HEXAGENIA-BILINEATA (EPHEMEROPTERA, EPHEMERIDAE) PERSISTS AT LOW-LEVELS OF ABUNDANCE IN THE LOWER FOX RIVER, WISCONSIN

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 All ee effect. In addition, it is possible that the population is still li mited by poor environmental quality, presumably in the upper layer of sediment inhabited by the larvae. Two other relatively sensitive speci es associated with benthic habitat, the sea lamprey (Petromyzon marinu s) 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 cle ar that any of the three have established populations capable of succe ssfully reproducing in the lower Fox River on a consistent basis.