Sorel, M.H., A.G. Hansen, K.A. Connelly, A.C. Wilson, E.D. Lowery, and D.A. Beauchamp. 2016. Predation by Northern Pikeminnow and tiger muskellunge on juvenile salmonids in a high–head reservoir: implications for anadromous fish reintroductions. Transactions of the American Fisheries Society 145:521-536.
The suitability of reservoir habitat for rearing anadromous salmonids and interactions with resident fish affect the feasibility of reintroductions above high–head dams. We evaluated the predation risk to anadromous salmonids considered for reintroduction in Merwin Reservoir on the North Fork Lewis River in Washington State for two use scenarios: year–round rearing and smolt migration. We characterized the role of the primary predators, Northern Pikeminnow Ptychocheilus oregonensis and tiger muskellunge Esox, lucius x E. masquinongy using stable isotopes and stomach content analysis, quantified seasonal per–capita predation using bioenergetics modeling, evaluated the size and age structures of the populations, and combined these inputs to estimate predation rates of size–structured population units. Northern Pikeminnow of fork length (FL) ≥ 300 mm were highly cannibalistic with modest seasonal per–capita ¬predation on salmonids, but were disproportionately much less abundant than smaller, less piscivorous conspecifics. The annual predation on Kokanee Oncorhynchus nerka (in biomass) by a size–structured u¬nit of 1,000 Northern Pikeminnow of FL ≥ 300 mm was analogous to 16,000–40,000 age–0 spring Chinook Salmon rearing year–round, or 400–1,000 age–1 smolts migrating April–June. The per–capita consumption of salmonids by Northern Pikeminnow of FL ≥ 200 mm was relatively low, due in large part to spatial segregation during the summer and the skewed size distribution of the predator population. Tiger muskellunge fed heavily on Northern Pikeminnow, other non–salmonids, and minimally on salmonids. Predation by tiger muskellunge, in addition to cannibalism, likely contributed to low recruitment of larger (more piscivorous) Northern Pikeminnow, with the effect being decreased predation risk to salmonids. This study highlights the importance of evaluating trophic interactions within reservoirs slated for reintroduction, as they can be functional migration corridors and may offer profitable juvenile–rearing habitats despite hosting abundant predator populations.