A Genetic and Behavioural Analysis of the Distinctiveness of the Cultus Pygmy Sculpin (Cottus aleuticus) and Implications for Its Conservation

The Cultus pygmy sculpin (Cottus aleuticus) is endemic to Cultus Lake, southwestern BC, and is listed as threatened under the federal Species at Risk Act (SARA). The Cultus pygmy sculpin was first discovered by Ricker in the 1930’s (Ricker 1960) and was described as a dwarf coastrange sculpin (C. aleuticus). It matures at a smaller size than the “normal” C. aleuticus; has a lacustrine rather than a fluvial life history; and appears to undertake diurnal feeding migrations into the water column to feed on the abundance of Daphnia. The Cultus pygmy sculpin also has different morphological features, including larger cephalic pores, shorter pelvic fins, and a greater number of pectoral fin rays, which may be advantageous for an open-water, planktivorous lifestyle. Little else, however, is known of the biology of this fish. I used mtDNA and microsatellite analyses to determine whether there is any genetic distinction between the pygmy sculpin and the coastrange sculpin. Potential behavioural differences between the two forms were also examined by comparing vertical depth selection in the laboratory, with and without Cottus asper, a benthic predator found in Cultus Lake. Results from combined microsatellite and mtDNA analyses indicate that there is no clear phylogeographic separation between pygmy and coastrange sculpin, but that there is genetic differentiation at the population level. Pelagic sculpin from Lake Washington were compared with the Cultus pygmy sculpin and showed divergence from it, which suggests that the pelagic life history has evolved independently in each lake. The Cultus pygmy sculpin was found significantly higher in experimental depth selection tanks than the stream forms of C. aleuticus, but only when C. asper was also present in the tanks. Taken together, molecular and behavioural results indicate that the Cultus pygmy sculpin is discrete from the coastrange sculpin, and validate its recognition as a conservation unit distinct from “typical” C.aleuticus. The presence of abundant planktonic prey within the water column of Cultus Lake, combined with the presence of predators on the lake’s bottom, may be important features of the lake that have promoted the evolution of pelagic behaviour by the Cultus pygmy sculpin.