An Assessment of the Impact of Non-Native Lake Trout Salvelinus namaycush and Mysis diluviana on the Growth and Survival of Pelagic Planktivores in Lake Tahoe

Abstract

Due to introductions of non-native species and the extirpation of native species, the food web of Lake Tahoe has undergone many changes over the past century. This study quantified temporal and ontogenetic trophic interactions in Lake Tahoe to determine if pelagic planktivore growth was limited by food supply or by predation. Field data on growth, diet composition, abundance, distribution, survival, and thermal experience were used in bioenergetics models to quantify the seasonal consumption demand by Kokanee Oncorhynchus nerka, mysids Mysis diluviana and Lake Trout Salvelinus namaycush. Seasonal and depth-specific biomass was estimated for the key planktivore prey, adult Diaptomus and Epischura, and Lake Trout prey, mysids, Kokanee, and Lake Trout. Kokanee fed primarily on the copepods Epischura and Diaptomus and mysids. Mysids became increasingly important for Kokanee > 200 mm, but particularly so for adults > 300 mm. Stable isotope values of δ15N indicated that mysids began to incorporate copepods into their diet between 8 - 12 mm TL, but mysids >12 mm TL still relied heavily on phytoplankton. When comparing the depth-stratified supply of zooplankton to the vertical distribution and consumption demand by mysids and Kokanee, the year round supply of zooplankton was sufficient for Kokanee and pelagic cyprinids, but still limited potential growth. The zooplankton supply was insufficient for mysids to consume the proportion of copepods modeled during spring, summer and winter. Vertical distribution and thermal segregation allowed planktivorous fish access to zooplankton prey that was unavailable to mysids. Lake Trout undergo an ontogenetic dietary shift wherein fish feed predominantly on mysids when fork length (FL) ≤ 500 mm, become increasingly piscivorous over 501-625 mm, and are primarily piscivorous at FL >625 mm with fish prey representing 80% of the diet. Lake Trout targeted predation on pre-spawning adult Kokanee through the summer and fall with the Lake Trout > 625 mm consuming 24% of the adult Kokanee biomass. In Lake Tahoe, Lake Trout can potentially regulate the Kokanee population, while also self-regulating their own population via cannibalism. Although heavy, Lake Trout predation does not limit the mysid population; mysid production (growth) throughout the year is sufficient enough to absorb the mortality sustained from Lake Trout predation. Understanding the factors that allow the co-existence of Lake Trout, mysids and Kokanee will be beneficial to managing these species in other systems.