The transition from the planktonic veliger stage to the benthic juvenile stage is called settlement and usually occurs within 3-6 weeks after veligers first appear in the water column. Higher water temperatures tend to shorten the time to settlement (see Life History and Biology section). This critical time in the life cycle is often accompanied by high veliger mortality. The presence of veligers in the water column does not always lead to successful zebra mussel colonization of a site. Detecting settlement may require the use of SCUBA divers for visual observation or the deployment of filamentous or solid substrates. Plankton nets are used to collect samp les at the planktonic stage, but are not useful in detecting the adult settlement stage.
Mussels will settle on and attach to virtually any available hard substrate. Monitoring may make use of any naturally available hard substrate and/or incorporate an artificial substrate. Sampling for adult mussels usually involves the deployment and retrieval of suitable hard substrates, as well as the enumeration of mussels based on the surface areas involved (e.g., number/mē). Habitat data such as depth, water temperature, current velocity, Secchi disk depth, calcium concentration, pH, and dissolved oxygen are useful additions to population data and should be collected if resources permit. Ways to record this data are discussed in the Importance of Maintaining Long-Term Records of Monitoring Datasection.
When deploying artificial substrates or collecting natural ones, remember that sample bias can occur, since adult mussels prefer dark areas, corners, crevices, and other bivalves as settlement sites. Settlement may also be influenced by temperature, depth, and water velocity. Any artificial substrates should be placed in areas where they are least likely to be disturbed by vandalism, sedimentation, or extremely high flow rates. They should be deployed in a variety of habitat areas with multiple replicates and should be allowed to "season" for several weeks prior to the initiation of adult mussel counts. If not allowed to "season," a biofilm does not have time to form on the substrate, and settling mussels will prefer not to attach to it.
Before, during, and after deployment of substrates and sampling, make sure that proper zebra mussel containment protocols (as described in the Protocol for Responsible Monitoring Procedures section) are followed for all sampling gear, as well as for storage and maintenance of samples prior to, during, and after analysis.
Sampling Juveniles with Filamentous Substrates