Operators should monitor for immature zebra mussels to determine when control strategies should be initiated. There are usually two (although in some instances, there has been one) high-density peaks of immature zebra mussels each season. However, immature zebra mussels can be found in the water all year when temperatures are above 12 ºC. Monitoring for immature zebra mussels is more difficult than for adults. This type of monitoring should be considered at pumping plants, especially where potable water supplies or electricity generation could be threatened. Monitoring to detect the presence of immature zebra mussels makes little sense at hydropower facilities where there is not a need to develop a strategy. Samplers constructed from polyvinyl chloride (PVC) plates (5 cm by 15 cm) suspended horizontally about 3 cm apart should be considered if information on density, growth, or time of settlement is required. However, these are unnecessary if only presence/absence information is required.
Presence/absence sampling for adults should be conducted to prepare for implementing strategies. The preferred substrates are PVC plates or pipes because they are lightweight, easy to obtain, inexpensive, and zebra mussels are easily seen or felt on their smooth surface. If PVC is unavailable, other appropriate substrates should be used to collect presence/absence information. Concrete blocks, ceramic tiles, or nylon sponges are also acceptable as monitoring substrate.
Conditions where zebra mussels are first found should be thoroughly documented, including water velocity, depth, and type of substrate on which the mussels are attached. Age can be estimated by measuring the length of five to ten of the largest individuals. Shell length is measured along the flat portion of the shell with calipers or a small rule. Zebra mussels will grow less than 1 cm/year in most areas. Density can be estimated by making several counts of mussels in a unit area (1-m2 sample size, if possible). Water temperature needs to be recorded and, if equipment is available, dissolved oxygen, pH, and total hardness.
Project personnel or divers can collect zebra mussels or other molluscs by hand. A simple procedure is to remove all shells and other material within a quadrat of a specific size and counts the number of individuals in a unit area. Quadrats can be fabricated or purchased ready-made. Quadrats have been constructed from aluminum stock approximately 0.3 cm thick and 10 cm wide or from 2.5-cm-diameter PVC pipe. The size of the quadrat square depends on the population densities of mussels. A 1-m square may be necessary to count reasonable numbers of mussels in areas of low density (<100/m2), whereas a 10-cm square may be sufficient in high density areas. All shells and materials collected from within the quadrat should be placed in a single container. Material can be examined directly or screened through 0.6-cm and 1.3-cm mesh to facilitate sorting, which is the preferred method for estimating mussel density.