Non-native fish showing up in Thunder Bay
Written by The Alpena News   
Thursday, 23 March 2006 03:35

People who fish Thunder Bay may be greeted with an unusual sight in the coming days and weeks. A fish that has previously been scarce in Thunder Bay is proliferating along the shore wherever there are warm water discharges.

These fish are gizzard shad and they’re expected to begin dying off by the thousands.

"Based on our past experience, these fish can’t survive here," said James Johnson, a Michigan Department of Natural Resources fisheries research biologist at the Alpena Fisheries Research Station.

Gizzard shad are members of the river herring family, and according to Johnson, likely aren’t native to the upper Great Lakes. Johnson believes the fish reached the Great Lakes through navigation projects approximately 150 years ago.

In the past, gizzard shad were common only in a few areas of Lake Huron, including Saginaw Bay.

Johnson hypothesizes the collapse of the alewife population two years ago and the abundance of algae mats — a favorite food of the shad — on zebra mussel colonies may be two explanations for why the fish population suddenly exploded in the area.

According to Johnson, before alewives disappeared, they were so numerous gizzard shad young didn’t have had a chance.

Johnson has been getting reports there are an abundance of the fish in the discharge water off of Lafarge North America, Alpena Plant. He estimates there might be millions of gizzard shads in Thunder Bay.

Most of the fish that have been observed so far are less than a year old, and most are about seven inches long, which is unusually large for a gizzard shad under a year, Johnson said.

But they’re days are numbered. Literally.

Generally, the fish don’t survive in water below 39 degrees for more than 100 days. The water dipped below 39 degrees in November, Johnson said.

"It’s a natural event," Johnson said. "Gizzard shad just naturally die after 100 days of winter."

In fact, the fish are dying now.

"You can’t fish Thunder Bay and not be aware of this," Johnson said.

Because the gizzard shad are larger than usual, though, more may make it through to spring.

"Because they’re so large, there’s a good chance there will be a higher survival rate than we’ve seen in the past," Johnson said. "Once they make it past their first year, they’re not so sensitive to winter."

If more gizzard shad survive, there may be an annual cycle of reproduction and death.

"I’m almost sure this is going to be an annual event," Johnson said. "I think as long as the alewives stay scarce, this is what we’re going to see."

 
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