Carrying Capacity of Reintroduced Anadromous Salmonids in Reservoirs on the Lewis River, WA

Increasingly, relicensing agreements of hydroelectric dams require managers to make habitat above many dams accessible to anadromous fish. In 2010 PacifiCorp Energy began trap and haul of Steelhead Trout (Oncorhynchus mykiss), Coho Salmon (O. kisutch), and spring Chinook Salmon (O.  tshawytscha) above Swift Dam on the North Fork Lewis River, WA. They are currently considering reintroduction in the lower two reservoirs (Yale and Merwin) as well. These species were extirpated when Merwin dam was constructed in 1931. Food web implications of reintroducing anadromous salmonids include seasonal availability of food and profitable foraging habitats. We are evaluating whether the three reservoirs will provide additional/alternative rearing and feeding habitat to lotic environments to better understand the feasibility and potential outcomes of reintroductions in each reservoir. In each reservoir we examined seasonal planktivore abundance and distribution, diets, and growth, as well as the thermal regime, to estimate consumption demand with bioenergetics models. We also examined monthly zooplankton density, biomass, and production. We estimated carrying capacity of the reservoirs by subtracting monthly consumption demand of zooplankton by resident planktivores from zooplankton biomass and production, and assessing the number of reintroduced juvenile salmonids that could be supported by the surplus food resources available.