October 23, 2008

The downturn in Lake Huron's alewife population has been linked to the decreasing fortunes of salmon fishermen. Generally speaking, the salmon fishery is struggling because the big fish simply do not have enough to eat.

But that same decrease in alewives may have been the catalyst for some of the best walleye fishing Saginaw Bay anglers have ever enjoyed.

"The walleye fishing on Saginaw Bay is absolutely world class right now," said Jim Baker, the Department of Natural Resources fisheries biologist who oversees Saginaw Bay. "It has been an outstanding summer based on what the creel clerks say. Any day when the weather is halfway decent you can catch walleyes, and if it's good you can catch a limit.

"We set a record the past two years in a row as far as summer catch goes in Saginaw Bay and I wouldn't be surprised to see another record this summer. The one thing that is hard to gauge is fishing pressure and the affect the economy might have on it. But for the guys who are going, it has been phenomenal."

Baker attributes the great fishing to the outstanding recruitment of walleyes in the bay over the past few years.

"Out of the last five years we've had four big-year classes -- 2003, 2004, 2005 and 2007," Baker said. "The 2007s are not in the fishery yet, but the others are.

"Throughout the '90s, you caught big walleyes out there but if you caught a 15- to 17-incher, it was the exception. Now we see considerably fewer of those big walleyes out there, but vast numbers of fish are 15 to 20 inches long.

"Back when we were shorter on walleyes and had a ton of alewives, we had these big hog fish out there. Things have really changed."

In fact, natural reproduction and recruitment have been so strong the DNR has not stocked the bay since 2005.

Saginaw Bay historically (before World War II) produced a commercial fishery of about a million pounds of walleye annually. In 2007, all facets of the fishery -- on open water, through the ice and in the rivers -- produced an estimated 300,000 fish that averaged roughly 2.25 pounds apiece. That's nearly 700,000 pounds of walleye caught by rod and reel, not gill net. Does that level of recreational catch correspond to a million-pound commercial catch? Perhaps it does.

But DNR biologists are using another variable to measure the carrying capacity of Saginaw Bay -- the growth rate of 3-year-old walleye.

Generally, as abundance of a species goes up, the growth rate goes down; when there are more mouths to feed, there is less food for each mouth.

Because the DNR is striving toward a balance of predator and prey populations, biologists set a goal for Saginaw Bay's walleyes to attain 110 percent of the state's average growth rate.

Two years ago, the growth rate was "right there at 110 percent," said fisheries research biologist David Fielder. "Last year it dipped down close to 100 percent."

According to Fielder, the fishery is largely now "on autopilot."

"We're not stocking," he said. "We can't control how many walleyes are entering the system. But eventually it will come into equilibrium and that is what we want: a self-sustaining, balanced population."

So how do alewives figure into all this?

Enter Diporeia, a shrimp-like critter that is the principal food of the alewife.

When Lake Huron became fully colonized by the exotic zebra and quagga mussels, these mussels changed the entire food web of the lake. Diporeia is one of the tiny animals in the food chain, and it declined to very low levels. With nowhere near enough food to eat, alewife populations collapsed in 2003.

"We know we have a surge in reproductive success in both walleye and yellow perch that traces back to the decline of the alewives," Fielder said. "What's really interesting is we knew the walleye population was limited and we thought that was because of a lack of spawning habitat. We thought a secondary factor was the effects of alewives had on the populations; alewives would come in and use the bay as a spawning and nursery grounds at the same time the walleye and perch fry were emerging. The alewives would feed on the fry.

"With the alewife collapse, they're out of the equation," Fielder continued. "Reproductive success just exploded. There are lots of young-of-the-year (yellow perch and walleyes) out there. Nature took care of it for us."

Unfortunately, the yellow perch aren't living very long.

"We see them as young-of-the-year when we trawl for them in the fall, but the next year they're gone," Fielder said, "What seems to be happening is high mortality. When we open up predators like walleyes, we see they're eating a lot of young yellow perch. They have become a forage fish in many respects in Saginaw Bay."

Fielder said the perch population is "as low as we have ever seen," and quickly adds it is an important issue on Saginaw Bay.

"It's easy to forget because everyone's so excited about the walleye fishery, but we need them both," he said.

So how did we historically have both perch and walleye on Saginaw Bay?

Fielder's theory is that the alewives served as a buffer. The walleyes generally fed on the alewives, which allowed the perch to thrive. Once the alewife population got so large, however, it started to control perch and walleye recruitment, either through predation on the young fish or perhaps just through competition for food sources among the young fish.

Fielder theorizes that before alewives became well established in the Bay (in the 1950s), there was another prey fish that served that same function as a buffer: lake herring (cisco). Although they served as prey for both perch and walleye, ciscoes spawned in the fall, not spring, so they were not a threat to newly hatched perch and walleye fry.

"Lake herring used Saginaw Bay for a spawning and nursery ground and that's probably what walleye fed heavily on, protecting yellow perch," Fielder said.

Fielder is kind of geeked about what he's seeing on Saginaw Bay.

"It's fascinating," he said. "We've quit stocking walleye in Saginaw Bay. I didn't think I'd see that in my lifetime. We haven't said, 'We'll never stock again;' it's on a year-to-year basis. But we've seen a recovery of some of the natural processes.

"The system is not fully restored. One of the ideas we've considered is what we can do to promote lake herring recovery to restore more of the true native ecosystem that was there. We still have much more to do."

Despite the ongoing systemic changes to Lake Huron's food web, Saginaw Bay's walleye fishery is a decided bright spot. Fielder expects the walleye population to continue to thrive as long as the alewife population is suppressed.