Zebra mussels (Dreissena polymorpha) and their larger cousins, quagga mussels (Dreissena rostrifurmis bugensis), are natives of the Caspian and Black Seas, but have spread across Europe, Canada and the US with alarming speed.
Zebra mussels first invaded the US in the mid-1980s, probably entering American waterways as eggs or larvae in ballast water. Quagga mussels were recognized as a distinct species in 1991. Today, these two mussels are found in the Creat Lakes, the Southwest, the California Coast and all major river drainages east of the Rockies.
The success of the tiny mollusks-which grow only to the size of a thumbnail but crowd out native species with immense, densely packed colonies-results from their remarkable adaptability. Though they are a freshwater species, they can tolerate salinity of up to 6,000 ppm or even higher, in certain conditions. This makes them a threat in some estuaries and brackish water sites.
They are also highly productive in water temperatures into the mid-8O'F range. In fact, warm water conditions in the West can multiply the mussels' reproduction rate by a factor of six.
Zebra and quagga mussels are extremely tiny and mobile during the early stages of their life cycle. The eggs are only 40 microns in size, with newly hatched trocophores (smal1, free-swimming, ciliated aquatic larva) starting out at 40 to 60 microns for a day or two before rapidly growing.
These mussels mature into veligers-a planktonic form in which they remain for weeks or months. After reaching 350 microns in size (about 0.014 inches in diameter), they settle down as juvenile mussels.
Many control measures for zebra and quagga mussels rely on chemical means, including chlorine and other strong oxidants. Ozone and UV light have also been shown to effectively control eggs, trocophores and veligers.
However, UV chemical and ozone measures can be costly and undesirable. This is especially true in areas where sensitive species-or sensitive stakeho'lders-are a concern. Fortunately, well-designed, automatic self-cleaning screen filtration systems at the 40-micron level have been demonstrated to effectively control zebra and quagga mussels.
An independent trial of 40-micron, automatic self-cleaning screen filters at Mississippi Power and Light's Gerald Andrus Station in Greenville. resulted in no viable life forms of the water supply's zebra mussel population in the fiitered water. The small proportion of eggs and veligers that slipped through the screen were torn, compressed or deflated-dead or dying, according to Acres International Corporation of Amherst, Nl which analyzed the samples in the study.
Automatic self-cleaning filters have since proven themselves in a variety of installations for zebra and quagga mussel control. Two specific examples are noted:
Mudd Creek Irrigation District, Bad Axe, MI. The district faced closure because authorities wouldn't a1low the district to connect Lake Huron to local surface water supplies without a 100-percent effective system to remove zebra mussels from the water. Three 10-inch (254 mm) diameter fiiters with 2S-micron screens handle a flow of 4,500 gpm (17,034 L/m) to keep Mudd Creek's farms supplied with mussel-free water.
Ed Week Fish Culture Station, Grand Isle, VT. The hatchery's shallow-water intake was invaded by zebra mussels in the mid 1990s, forcing managers to draw from a deep-water intake. Because the deeper water is 30'F cooler than the shallow water, switching to deep intakes multiplied the hatchery's heating bill tremendously.
Since 1997, nine of the 1O-inch filters have put 8,800 gpm (33,311 L/m) of shallow-intake water through 25-micron screens to protect the hatchery while also protecting its bottom line. In that time, the filters have paid for themselves time and again in reduced heating costs.