An eagle-eyed St. Paul resident traipsing through nature spotted hundreds of dead fish floating Wednesday in an icy Mississippi River bay near Shepard Road and Randolph Avenue. The site was both alarming and unprecedented for her, but the Minnesota Department of Natural Resources maintains it’s not unexpected.

In the waning days of winter, a particular species of fish ill-suited for the Minnesota cold — the gizzard shad — is prone toward going belly-up on the Mississippi in the southern part of the state.

“We are observing a large shad die off on the river in the last couple of weeks,” said DNR spokesperson David Tauchen, in an email Thursday. “We do expect some additional reports in the next coming days as the dead and dying shad float to the surface and decompose.”

The gizzard shad in Minnesota are at the northern-most reach of their native range, making them a fish out of water, so to speak, during Minnesota winters, where they tend to fend poorly against the stress of the cold. Shad like to gather in dense shoals in river backwaters, especially areas with warm water outflows from industrial operations, according to the DNR.

During the winter, even a temporary shutdown of a warm water source can be enough to kill off smaller shad. Fluctuating temperatures like those the metro experienced over the past week also contribute to their die-offs.

Despite their sensitivities, shad reproduce quickly and host “recruitment events” in southern Minnesota almost annually, making them plentiful in certain areas. Receding ice during spring thaw can free carcasses to float to the surface or the shoreline.

The DNR Fish Health Laboratory tests shad from reported fish die-offs for a widespread virus known as VHSV as part of annual monitoring, but VHSV has never been detected in association with a gizzard shad die off in Minnesota, according to DNR officials.

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