Invasion success and population characteristics of the opossum shrimp, Mysis diluviana, in Wyoming, USA

Studying the colonization, distribution, demographics, and abundance of invasive species is important for understanding their invasion biology, including the conditions required for establishment This information can also be used to reduce their risk of spread. Opossum shrimp Mysis diluviana Audzijonyte and Vainola, 2005, is an invasive species in lakes and reservoirs of the western United States and Canada Four lakes in western Wyoming, USA, were stocked with this nonnative crustacean in 1971, but no Mysis surveys have been conducted in Wyoming since 1981. We determined presence/absence, demographics, and abundance of Mysis in these and six nearby lakes that could have been invaded using vertical net tows and environmental DNA analysis. Environmental conditions were compared in lakes with and without Mysis, and we evaluated the potential for Mysis to disperse downstream. Mysis (> 500 individuals/m(2)) persisted in two of the four stocked lakes and nowhere else. Both of the lakes with established populations had daytime light levels on the bottom below the visual feeding threshold for fish, and the hypolimnia were oxygenated. Hierarchical cluster analysis of lake physicochemical conditions grouped these two lakes with four others, all of which were deep (46-185 m), with high oxygen concentrations (> 3 mg/L) on the bottom, and relatively low light intensities (< 0.2 lx) near the bottom. A second cluster of lakes that all lacked Mysis, and appeared to be less suitable, were shallow (< 20 m), had severe hypolimnetic hypoxia, and higher light levels (>= 590 lx) near the bottom. The interaction of strong light penetration with lake depth, compounded by strong clinograde oxygen profiles, would prevent the formation of a daytime predation refuge from fish in these lakes, reducing the likelihood of Mysis invasion. Given that only half of the purposeful introductions in Wyoming were successful, and that there have been no new invasions in nearly 50 years, future range expansion by the species in the region is unlikely without human facilitation.