Using Stable Isotope Analysis and Multiple-Source Mixing Models to Assess the Influence of Hypolimnetic Oxygenation on Diet in a Mixed Cold- and Warm-Water Fish Community

A temperature/dissolved oxygen “habitat squeeze” is a common problem impacting cold-water fisheries in deep, eutrophic lake systems. A habitat squeeze may increase competition, reduce access to food sources, increase physiological stress, reduce fecundity, and generally negatively impact fish.  Hypolimnetic oxygenation (HO) is an increasingly popular management tool that may be applied to address the temp/DO squeeze. Our previous studies demonstrate cold-water fish respond rapidly to utilize habitat and prey sources made available by HO. However, previous studies did not utilize stable isotope analysis to assess diet and prey assimilation over time. The present study examines the effects of HO on the diets of largemouth bass (Micropterus salmoides), golden shiner (Notemigonus crysoleucas), rainbow trout (Oncorhynchus mykiss), and brook trout (Salvelinus fontinalis) in Twin Lakes, Washington.  Fish diets in treated North Twin Lake and untreated South Twin Lake are compared with stable isotope analysis and multiple-source mixing models. Rainbow trout diets are also compared monthly between lakes.  We anticipate observing a difference in trout diet between treated North Twin and untreated South Twin Lakes as trout access habitat created by HO, however, it is still unclear what effect HO will have on largemouth bass and golden shiner populations in the lakes.