An Upside to the Invasive Zebra Mussel?

The dreaded zebra mussel — an invasive species — is killing off native species in lakes and rivers all over the country. But in Lake Ontario, it turns out the zebra mussel has had a positive impact for salmon fishermen. It's producing fatter, faster-growing salmon.

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Many coastal towns around the United States have been trying to deal with a big problem in a small package. The zebra mussel is only about the size of a dime but it breeds quickly, consumes a whole lot of algae and practically takes over American waterways. But could one of the nation's most notorious invasive species be a good thing?

It apparently is for anglers who like to catch big salmon, at least according to a study that links zebra mussel infestations to exceptionally healthy salmon found in one of the Great Lakes.

NPR's John Nielsen has more.

JOHN NIELSEN: The zebra mussel infestation started fouling the Great Lakes in the late 1980s. As they've spread, these wildly prolific foreign bivalves have wracked native ecosystems and done huge amounts of economic damage, according to biologists like Hugh MacIsaac of the University of Windsor in Canada. MacIsaac says zebra mussels are especially good at clogging up factory intake pipes.

Dr. HUGH MacISAAC (Biologist, University of Windsor): Companies that are affected by this have to shut the plant down and put a coring machine into the pipe to scrape this material out and there are many photographs that you can find on the Web of, like, dump truck loads of mussel shells being taken out.

NIELSEN: But some of the other changes linked at least in part to zebra mussels haven't been so easy to despise, according to scientists like Lars Rudstam of Cornell University. For example, Rudstam says, taking away these mussels have helped clear up the once-murky waters found in Lake Ontario.

Dr. LARS RUDSTAM (Scientist, Cornell University): Well, they filter the water so they take up algae that they feed on. And it's an amazing water clarity now. You probably see twice as far down into the water now. Good for scuba divers.

NIELSEN: And good for local salmon fisherman, says Rudstam. Now, to understand how this could be true, let's begin by stepping back and reviewing a simple version of the local food chain in Lake Ontario. First, there's people who eat salmon, then there's salmon. They dine primarily on smaller fish called alewife. Then there's alewife, which hunt for small but tasty bottom shrimp called mison(ph). The salmon and the alewife hunt primarily by sight and when the lake was murky that was not an easy thing to do.

When the sun was up, the shrimp could hide out in the murky depths at the bottom where the alewife rarely found them, according to Redstom. But then the zebra mussels came along and helped clear up the murky waters. Suddenly it got much harder for the shrimp to hide.

Dr. RUDSTAM: So they are now in high light environments where alewife are more efficient at feeding on them. So we have seen an increase in alewife feeding on misons in the last couple of years.

NIELSEN: Rudstam says the alewife in this lake have gotten noticeably fatter since they started pigging out on the defenseless shrimp. That in turn appears to have been good news for the salmon that have been chowing down on this super-sized alewife. Rudstam says this may be why salmon populations found in Lake Ontario are in much better shape than salmon found in nearby waterways.

Rudstam reports on this research in the Journal Limnology and Oceanography. Hugh MacIsaac of Windsor University calls the work provocative partly because salmon might not be the only fish that have learned to benefit from the zebra mussel infestations. He says there's evidence that fish like yellow pert have learned to feed directly on the filter-seeking bivalves.

Dr. MacISAAC: Typically, when they're small they'll feed on plankton but when they get bigger, in some cases, they feed on other fish. But often we'll find that they too have been feeding on mussels.

NIELSEN: But does this mean that what this country really needs is more zebra mussel infestations? To paraphrase MacISAAC and every other invasive species experts in the country, heck no. First of all, the extra money these big salmons may be generating is but a speck compared to the billions worth of damage zebra mussels do every single year. Secondly, according to MacISAAC, these mussels produce waste that has been linked to all kinds of problems, including giant blooms of toxic algae.

Dr. MacISAAC: And in the case certainly of waters around Ohio and Lake Erie, we've cut these huge slicks of (unintelligible) bacteria forming in mid-summer and some of these slicks do produce compounds, which you really don't want to come in contact with.

NIELSEN: Lars Rudstam of Cornell says he agrees with all of that and adds that his fat salmon may not stay fat for that much longer. He says it's possible that the clearer waters in Lake Ontario could soon cause the salmon alewife shrimp food chain to collapse.

Dr. RUDSTAM: Alewife is their own worst enemy. They eat their own young, the larvofish(ph). And as the water has gotten clearer they have an easier time to catch these larvae. So it's good for individual alewife but it might not be good for the population.

NIELSEN: Or for the anglers who like to catch big salmon.

John Nielsen, NPR News, Washington.

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