After burrowing mayflies (Hexagenia bilineata) were first noted in the vicinity of the DePere Dam on the Fox River in 1991, adults have been observed in small numbers each summer since then. It is possible that the Fox River population has remained at low levels because of an Allee effect. In addition, it is possible that the population is still limited by poor environmental quality, presumably in the upper layer of sediment inhabited by the larvae. Two other relatively sensitive species associated with benthic habitat, the sea lamprey (Petromyzon marinus) and the lake sturgeon (Acipenser fulvescens), have been observed in the Fox River in recent years. Collectively these species provide an indication of improved environmental conditions, but it is not yet clear that any of the three have established populations capable of successfully reproducing in the lower Fox River on a consistent basis.
Cochran, Phillip A. and Kinziger, Andrew P.
"Hexagenia Bilineata (Ephemeroptera: Ephemeridae) Persists at Low Levels of Abundance in the Lower Fox River, Wisconsin,"
The Great Lakes Entomologist, vol 30
Available at: https://scholar.valpo.edu/tgle/vol30/iss2/4