Tiny mussels cause big problems
By Sue Erickson, Staff Writer
Oodenaang (Odanah), Wis.—The tiny, striped and notorious zebra mussel, Dreissena polymorpha, averages only about an inch in size and seems quite harmless.
But put together an army of these prolific mussels and major problems can happen in the nation's freshwater lakes, including Mille Lacs Lake in Minnesota where the count leaped from 14 mussels per square foot in 2010 to over 900 per square foot in 2011.
Known as a premier walleye lake, Mille Lacs Lake has been experiencing a rise in the zebra mussel density since 2005. Zebra mussels reproduce prolifically and have few predators. A female zebra mussel can produce 30,000 to 1,000,000 eggs in one year! What can be done about it? "Nothing can be done once they enter a lake," according to Tom Jones, Aitkin Fisheries Large Lake Specialist, Minnesota Department of Natural Resources, "However, we should do what we can to prevent the spread to new lakes!"
Next question: What is the potential impact on the fishery? A number of problems can arise, especially related to the forage base for fish.
Zebra mussels feast on phytoplankton (mostly tiny, single-celled, aquatic plants), but phytoplankton is also a food source for zooplankton (microscopic, free swimming animals), which are in turn a forage base for small perch and walleye. So disappearance of a food source like phytoplankton at the bottom of the food chain can start a domino-like event up the food chain, impacting larger fish like walleye and northern pike that depend on the smaller fish for food.
However, for Mille Lacs Lake, Jones says a Cornell University study suggests that the phosphorus level in the lake might be sufficient to help sustain a zooplankton population regardless of the zebra mussel intrusion.
Jones also notes that there is concern about the impact on mayflies. If the zebra mussels cover mayfly burrows, they will not be able to emerge, and mayflies are another important component of fish diets.
Other concerns cited by Jones include intrusion on the walleye spawning grounds, although he says the ice seems to keep the zebra mussels out of the very shallow spawning sites. However, where walleyes spawn deeper than about four feet, ice will not scour the mussels.
The zebra mussels are highly likely to appear in nets and on aquatic vegetation or can be stuck on small rocks. He encourages fishermen to check all gear thoroughly and clean nets, boats, gear before leaving the lake to prevent transporting them to other water bodies.
Zebra mussels can also be transported in the veliger stage. Once zebra mussel eggs are fertilized, they develop into veligers, so tiny they are difficult to see. Veligers feed on tiny phytoplankton while they grow their shells. Swimming freely as veligers, they can also travel great distances carried along by water currents. Of course, man-made transportation can also help, such as being caught on boots or in fishing gear or boats.
After about a month, the weight of the veligers' shells causes them to sink to the bottom where they find something to which they can attach. This could be another mussel, a crayfish, intake pipes, piers, boat bottoms, whatever.
Damages caused by the attached mussels include killing native mussels by smothering them, clogging pipes, and cleaning the water.
While clean water sounds like a good thing, there are negative impacts, such as the removal of plankton and allowing more light through to the bottom of the lake, which may stimulate more growth of aquatic vegetation, changing the nature of a lake's habitat.
The zebra mussels are highly likely to appear in nets and on aquatic vegetation or can be stuck on small rocks. Tom Jones, Aitkin Fisheries Large Lake Specialist, Minnesota Department of Natural Resources, encourages fishermen to check all gear thoroughly and clean nets, boats, gear before leaving the lake to prevent transporting them to other water bodies.
