PARENT SESSION
Tuesday, August 8, 5:00-6:30 pm
Poster Session 8 - Aquatic ecology
Exhibit Hall, Ballroom Level, Cook Convention Center

What factors mediate the impacts of introduced species? Investigating the success of kokanee since establishment of lake trout and Mysis relicta in a large, oligotrophic lake.

Schoen, Erik^*,1, Beauchamp, David¹, Overman, Nathanael¹, Sergeant, Christopher¹, ¹ University of Washington, Seattle, WA

ABSTRACT- In aquatic systems, the consequences of species introductions can vary widely among host communities, making outcomes difficult to predict. In many montane lakes across western North America, introductions of a top predator, lake trout (Salvelinus namaycush), and a planktivore, Mysis relicta, are implicated in the reduction or extirpation of planktivorous fish populations. However, in fewer lakes, these same species appear to produce few or no deleterious effects on local planktivores. We modeled population dynamics and trophic interactions in Lake Chelan, Washington, USA, where kokanee (Oncorhynchus nerka) populations have increased since the establishment of lake trout and Mysis. We evaluated three hypotheses for the success of kokanee: 1) Hatchery supplementation of kokanee sustains the spawning population; 2) Mysis consumes little of the zooplankton prey base, leaving ample prey for kokanee; 3) kokanee and/or Mysis are not readily available to lake trout and other predators, limiting predation and apparent competition. A bioenergetic assessment of predation on stocked kokanee fry suggested that hatchery supplementation alone is unlikely to explain the success of naturally spawning kokanee. We compared bioenergetics-based estimates of Mysis and kokanee consumption with spatial and temporal patterns of zooplankton abundance to determine whether resource competition was likely to limit kokanee. Lake trout diets contained large proportions of Mysis in a relatively shallow lake basin but not in the main, deep lake basin. Mysis densities were relatively high throughout the lake, but hydroacoustic evidence suggested that Mysis were less aggregated in the deeper basin, and thus less available to lake trout. Lake trout catch rates were much lower in the deeper lake basin than in shallower areas, suggesting that low Mysis availability may limit the lake trout population across most of the lake, and thus result in decreased predation on kokanee. Knowledge of the ecological factors that mediate the impacts of lake trout and Mysis can aid the conservation of native species in systems where introductions have already taken place and may allow scientists to identify particularly vulnerable systems to protect from future introductions.

Key words: aquatic ecology, food webs, kokanee (Oncorhynchus nerka)