoring effort included soil and water quality information and fish gut contents. The soil and water quality data was collected in conjunction with the benthic invertebrate sampling. Fish gut contents verified Chinook diets. In all, 118 fish gut samples were collected in April-July of 2012.

Invertebrate species were monitored across the Nisqually Delta. Sites located east of the Nisqually River were restored in 2006, while those on the west of the river were restored in 2009. Benthic invertebrates were monitored based on distance from delta.

Invertebrate species were monitored across the Nisqually Delta. Sites located east of the Nisqually River were restored in 2006, while those on the west of the river were restored in 2009. Benthic invertebrates were monitored based on distance from delta.



The delta restoration has the potential to enhance salmon habitat at the Nisqually River Delta. However, the available habitat is only a small part of the components necessary for salmon recovery. Food sources are vital too. The Nisqually River supports several different species of salmon, all of which migrate at different times of the year, and prefer different invertebrate species. This study has helped scientists understand what invertebrates are present during different times of the year. The information will help scientists understand more fully the capacity of the Nisqually River Delta in supporting various salmon species.

In an even greater context, the invertebrate communities across the delta form the basis of the food chain for many migratory bird species. Understanding the types of insects and invertebrates present during key foraging seasons helps land managers better understand the ability of the delta to provide food for bird species.

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