If a single long-term application is being used to kill all mollusks present, it should be made following the reproduction period to ensure that no additional veliger settling will occur. Reproduction in zebra mussel is also dependent on water temperature, and this timing will be keyed in to the time of year.
A one-time application of this kind can be done if infestation is low enough that killed material will flush out without blocking the system. If infestation is greater and may block any parts of the system, twice-a-year chlorination may be required.
Winter treatment with low levels of chlorine may be useful for ridding systems of adult zebra mussels, although the process is slow (Van Benschoten et al. 1993).
For veliger control there is no need to chlorinate during winter when temperatures are too low for mussel reproduction, and chlorination can be suspended during periods when veligers are not present in intake water (Payne and Lowther 1992; Claudi and Mackie 1994)
Zebra Mussel Chemical Control Guide
Hypochlorite and Chlorine Gas