Considerable lag times between application and adult zebra mussel death have been observed, presumably due to shell closure when the presence of oxidant is sensed. Lag times of from 2 to 18 days are noted and generally decrease as chlorine concentration increases (Van Benschoten et al. 1993).
Resistance to chlorine can vary with age, size, and developmental stage of the mussel, with older and larger individuals being more resistant; veligers are much more susceptible than adults (Claudi and Mackie 1994).
Zebra Mussel Chemical Control Guide
Hypochlorite and Chlorine Gas