Food–web implications of reintroducing anadromous salmonids in reservoirs on the North Fork Lewis River, Washington
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Reintroductions using active transport (trap–and–haul) of anadromous salmonids around impassable high–head dams are being proposed with increasing frequency in the Pacific Northwest. Such reintroductions have already begun or are proposed above three dams on the North Fork Lewis River in Washington State. Spring Chinook Salmon Oncorhynchus tshawytscha and Coho Salmon O. kisutch have been observed rearing in several lakes and reservoirs elsewhere, raising the question of the potential for reservoir habitat to contribute to smolt production in the Lewis River. To determine whether the Lewis River Reservoirs can support reintroduced populations of juvenile salmonids, we evaluated prey supply and consumption demand of resident fishes to estimate the carrying capacity for planktivores. We also evaluated the potential predation impact of Northern Pikeminnow Ptychocheilus oregonensis on reintroduced anadromous salmonids in Merwin Reservoir, in response to concern that predation by the large population of native piscivores could undermine the reintroduction effort. To achieve these objectives, we examined the thermal regime, food–web structure, invertebrate abundance and distribution, and fish abundance and distribution in a spatiotemporally explicit manner in each reservoir. We then quantified trophic interactions with bioenergetics and population modeling. In all three reservoirs on the North Fork Lewis River, estimates of prey supply suggested that a moderate surplus of food was available to support the addition of juvenile anadromous salmonid consumption demand. The preliminary estimates for the abundance of sub-yearling salmonids that could be supported ranged 130,000–330,000 for each of the individual reservoirs; however, investigation into the behavior and growth of juvenile anadromous salmonids in the reservoirs would refine these estimates. Estimates of predation rates on resident salmonids were used to infer potential predation rates on reintroduced anadromous salmonids rearing year–round or migrating through Merwin Reservoir. We found that per–capita predation rates by Northern Pikeminnow were relatively low due to non–overlapping distributions during much of the growing season and the size structure of the predator population. The annual predation of Kokanee Oncorhynchus nerka by a size–structured population of 1,000 Northern Pikeminnow ≥ 300 mm was analogous to predation losses of 16,000–40,000 sub–yearling spring Chinook rearing in Merwin Reservoir year–round. Estimated salmonid consumption by 1,000 Northern Pikeminnow during peak smolt out–migration (April–June), if directed solely at migrating smolts, was analogous to 400–1,000 yearling spring Chinook Salmon (18–45 g each). These results can help managers set goals for the ongoing and proposed reintroductions of anadromous salmonids in the Lewis River that are desirable to stakeholders and realistic given the constraints of habitat, food supply, and predation losses.
- Fisheries