Zebra Mussels Invade the South

From May, 1995 (Update #24)

BATON ROUGE, La. - What started out as a Great Lakes problem may ultimately prove to be a bigger headache in the southern reaches of the Mississippi River. Zebra Mussels, according to Louisiana Sea Grant researchers, appear to love the warmer southern waters into which they've moved. Zebra mussel veligers were first reported in Louisiana in 1993. By summer 1994, Louisiana Sea Grant researchers John Lynn, Tom Dietz and Harold Silverman were recording zebra mussel densities up to 200,000 per square meter on hard surfaces in the Mississippi River. By fall, those densities had doubled to around 400,000 per square meter. "Our zebra mussel population has grown by leaps and bounds," Lynn said. "I'd say we're in the big leagues now." The population growth most likely was the result of last summer's spawn, Lynn said. "Not only are the veligers settling, but the settlers are growing up," he added. "We've seen a shift in the mean average size of the zebra mussels from one millimeter to 3.5-4.0 mm in the last three months." One reason the mussels are thriving is the water flow rate in the Mississippi River, Lynn said. "We have millions of gallons of water flow per second," he said. "That's a lot of fresh water to provide zebra mussels with a steady food source and to flush away any potential toxins." Southern conditions also are ideal for reproduction. Though nobody has yet determined exactly how many times zebra mussels can spawn in a year, Lynn said most people believe they spawn more than once. "I'd even speculate that under optimum conditions of temperature and food supply, they might be able to spawn every three weeks or less," he said. Louisiana State University researchers also are studying the salinity concentrations mussels can tolerate. "We've seen long-term mussel survival in water with a sodium chloride concentration as high as 12 parts per thousand," he said. That concentration would translate to a mix of about 30 percent seawater, 70 percent freshwater. "We've worked on duplicating the chemistry of water in the tidal flats," Lynn explained. "That involves manipulating ion ratios of sodium, potassium, calcium and magnesium. It's the ion ratios that are most critical to the mussels." Lynn hopes this work will help researchers' understanding of mussels in fresh water, too. "The common thought is that a high level of calcium is conducive to zebra mussel survival," he said. "That's true in part, though we're finding that it's more specifically calcium in conjunction with magnesium, potassium and sodium that's important. Even then, the ratios must be in the proper balance." Lynn doesn't believe the mussels will be contained in the Mississippi, either. Over the next year he envisions them continuing to spread in the Atchafalaya basin, possibly invading the Tombigbee River basin and entering Texas waters through the Toledo Bend Reservoir.

ID: 199505-6.

The Zebra Mussel Update was a 4- to 8-page quarterly national newsletter published by the University of Wisconsin Sea Grant Institute from May 1990 through May 1997. The ZMU documented the spread of the zebra mussel -- an exotic nuisance mussel -- through North America's freshwater environments, especially the Great Lakes, and on efforts to control it.