Veligers
Zebra mussel veligers viewed with the assistance of cross-polarized light.
Periodic monitoring and sampling provide researchers with the information required for a better understanding of zebra mussel biology, including their population dynamics, spawning, and settlement.
Depending on the location, the following methods are suggested for the initial detection of veligers:
1) Sampling in Open Areas:
Tow a plankton net at a slow speed (mesh opening 63-75 mm and diameter of 50 cm). The tow may be vertical or horizontal. If horizontal, ideally tow below the photic zone, but above the thermocline, with towing above the thermocline taking precedence when overlap occurs.
2) Sampling in Closed Areas:
Using a plankton net of the same standard as above, pass water through the net for an extended period of time. This can most easily be accomplished by finding a tap on a raw water line which is flowing and untreated. This procedure can also be accomplished within a facility where a wet well is available to access the raw water supply.
To Process the Sample:
Regardless of whether sampling is done in an open or closed area, remove the net from the water, and flush all net contents that may stick to the edges into a collection cup. Empty the cup and examine its contents using a dissecting microscope of ³40X magnification. It is preferable to use one with cross-polarized light to aid in veliger detection. For more information on the use of cross-polarized light, see Use of Cross-Polarized Light for Veliger Detection.
Settled Juveniles and Adults
Recently settled juveniles may be attached to any hard substrate, including other zebra mussels. Here they appear as white grains of sand.
Two suggested methods of sampling for the detection of settled juveniles and adults are:
1) Visual Observation:
Divers
Visual observation of infrastructure by SCUBA diver.
Visual observation of infrastructure or cameras can inspect underwater substrates, including attached submerged aquatic vegetation, for settlement and colonization. Generally settlement occurs in the greatest densities between depths of 2-15 m, but zebra mussels have been located at over 50 m in depth and also very near the water's surface. Settled juveniles and adults may also be observed visually using artificial substrates that have been deployed at least 2 weeks prior to the spawning season. This early deployment allows a biofilm to accumulate upon the substrate making it a more attractive environment to the settling mussel.
2) Metal Box Scraper:
Metal Box Scraper
Construct a metal box scraper using a medium-sized metal box with the top and bottom removed. Drill holes along the bottom and sew on a mesh net bag using any heavy or nylon thread. (The mesh on the net is effective in the 1/4-to 1/16-in. range.) File one of the top edges of the box to sharpen it and firmly attach a long pole to the other side of the box. Lower the metal scraper box below the surface of the water and scrape firmly on the substrate so that any mussels that are there will become dislodged and fall into the mesh bag.