North Platte Reservoirs Stocked Salmonids Fate
July 2021 - June 2024
- Wyoming Game and Fish Department
Pathfinder Reservoir and Alcova Reservoir (Upper North Platte Reservoirs; UNPR) maintain important recreational fisheries for Walleye (WAE), Brown Trout (BNT), Rainbow Trout (RBT), Kokanee Salmon (KOE), and Cutthroat Trout (CUT). WAE recruitment is maintained through natural reproduction, while recruitment of salmonids (other than BNT) is primarily driven by stocking, though some natural reproduction does occur. In several recent years, WGFD spring gillnet surveys in Pathfinder Reservoir suggest near 100% mortality of salmonids stocked the previous fall. Similar surveys in Alcova Reservoir suggest reliable and stable recruitment of stocked salmonids following their first winter, although it varies by species. WGFD biologists suspect WAE predation on newly-stocked salmonids (SSD) is the primary driver of stocking failures; however, verifying this hypothesis and quantifying the seasonal magnitude of WAE consumption of SSD is required to develop and evaluate alternative management actions. The goals of this proposed work are to 1) quantify the extent and timing of WAE consumption of SSD in the NPR, and 2) to evaluate how resource use by SSD post-stocking affects the vulnerability of SSD to WAE predation. WAE, SSD, and other fish species will be sampled during the open water period using experimental gill nets, midwater curtain gill nets, and nearshore sampling gears (electrofishing, trapnets, seines) to quantify population demographics (relative abundance, age/size structure), foraging dynamics (stomach content and stable isotope analyses), and indices of condition. These data will be integrated with temperature data and coarse estimates of low, medium, and high population size into a bioenergetics model to estimate consumption of stocked SSD by WAE. Estimates of consumption will be compared to SSD resource use and stocking rates to assess the relative importance of WAE predation as a source of mortality for different SSD species. Results from this project will provide the WGFD with an improved understanding of the timing and magnitude of WAE consumption of SSD in the UNPR. This information will inform harvest regulations for WAE, SSD, and other fish species, and could guide development of a future SSD stocking evaluation to identify best practices to maximize survival post-stocking (e.g., composition, fish size, density, location, and timing). This research is a collaboration of researchers across multiple agencies and includes the Wyoming Game and Fish Department, Casper Regional Fisheries Management Crew, Aquatic Assessment Crew, and the University of Wyoming.