Documentation of a Mass Emergence of Hexagenia Mayflies from the Upper Mississippi River

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Abstract
Hexagenia mayflies have been virtually eradicated by pollution from Green Bay of Lake Michigan, western Lake Erie, most of the Illinois River and from segments of the Upper Mississippi River. Hexagenia mayflies are still abundant throughout the less polluted portion of the Upper Mississippi River. On July 8-10, 1966, a spectacular mass emergence encompassed 550 miles of river. The emergence was documented by personal observation and by 59 observations made by cooperating personnel such as ship captains and lock-mas te r s.