Large Mayfly Hatch Prompts Public Concern

Discarded skin casings from mayfly larvae line the shore.

A mayfly exuvia up close.

An adult mayfly.

On June 29, 2007, Ted Halpern with the Minnesota DNR, phoned Minnesota Sea Grant to pass along a report from the public of a possible fish kill at the Duluth Rowing Club on the bay side of Park Point. Halpern said that a woman who was visiting the area with her daughter gave an account of "hundreds of two-inch fish" washing up on the shoreline.

Making independent visits, Halpern and Doug Jensen, Minnesota Sea Grant's Aquatic Invasive Species Program Coordinator, didn't find fish bodies, but they did find animal remains. Due to a massive mayfly (Hexagenia sp.) hatch over the last few days, tens of thousands of cast-off skin casings (exuviae) of the emerged mayflies washed up along the shoreline. At a glance, the exuviae could be confused with dead fish.

"Even though the report was incorrect, it signifies the public's concern for the St. Louis River and aquatic resources," said Jensen.

"Based on the quantities of exuviae washing up along the shoreline, this may be one of the largest mayfly emergences in the past 20 years in this area," said Jensen. "It reflects improved water quality in the St. Louis River."