AN ISOLATED POPULATION OF FOURHORN SCULPINS (MYOXOCEPHALUS-QUADRICORNIS, FAMILY COTTIDAE) IN A HYPERSALINE HIGH ARCTIC CANADIAN LAKE

Freshwater sculpins probably evolved from marine ancestors which enter ed bodies of water such as proglacial lakes or lakes which were gradua lly isolated from the sea by isostatic rebound. Sculpins in fresh wate r lakes (Myoxocephalus thompsoni [Girard]) lack cephalic horns and liv e well below a depth of 10 m. Those in the sea (Myoxocephalus quadrico rnis [Linnaeus]) typically live above 10 m and possess a well develope d set of four cephalic horns. The sculpins in Garrow Lake, North West Territories, are intermediate between the marine and fresh water forms with respect to their depth distributions and their cephalic horns (s pines). As a consequence, Garrow Lake, which separated from the sea so me 3000 years ago, serves as an excellent 'laboratory' for studying ev olutionary changes in this sculpin. The age of the lake was based on c arbon-14 dates of the fossil pelecypods from raised beaches around the lake and from observations of rates of isostatic rebound in the area as reported by Dickman & Ouellet 1983 and Page et al. 1984. During the last 3000 years, the surface waters of Garrow Lake have freshened and its sculpins have apparently adapted to this top down freshening by o ccupying a depth where the salinity of the lake approaches that of sea water. As a result, the sculpin population in Garrow Lake lives deepe r than the sculpin population in the nearby Garrow Bay. Thus, the deep er dwelling Garrow Lake sculpins appear to be less vulnerable to avian predation than their shallow water dwelling marine ancestors. It is h ypothesized that reduced avian predation of the Garrow Lake sculpin po pulation is associated with the observed reduction in their cephalic h orns which impart a certain degree of disruptive colouration and disru ptive pattern outline allowing the shallow dwelling marine species to blend in with its background in a manner which appears to make it less visible to avian predators. It is unfortunate that the three thousand year old Garrow Lake sculpin population is now endangered by mine tai lings entering the lake from the nearby Cominco Ltd. mine. The entire food chain of the lake appears to have been severely impacted by lead and zinc mine tailings entering Garrow Lake at a rate of 100 metric to nnes per hour.