The introduced aquatic macrophyte, Myriophyllum spicatum, as habitat for fish and their invertebrate prey

The exotic macrophyte Myriophyllum spicatum spread widely in Lake Opinicon, Ontario, after 1974, dominating the 2.0- to 3.5-m depth zone which had previously been largely open water. In the shallows it formed beds within the species-rich native Potamogeton–Vallisneria community. This paper compares fish and prey-invertebrate abundance and distribution (i) before and after the milfoil invasion for a series of depth zones (1972 relative to 1979–1980), and (ii) in communities of M. spicatum relative to native macrophytes in the littoral zone shallows in the summer of 1980. At a depth of 2.0–3.5 m (major M. spicatum zone), there were significantly fewer bluegill but more black crappie and golden shiner in 1979–1980 compared with 1972, and significantly fewer isopods, chironomid and trichopteran larvae, and ephemeropteran nymphs. Differences were greatest in late summer. Inshore and offshore from the M. spicatum zone, differences were minor, suggesting that the invader, not time, was responsible. At a 1.0- to 1.3-m depth in the summer of 1980, three to four times as many fish occurred during the daytime feeding period in the Potamogeton–Vallisneria community than in the M. spicatum area. In the benthos, beneath the native beds, five major taxa of prey invertebrates were three to seven times more abundant. The foliage of Potamogeton plus Vallisneria supported twice as many invertebrates per square metre in late summer than did M. spicatum. Insect emergences over the summer (May–September) were twice as high. Up to the present, the net impact of the habitat change on the fish populations of Lake Opinicon has not been great. If in the future M. spicatum were to replace the rich native macrophyte beds in the shallows, this would be cause for concern.