The initial inspection of a site or facility for the presence of zebra mussels is intended to determine if infestation currently exists, where the mussels are located, and if population densities warrant control measures. Information gained during initial inspections is primarily qualitative rather than quantitative, and the focus is typically on examining large volumes of water for veligers (i.e., planktonic mussel larvae) and/or inspecting as much surface area as possible for attached mussels.
A thorough and careful initial inspection is important, as young mussels can be easily missed. Mussel veligers in a water column, for example, cannot be seen by the naked eye, and recently settled mussels are less than 1 mm in length, appear like tiny sand grains, and may be located in small crevices or seams.
If the site has been deemed very sensitive to mussel infestation, it would be advisable to monitor for both veligers as well as settled juveniles and adults. To be cost-effective, monitoring at sites that have been deemed less sensitive (i.e., which can incur substantial mussel fouling) could be limited to the detection of attached juveniles and adults.
It is important to note that the presence of one stage of the life cycle does not ensure the presence of another. There are many sites where veligers have been frequently found in the water column, but settled mussels have never been observed. Conversely, there are water bodies in which high densities of adult zebra mussels have been recorded, but no successful settling of new mussels was seen for several years. When considering sampling for veligers, it is important to keep in mind that adult mussels generally do not spawn until water temperatures rise above a particular threshold, often 10-12°C for the zebra mussel, Dreissena polymorpha, and 8-10°C for the quagga mussel, Dreissena bugensis. Thus, sampling for veligers at lower temperatures would not likely be cost-effective.
All individuals participating in a monitoring program should be knowledgeable about and familiar with the following:
1) The life history, biology, and environmental needs of the zebra mussel in all of its life stages.
2) The environmental problems that could occur if live mussels are released into uninfested open waters.
3) The correct procedures for handling specimens.
4) The correct procedures for decontaminating surfaces and objects.
5) The correct procedures for the disposal of specimens and field materials.