Lake Huron numbers fall -- fish go to Canada
Written by Detroit Free Press - Puckstop   
Thursday, 21 April 2005 13:57
Talk about the rule of unintended consequences.

Five years ago, about 15 percent of the chinook salmon that biologists sampled from Lake Huron were wild fish, and the other 85 percent were produced by hatcheries in Michigan.

Today, that number has turned around, with 20 percent of the fish coming from hatcheries and 80 percent from salmon that run up tributary streams to produce the next generation without human help.

In most parts of the country that would be good news. But not on Lake Huron, where the glut of salmon has overwhelmed the alewives that once formed their prey base. And most of the salmon are produced not by Michigan streams but in rivers on the Canadian side of the lake 80 to 100 miles away, which means that in August, the adult chinooks head back to their native streams to spawn and largely disappear from the Michigan side.

Chinook catch rates in Michigan waters of Lake Huron have dropped significantly in the last three years. That has been accompanied by a decline in the average size of the fish, which has dropped from about 13 to eight pounds in the same period. At that weight, the fish show obvious signs of starvation, said Jim Johnson, a Lake Huron fisheries researcher for the state Department of Natural Resources.

When there was a similar decline in prey fish in the late 1990s, biologists responded by reducing the number of salmon they stocked. But with most of the fish now being produced naturally in rivers in Canada, there is no way to shut off the spigot. The actual number of salmon in Lake Huron a few years ago might have been two or three times the official estimate of 4 million to 5 million, Johnson said.

The disappearance of salmon seems to be resulting in a big increase in the number of lake trout in Lake Huron. The disappearance of alewives also might help the lakers, walleyes and perch by drastically reducing predation on sport-fish fry.

Last summer, researchers captured 22 young-of-the-year lake trout in their trawl nets, Johnson said. That might seem like an insignificant number, but in the previous 30 years the nets had caught a grand total of five juvenile lakers.

"Lake trout fishing last year was fantastic, especially that early fishing in May and June," said Terry Walsh, who runs the Termar charter boat out of Oscoda. "We were getting fish right offshore, in 30-40 feet of water, and we were getting a mixed bag of lakers, salmon, brown trout and steelhead.

"I rarely had to run more than four to five miles before we hit fish. And I remember when it used to be an early morning and evening bite, without much in between. But last summer you could catch fish all day. When the prey base isn't there, they are hungrier and they have to work harder to get food."

The alewife is an exotic species from the Atlantic Ocean that probably reached the Great Lakes through shipping canals. Smelt were the primary prey species for Lake Huron salmon until the 1980s, when alewife numbers exploded and smelt began a steep decline.

Lake Huron is also feeling serious effects from another invader -- the zebra and quagga mussels that arrived in the ballast of ships from the Baltic Sea in the past two decades.

After the mussels were found in the Great Lakes, scientists noticed that a small freshwater shrimp called diporeia was disappearing at an alarming rate. This was worrisome because diporeia were a major link in the chain that cycled energy through the lake, including energy that went into building the flesh on sport and commercial fish.

"The diporeia live near the bottom in daytime and feed on detritus that drops from above," Johnson said. "But at night, they move up into the water column where they are available as food to a lot of other species," including smelt, alewives and other prey fish eaten by salmon.

The disappearance of diporeia and a couple of cold winters have resulted in the near disappearance of alewives from the lake, and while there are some signs that smelt might be coming back, no one knows yet if they will fill the prey fish niche.

Diporeia also were a primary element in the diet of lake whitefish, the most important commercial species in the Great Lakes, and the reduced numbers of diporeia probably accounts for the decreased numbers and size of whitefish in Lake Huron.

Johnson said there are several possible scenarios for the future of Lake Huron, all of which are largely out of the hands of biologists. Decreased predation by salmon and mild winters could result in a resurgence of alewives, followed by a resurgence of salmon in boom-and-bust cycles.

The DNR is considering reintroducing lake herring, a native prey and food fish that was largely wiped out by the changes in recent decades. This would lead to lake trout as the primary sport fish, with salmon playing a lesser role. Johnson said that's what happened in Lake Superior when the lake trout was brought back from near-extinction.

"Lake Huron really is a lot like Lake Superior," Johnson said. "Most of it is very deep and very cold, with relatively few shallow shelf areas. That's why Huron is much less productive than Lake Michigan."

 
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