The family Dreissenidae consists of three genera of mussels: Congarea, Mytilopsis, and Dreissena. Collectively, these are known as dreissenids.
This toolkit is focused on two members of the genus Dreissena; the zebra mussel (Dreissena polymorpha) and quagga mussel (Dreissena rostriformis bugensis). Although there are differences in the biology of these two species, they share many similar life history traits and cause similar adverse environmental and economic impacts. Both species have European origins and were introduced to the United States in the 1980s as the result of ballast water discharge. Both zebra and quagga mussels attach to a broad range of surfaces, including pilings, pipes, rock, cement, steel, rope, crayfish, other bivalves, aquatic plants, and each other, forming dense colonies.
Zebra and quagga mussels seem to have divergent spatial distributions; zebra mussels are primarily warm, eutrophic, shallow water inhabitants whereas quagga mussels prefer deep, oligotrophic, cold-water (MacIsaac 1994). Although this toolkit includes some references specific to zebra mussels (reflecting a larger national focus on the spread of this species), its objectives and tactics also apply to quagga mussels and other dreissenids.
Conrad’s false mussel (Mytilopsis leucophaeta), also known as the false dark mussel, is another invasive member of the family Dreissenidae, and occasionally is found on boats entering the Pacific Northwest. This mussel represents a threat to brackish waters. Conrad’s false mussel has a planktotrophic life-stage and may be difficult to differentiate from Dreissena species using visual identification techniques.
The golden mussel (Limnoperna fortunei), an invasive freshwater mussel in the family Mytilidae that shares many characteristics with zebra mussels, including planktonic larvae and byssus threads, is also a potential species of concern. We refer to “zebra mussels and other dreissenids” with the understanding that golden mussels may be included in this designation, although they are not members of the family Dreissenidae.
Density and Food Availability
Zebra mussel densities within the CRB could vary widely depending on water chemistry, food availability, and breeding population. After their initial introduction, zebra mussel populations can rapidly increase by orders of magnitude, and then similarly decrease. Eurasian zebra mussel population densities range up to 40,000 mussels per square meter (Neumann et al. 1993). Under ideal conditions in the Laurentian Great Lakes, zebra mussel densities reach 700,000–800,000 per square meter (Kovalak et al. 1993). In the lower Mississippi River, where the zebra mussel has been introduced, densities of 400,000 per square meter have been reported (Kraft 1995). The Mississippi has an ideal environment for zebra mussels, in part because food resources are abundant (Kraft 1995). The Columbia River’s lower plankton densities in comparison to the Mississippi or Great Lakes, may limit zebra mussel population densities, though this has yet to be quantified.
Dreissenids can tolerate a wide range of water temperatures from roughly 32° to 86°F (0 °F to 30° C) (Ohio Sea Grant 1997). North American zebra mussel spawning (release of gametes into the water column) will not generally occur at temperatures below about 12 °C (Claudi and Mackie 1994). There is evidence, however, that quagga mussels in deep waters of the Great Lakes are capable of spawning at temperatures near 5 °C (Roe and MacIsaac 1997) and 9 °C (Claxton and Mackie 1998).
Based on these parameters, a water temperature profile created from data recorded at the smolt monitoring facilities at Bonneville and John Day Dams shows the potential for quagga mussel egg release for approximately 7 months of the year (late March to late-November). However, peak spawning temperatures of 68 °F (20° C) and above occur for 2 months during mid-July to mid-September.
North American zebra mussel populations require 10 mg Ca2+/l to initiate shell growth and 25 mg Ca2+/l to maintain shell growth. Larval development is inhibited at pH of 7. 4. Higher rates of adult survival occur at a pH of 7. 0-7. 5, but populations have been found in the hypolimnetic zone of lakes with a pH of 6. 6-8. 0, and in the epilimnetic zone with a pH of 7. 7-8. 5. Optimal larval survival occurs at a pH of 8. 4, and optimal adult growth occurs at pH 7. 4-8. 0. (Benson and Raikow 2007).
Calcium concentrations could be a factor limiting dreissenid densities in the Columbia River Basin. Large populations of zebra mussels are not expected where calcium levels are less than 25 mg/l (Hincks and Mackie). Cohen and Weinstein (2001) found little evidence that zebra mussels can become established at ambient calcium concentrations below about 20mg/l. Calcium thresholds in the Columbia River West of the Cascades and in particular the Willamette River may be suboptimal for establishment of dreissenid populations (Whittier et al. 2008).
It should be noted that calcium may be elevated near concrete structures (Cohen and Weinstein 2001). This needs to be studied further in relation to the Columbia River Basin with its numerous hydroelectric projects made of concrete, including concrete fish passage facilities such as fish ladders. There are also cases where dreissenid populations have become established in calcium-limited water bodies at locations that have input from other water sources with higher calcium levels (Cohen and Weinstein 2001).