Shrinking salmon: Lack of prey contributing to decline

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Lake Michigan anglers can expect to catch fewer chinook salmon this summer, and those that are caught likely will be smaller.

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[GLF]

SHRINKING SALMON: Lack of prey contributing to decline

February 7, 2008

BY ERIC SHARP

FREE PRESS OUTDOORS WRITER

Lake Michigan anglers can expect to catch fewer chinook salmon this summer, and those that are caught likely will be smaller. Prey fish numbers have decreased for the third year in a row to about one-third of 2005 levels (and only 8% of the peak in 1989), and salmon will find it tougher to locate the alewives that are their primary food source.

What researchers don't agree on is whether Lake Michigan's chinook can hang on at present levels or if the new data presage an imminent collapse of the salmon fishery like the one that occurred in Lake Huron three years ago.

The key to the lake's future is held by a nickel-sized mollusk called a quagga mussel, another invader from Europe that has nearly eliminated earlier-invading zebra mussels in both Lakes Michigan and Huron.

Quaggas can colonize sand and mud bottoms (zebra mussels need rock or other substrate) and live much deeper than zebra mussels (now past 300 feet and still going down). They're bigger and more efficient filter feeders than zebras, removing more nutrients from the water, and in Lake Michigan their numbers are an astonishing seven to eight times greater than they were in Lake Huron when the salmon fishery collapsed there.

Quagga break the food chain by eliminating tiny creatures that provide food for huge numbers of small crustaceans and fish from many species, including the alewives that are the chinooks' nearly exclusive prey base. Energy that once moved up the links of the food chain through invertebrates to baitfish to salmon is now locked up on the lake bottom in mussel shells, which are eaten in small numbers by only a few fish.

Charles Madenjian, a biologist who surveys prey fish populations for the U.S. Geological Survey's Great Lakes Research Laboratory, thinks predictions of a Lake Huron future for Lake Michigan aren't supported by the available data, at least not yet.

"I don't agree that it's necessarily a gloom-and-doom situation," Madenjian said. "I know some people are convinced that we can blame everything on the mussels and that it's just a question of time, but I think it's a lot more complex than that. Lake Huron is very different from Lake Michigan, and I think the basic problem in Lake Huron was too many salmon munching up too many alewives."

"We could see a similar (salmon collapse) in Lake Michigan, but I don't think we have the information we need to say it's going to happen," he said, adding that Lake Michigan's baitfish population could stabilize or even grow slightly -- and the salmon population follow suit -- with another good alewife hatch.

Madenjian's estimate of Lake Michigan's prey base -- alewives, bloater chubs and other baitfish eaten by predators like walleyes, steelhead, lake trout, salmon and whitefish -- dropped from 61 kilotons in 2006 to 30 kilotons in 2007.

The prey base was more than 80 kilotons in 2005 and a record 400 kilotons in 1989, the year the first quagga mussel was seen in the Great Lakes. But trawling data from the USGS research ships in 2007 estimated that the mass of mussels (98% quaggas) in Lake Michigan alone was a staggering 245 kilotons -- a half-billion pounds. That's eight times the mass of the available prey fish there.

Thomas Nalepa, a researcher at the National Oceanic and Atmospheric Administration's Great Lakes Environmental Research Laboratory and sometimes Madenjian's collaborator, is more pessimistic.

"Logically, I ask myself where is all the energy to support all that mass (of mussels) coming from? It has to come at some cost, and logically, it's from the fish. We're trading fish biomass for mussel biomass," Nalepa said. "I can see Chuck's point that we may be looking at a blip in Lake Michigan, but I also know that it's following the same trend we saw in Lake Huron. Overall, Lake Michigan has a slightly higher biomass of quaggas than Lake Erie. That's pretty mind-boggling. We see them going very deep in (Huron and Michigan), and that means they seem to be able to do very well at low-food concentrations."

The future of Lake Michigan's salmon is important for several reasons, not the least being estimates that the lake accounts for about $1 billion of the $4 billion Great Lakes sportfishing expenditures.

Randy Claramunt, a Michigan Department of Natural Resources fisheries research biologist, appeared optimistic.

"Lake Michigan is very different from Huron," he said. "I think that it is a lot richer (biologically), and the southern basin may be productive enough to sustain the fishery. But I agree that we'll probably see a decrease in the number of fish caught, although that's still enough for a great fishery. Even with the numbers down, Lake Michigan is still producing a lot of salmon."