The invasion of a non-native species into pelagic fish assemblages ORConceptual and empirical analyses of biological invasion : non-native fish invasion into North Temperature Lakes
Every summer from 1981 to 1991 pelagic fish assemblages were sampled in lakes at the Long Term Ecological Research (L TER) North Temperate Lakes site in Vilas County, Wisconsin. During that period a non-native fish, rainbow smelt (Osmerus mordax), invaded two study lakes. These two invaded lakes, along with a third uninvaded one, comprised a natural experiment, testing whether thermal preferences would direct spatial distributions of fish and determine the degree of interaction between invader and native species. The native species were cisco (Coregonus artedii) and yellow perch (Perea f/avescens). Water temperature was a good predictor of spatial distribution, and overlap between smelt and cisco (both coldwater species) was high, while overlap between smelt and perch (a coolwater species) was low. In the presence of smelt the cisco population declined sharply, but perch with smelt did not, nor did cisco alone. Next I examined the nature of the interaction between smelt and cisco. Evidence of food resource limitation was negative. Fitness measures of cisco (fecundity, growth, and condition) did not decline as would be expected if interspecific competition between the species were responsible for the cisco crash. An opportunity for adult smelt predation on young cisco occurs in June as larval cisco move to cooler temperatures occupied by smelt and adult cisco. I calculated that individual smelt in the lake in 1988 had to eat only 1 cisco each every 4 days during June to eliminate the cisco year class. The cisco decline can be accounted for by this predation gauntlet. Increasing occurrences of biological invasion require better tools to predict their consequences. Generalizations from ecological theory cannot address specific cases. In invasions by freshwater fishes, predation is the dominant mechanism creating change in the native assemblage. Focusing on predation opportunities, such as the predation gauntlet in the smeiUcisco interaction, is a pragmatic approach to dealing with fish invasions. Understanding how these predation opportunities differ among lakes may provide a useful predictive tool.