Abstract

Stocking exotic trout species in high mountain lakes was a common practice in Rocky Mountain Lakes in the 20th century. Currently, restoration action to remove trout populations favours the conservation of native food webs. Meanwhile, little is known about the self-sustaining trout populations, particularly in our study area. We assessed impacts of stocking trout on naturally fishless foodwebs of mountain lakes. Results were applied to management goals in Waterton Lakes National Park. A comparison of mean abundances of zooplankton in fish and fishless lakes revealed differences in less than half the taxa encountered. Principle component analysis did not show an association between zooplankton community structure and fish presence. Paleolimnological analyses suggested an increase in relative abundance of large-bodied cladocerans, but statistical power was low. The results show the importance of identifying explicit restoration objectives because impacts may not be compelling. If restoration by trout eradication is pursued, demographic characteristics can aid in selecting which populations are more easily depleted by gillnetting. We used generalized linear models to examine four population characteristics associated with population decline: 1) catch per unit effort (CPUE), 2) proportion of females, 3) proportion of mature individuals and 4) length of mature individuals. There were significant differences between populations in CPUE and length at maturity, but not on the proportion of females or mature individuals. We thus incorporated the former characteristics into a basic assessment system and ranked the 11 salmonid populations by their susceptibility to eradication. We presented a simple yet meaningful step in facilitating management actions commonly constrained by a lack of biological knowledge.