HISTORICAL STATUS OF FISH POPULATIONS IN CANADIAN ROCKY-MOUNTAIN LAKES INFERRED FROM SUBFOSSIL CHAOBORUS (DIPTERA, CHAOBORIDAE) MANDIBLES

We used subfossil Chaoborus mandibles preserved in sediments of Cabin, Caledonia, and Celestine lakes, Alberta, to determine whether fish we re present in the lakes (which now contain rainbow trout (Oncorhynchus mykiss) and lake chub (Couesius plumbeus)) prior to the first fish st ockings early this century. We first tested whether Chaoborus were goo d indicators of the presence of fish in montane lakes in a survey of 4 3 lakes in Jasper and Banff national parks. Chaoborus americanus was t he only species inhabiting fishless lakes and was also found in three lakes with low fish density (co-occurring with C. flavicans in two of the three lakes). Other lakes containing fish had either C. flavicans or no Chaoborus species. Subfossils revealed that C. americanus was th e predominant species prior to this century in Cabin and Celestine lak es, suggesting that these lakes were originally fishless; C. americanu s (and C. trivittatus in Cabin Lake) were eliminated early this centur y, contemporarily with the first recorded fish stockings. Chaoborus fl avicans was the only species recovered from Caledonia Lake, suggesting that the lake had fish prior to this century; while it is not possibl e to identify the species originally present, historical evidence sugg ests that rainbow trout is native to Caledonia Lake.