Quantifying Latent Impacts of an Introduced Piscivore: Pulsed Predatory Inertia of Lake Trout and Decline of Kokanee

Abstract

AbstractIntroduced long‐lived predators often cause significant impacts on their prey, but these impacts can be masked from detection due to high “predatory inertia”: time lags in population growth and dietary ontogeny. We evaluated whether predation by introduced lake trout Salvelinus namaycush could explain the 88% decline in escapement of kokanee Oncorhynchus nerka during 2005–2009 in Lake Chelan, Washington. We quantified the strength and trend of predation impacts with field sampling, a hydroacoustic assessment of kokanee production, and bioenergetics and age‐structured population models of lake trout. Lake trout consumption of kokanee exceeded kokanee production, indicating strong predation impacts at the start of the decline. Fully piscivorous lake trout (>550 mm fork length) were responsible for 83% of this predation. The population model predicted that a pulse of strong stocked cohorts crossed this piscivorous size threshold, causing the biomass of fully piscivorous lake trout to expand by roughly 70–300% during 2004–2009 and driving predation pressure to peak levels. Together, these results suggested that lake trout predation was a large and growing source of kokanee mortality during the decline. Counterintuitively, predation pressure was projected to increase even if the numbers of harvestable lake trout declined, as strong cohorts grew to piscivorous size while succumbing to mortality. Angler catch rates of lake trout declined by 40% during 2004–2007, as was predicted by the population model; this decline in catch masked the rise in predation pressure. This analysis demonstrates the potential for introduced predators exhibiting high predatory inertia to cause strong, latent impacts on prey that would be unexpected based on harvest trends and prior dynamics alone. Forward‐looking monitoring and modeling analyses are clearly advantageous for managers who seek to maintain ecosystems in long‐term “balance” by detecting and reversing incipient changes in predation.