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Examination of 167 specimens shows that immature fish feed upon planktonic forms while mature fish feed upon larger bottom organisms and bottom detritus. The diet varies with habitat and season, depending upon what is available. Since it eats the spawn of salmonids and takes much the same food organisms as they do, its presence in waters containing important salmonids is undesirable.

Cowichan Lake is 34 kilometres long, up to 4 kilometres wide and up to 150 metres deep, with an area of 62 square kilometres. Its average depth is 51 metres. Its shores are of precipitous rock for over half their length, the remainder being boulders or gravel, with a very little marsh.Summer surface temperatures are usually close to 20 °C., while the winter minimum in 1938 was 5.0°. Oxygen is generally abundant in the lake, the lowest value observed being 3.4 cc. per litre, just before the fall overturn (November 24, 1939). The reaction of the water is alkaline (pH 7.0–7.8), and its bicarbonate content was 18–22 mg. per litre (as CaCO3).Lists are presented of organisms collected in the lake. The plankton is reasonably rich in variety but was poor in quantity, in 1940 at least. Bottom organisms also are not abundant. Two species of lampreys and three fishes were found in the lake, plus a number of Salmonidae whose occurrence is described elsewhere.