| Zusammenfassung: | The triclad flatworm Dugesia tigrina (Girard) is abundant in Lake Wabamum, a west-central Alberta lake receiving thermal effluent. A 14-month study was carried out to investigate D. tigrina's population ecology and the effect a thermally altered environment might have on Dugesia. Our data support the hypothesis that D. tigrina is limited by intraspecific competition for food during and after reproduction. In spring, both the normal water and heated water triclad populations grew rapidly and produced cocoons. Recruitment of young intensified food competition, causing shrinkage and mortality. The spring gain in length and weight was lost through shrinkage of individuals, and numbers of triclads in autumn were similar to prebreeding numbers. Hence the hypothesis is supported for populations in the thermally altered and the normal lake habitats. Triclads from the thermally enriched environment were found to be 2x as fecund as normal lake triclads. Cocoon production was probably increased to some extent by thermal effluent. However, the elevated mortality rate caused by the mechanical harvesting of abundant macrophyte populations in the warm water area, estimated at 1-3x the standing crop of triclads, probably accounts for much of the increased fecundity.
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