Trawling trips gives biologists an idea of amount of prey fish in lakes
Written by Booth Newspapers   
Sunday, 22 May 2005 10:33
Back in the 1960s, when Michigan fisheries biologists began planting salmon in the Great Lakes, they knew they had plenty of forage for the big predators.

Dead alewives annually littered the beaches and one of the big benefits of the salmon program is that the game fish took care of the alewives.

But plenty is a relative term; fisheries managers had no concept of how many there really were. So in the early 1970s, the Department of Interior began surveying the lakes to see what was out there.

Now, the annual trawling expeditions conducted by the U.S. Geological Survey provides state management agencies with an index of the relative abundance of prey fish in the lakes.

Jeff Schaeffer, a 48-year-old biologist headquartered in Ann Arbor, is in charge of the Lake Huron survey. Schaeffer and his crew recently completed the spring survey on the Grayling, a 75-foot steel-hulled converted shrimp boat. And what they found just adds to the woe surrounding the collapsing salmon fishery.

"What we've seen between fall of 2001 and fall 2004 is about a 65 percent decline in prey fish biomass," Schaeffer said. "That decline is almost completely due to the decline in alewives."

Alewives are ocean fish that gained entry to the Great Lakes through shipping canals. First noted in Lake Huron in 1933, their population boomed as the lake trout population dropped. Alewives have been a common species in Lake Huron for 30 years, but they've never been as low as they are now, Schaeffer said.

Here, the third stop on the spring survey, the Grayling crew drags the trawl on the bottom in 82 meters for 10 minutes, then hauls it aboard. The trawl yields nearly 30 gallons of fish -- more than 90 percent bloater chubs, some smelt, a couple of whitefish and a large lake trout. Alewives? Not a one.

At a depth of 72 meters, maybe 30 gallons of fish are collected. This batch is more than 90 percent smelt. That's good, Schaeffer says.

"Smelt numbers are coming up, but it's mostly young fish," he said. "Smelt have high mortality, probably due to predation.

"Smelt can support salmon and they are increasing, but there just aren't many big ones out there."

Although pleased with the smelt, Schaeffer warns against over-enthusiasm.

"That was one good tow of smelt, but just one," he said. "We've already finished two ports and we haven't seen anything close to this."

The next trawl, nine meters shallower, produces less than 15 gallons of fish. And though they are largely small smelt, other species -- trout perch, sticklebacks, sculpins -- are more in evidence.

"As you move in you can see the fish community change," Schaeffer said. "It's neat."

It's also significantly less dense. As the crew continues to trawl at nine-meter intervals, the catch declines precipitously. By the last trawl, at 27 meters, the net yields just 15 fish -- 12 smelt, two sticklebacks and a deepwater sculpin.

But most importantly, a whole day of trawling failed to produce a single alewife.

At Hammond Bay, a total of eight tows produced 60 alewives. At De Tour, seven tows yielded five alewives.

"Normally, we get that many in one tow," Schaeffer said. "Keep in mind, Hammond Bay and De Tour are our closest ports to Lake Michigan where we still have substantial numbers of alewives."

Although the snapshot of alewives is bleak, Schaeffer and crew have found some good news in recent surveys. For instance?

"Last year, we caught 22 young-of-the-year lake trout," Schaeffer said. "In the previous 33 years of this survey, we caught six (total)."

So lakers are on the rise, reproducing again. And, as further evidence, the crew netted a 6-pound laker at 55 meters with no visible fin clips -- possibly a naturally produced fish. It was weighed and released immediately.

"When we get a nice healthy fish like that, we'll forego the data and try to put it back," Schaeffer said.

Although Lake Huron lake trout fishing has been good in recent years, salmon fishing has tanked. But Schaeffer says it isn't because the salmon aren't there.

"I think everyone agrees the salmon population is higher than its ever been," he said. "There are plenty of salmon out there."

What they've done, however, is change their habits. Because they can't find alewives, salmon have either switched to more abundant prey (smelt, for instance) or in some cases, cruised over to Lake Michigan where they can still find their preferred meals.

"I've talked to a few salmon anglers who have done well and basically, they fish deep," Schaeffer said. "And that's probably something anglers need to do this year, too."

As for the future, Schaeffer isn't making any predictions.

"Just when I think I've seen it all, Lake Huron surprises me," he said. "It's a very dynamic system. It changes from year to year."

And that means alewives could return at any time.

"Although alewives are down, they're not anywhere near extinct," he said.

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