Quantifying Effects of Lake Trout and Northern Pike On Stocked Salmonids in a Coldwater Reservoir

Recent sampling surveys in Pactola Reservoir, a coldwater fishery in the Black Hills, South Dakota, and anecdotal information from anglers have reported lowered catch rates of stocked salmonids.  Concomitantly, northern pike (Esox lucius) have been illegally introduced into the system.  The primary objective of this study was to quantify prey consumption by lake trout (Salvelinus namaycush) and northern pike in Pactola Reservoir, with emphasis on stocked salmonids. Fish were collected with gillnets in spring through fall 2012.  Diet analysis was conducted and seasonal feeding rates were estimated using bioenergetics modeling.  Other inputs in the bioenergetic models included water temperature and energy density of prey and predators. We estimated total, age-specific energy requirements for the two species.  Annual, energetic contribution of rainbow trout (Onchorycus mykiss) was then determined.  Similar calculations were performed for rainbow smelt (Osmerus mordax) and other forage.  Cumulative-annual consumption of rainbow trout (g/y) was calculated for each age class and used to estimate total number of rainbow trout consumed by an average individual.  These estimates allow for a comparative, economic analysis on the relative predation effects of lake trout and northern pike on rainbow trout.