Quantifying the impact of two native predators on juvenile sockeye salmon survival in Lake Washington
Clark, Casey Porter
MetadataShow full item record
Understanding the mechanisms regulating population fluctuations, such as births, reproduction, and deaths, remains a persistent question in ecology. Mortality can be incurred at any point during the lifecycle, but mortality rates can be high for juvenile animals in particular. One source of this early life history mortality is via predation. Accurately measuring predation rates requires intensive studies, and is further complicated by variability in habitats and changes in the extent of spatial and temporal overlap between predators and prey. In this study, we examined abundance, spatial distributions and diets of two piscivorous fishes and their predation impact on juvenile salmonids in a well-studied large western lake, Lake Washington. We addressed the following questions: (i) What is the abundance of cutthroat trout and northern pikeminnow in Lake Washington? (ii) At what rates do these predators consume juvenile salmon throughout the year, and how many total juvenile salmon are consumed given the predator population size? Understanding which factors influence predation and the extent of this predatory impact requires temporally and spatially explicit data on the interaction between predators and prey, including both juvenile sockeye salmon and the main alternative prey fish species. To observe these interactions, we used several overlapping sampling types across trophic levels. We used a Chapman-modified population estimation procedure to estimate a cutthroat trout population size of 22,791 ≥300 mm FL and the same procedure to estimate a northern pikeminnow population size of 13,582 ≥300 mm FL. We also used a relative catch method to estimate a northern pikeminnow population size of 112,816 ≥300 mm FL. The magnitude of predation on juvenile salmon and other prey fishes varied considerably among months and between cutthroat trout and northern pikeminnow, between small and large size classes of each predator species, and between years. In 2015, predation mortality of lake entry 2015 (sub-yearling) sockeye salmon was 20% of fry production in that year, and predation losses of lake entry 2014 (yearling) sockeye were 56% of pre-smolt production. In 2016, mortality of lake entry 2016 (sub-yearling) sockeye was 44% of fry production, and predation losses of lake entry 2014 sockeye were 473% of pre-smolt production of that year. Our work shows that the current predation rate is high enough and these predator populations are of sufficient size that predation is a significant source of mortality for juvenile sockeye and Chinook salmon in this system. This work highlights a scenario of combined physical and biological factors that influence mortality in juvenile fish, and can potentially inform the potential for predation mortality in other lake systems.
- Fisheries