Sampling for veligers is a critical strategy for sensitive facilities that can tolerate no zebra mussel infestation. This technique provides the earliest warning of zebra mussel presence at a site. However, this can often be the most expensive and labor-intensive form of monitoring, as it requires microscope work by trained personnel. Two basic types of gear are used: plankton nets (towed or pumped) and filamentous substrates. The use of polarized light (see Veliger Analysis Techniques) substantially reduces sample processing time and expense, and this technique is highly recommended.
Zebra mussel veligers are planktonic, free-swimming, of microscopic size, and sometimes very abundant (at times greater than 1 million/m³). In infested waters, zebra mussel veligers will generally be present in the water column only after the water temperature has risen above 10-12 °C. Once water temperatures rise above this threshold, any areas downstream of the infested area are at risk from planktonic veligers due to mussel spawning. This risk continues to some extent until water temperatures drop consistently below 10-12 °C. See the Life History and Biology section for further details.
In areas where veligers have not been found, sampling methods generally involve the clearance of at least 1000 L of water through a filter that can strain veligers out of the water. For any sampling method, at least three replicate samples should be taken from as many different habitat areas as possible. When sampling within a facility, modifications of habitat sampling techniques appropriate for sidestream monitoring are described. When collecting water samples in shallow water, equipment must always be at least 0.5 m above the bottom, and sediment must be kept out of in the sampling gear. In deeper water, sampling should be done below the photic zone if possible, but always above the thermocline.
Additional habitat data - such as water depth, water temperature, current velocity, Secchi disk depth, calcium concentration, pH, and dissolved oxygen - are useful in tracking long-term changes in veliger populations and should be collected if possible. Ways of recording these data are discussed in the Importance of Maintaining Long-Term Records of Monitoring Datasection.
Before, during and after sampling make sure that proper zebra mussel containment protocols (described in the Protocol for Responsible Monitoring Procedures section) are followed for all sampling gear, as well as storage and maintenance of samples prior to, during, and after analysis.
Sampling Veligers with Filamentous Substrates
Sidestream Sampling of Veligers
Automatic Plankton Samplers