Zebra Mussel Numbers Way Down on the Croix

[DonBo]

St. Croix River: Zebra mussel population continues to plummet

By Dave Orrick

[email protected]

Posted: 03/26/2013 12:01:00 AM CDT

March 27, 2013 3:16 AM GMTUpdated: 03/26/2013 10:16:15 PM CDT

For the second straight year, populations of zebra mussels have plummeted in key portions of the St. Croix River.

The findings prompted a leading biologist to suggest the St. Croix has a "unique second chance" at an existence nearly free of the invasive mussels -- as long as humans don't reintroduce them.

The promising news could have ramifications for a range of river users, from recreational boaters on Lake St. Croix to barges scheduled to be steamed upriver this spring for the new St. Croix River bridge construction project. For beachgoers, the days of bare feet being sliced by a bed of sharp shells are ending, at least for now.

The population crash data, contained in a report prepared this week by the National Park Service for the U.S. Army Corps of Engineers, paints a picture of a previously encroaching foe suddenly on its heels after being pounded by predators and flushed out by the very river system it infested.

The numbers are striking.

"We found 22 live mussels, that's all. That's the whole shebang for all ," said Byron Karns, an aquatic biologist with the Park Service and the author of the report. Karns cautioned that those 22 mussels were found in eight specific locations between Stillwater and Prescott, Wis. The total river system in that area undoubtedly contains thousands of mussels, he said.

But not millions, as might have been the case as recently as 2008.

"I'm pretty good at finding these things, and one time while I was diving and looking for them, it took me 45 minutes to find five live mussels," said Karns, referring to a hunt for zebra mussels he conducted last year for a related study. "That was in a place where a few years ago we could find tens of thousands per square meter."

Credit goes largely to the river itself, which in 2011 and 2012 experienced unusually high flow levels in June and July, the very time when zebra mussels are trying to reproduce.

Zebra mussels, which evolved in lakes, reproduce by hatching larvae, known as veligers, that suspend within the water column for weeks. The veligers don't swim, so they're vulnerable to being flushed downstream by a swift current, and scientists are now beginning to see this vulnerability manifest itself in zebra mussel crashes.

"In stretches of free-flowing rivers, zebra mussels don't do very well," said Gary Montz, an aquatic invertebrate biologist with the Minnesota Department of Natural Resources who didn't participate in Karns' research but is familiar with the results. "We've seen this now in the St. Croix for two years and the Mississippi River in some parts of the metro."

In addition, common carp have been eating the mussels, said Karns, referring to a U.S. Fish and Wildlife Service research project not yet published. High summer temperatures might also have played a role in zebra mussel die-offs, a phenomenon witnessed in the Mississippi, according to Karns and Montz.

"You've got a population already stressed by predation, and then you have two years of this weird water flow, even though we're in a drought, as well as high temperatures, and it's like a perfect storm," said Karns.

Hitchhikers on boats and piers, zebra mussels have been spreading throughout the state waters, prompting increasing levels of alarm. While their long-term effects on native ecology isn't known, the mollusks alter the chemistry of lakes through their filtering and often crowd out native mussels. The St. Croix River is a haven for native mussels, including five species of federally endangered mollusks. Karns and Montz said native mussels appear to have held their own on the St. Croix during the zebra mussel infestation, which appears to have peaked in 2007 and 2008.

Montz said he's not optimistic the population won't have a chance to rebound, aided by people.

"They got up there at first, and they didn't get up there on their own," Montz said. "The odds are they'll be brought up there again."

Karns said he's begun to press officials at the Minnesota Department of Transportation to try to reduce the likelihood of the St. Croix River bridge project reintroducing the invasive species.

"We've been given this unique second chance," said Karns. "The thing I don't want to see is a barge sitting in the river for 6 months with an abundance of zebra mussels on its hull."

[Nick Kuhn]

Cool. I was wondering if sheephead were munching on them but I suppose there's nothing wrong with a bunch of fat carp.

[maxpower117]

Quote:

Hitchhikers on boats and piers, zebra mussels have been spreading throughout the state waters, prompting increasing levels of alarm. While their long-term effects on native ecology isn't known,

So remind me, what are we spending all this AIS money on? Why are we being treated like criminals when we want to launch our boat?

Can someone please point me towards a story that says the entire ecosystem has been ruined by zeebs?

Oh, I forgot, Mille Lacs walleye population is being desimated by smallmouth, pike and zebra mussels.

[BrianLucky13]

Can someone please point me towards a story that says the entire ecosystem has been ruined by zeebs?

Oh, I forgot, Mille Lacs walleye population is being desimated by smallmouth, pike and zebra mussels.

Zebra mussels eat walleyes???

[Scott M]

A couple periods of heavy flows and some predation reduced their population...guess that means we can celebrate and unveil the mission accomplished banner.

A diminished carrying capacity is good for walleye fishing, right?

[maxpower117]

Quote:

Zebra mussels eat walleyes???

Thats what I'm wondering. Somehow they are sharing the blame for the reduced walleye numbers which only puts the zeeb infestation at heightened awareness. This is all because they claim that they don't know the long term effects zeebs will have on an ecosystem.

I'm going to guess that zeebs have been in US freshwater ecosystems for at least 50 years. Where are the horror stories?

If the sky isn't falling in those areas, why is the sky falling here in MN?

[BrianLucky13]

Overharvest is to blame a billion times more then zebras that's for sure...