Once a particular facility has gained a multi-year history of spawning cycles, that database will assist in maintaining cost-effective monitoring and control strategies.
Fertilization occurs externally in the water.
Adult zebra mussels reach maturity and are capable of spawning in their first year of life. Most males reach maturity at 6 mm and females at 7 mm. Although the sexes of the zebra mussels are separate, fertilization occurs externally. Gametes are released into the water column at the same times in both sexes and this is temperature-dependent. In general, water temperatures must remain above 10-12°C for 1 to 2 weeks, although several variations of this have been noted. Quagga mussels may require lower temperature thresholds (i.e., 8-10°C) (see Life History and Biology and the Risk Assessment sections for further details). Females are capable of spawning again after 8 weeks and can continuously spawn throughout the year in warmer areas. The average female can easily release between 30,000 and 40,000 eggs per year over a period of several weeks, but exceptions to this have also been noted.
Sampling for spawning data
Since spawning seasons are directly related to water temperature, it must be established when they are most likely to occur. After determining a general time frame, it would be advantageous to employ a veliger sampling program at periods twice weekly for early veliger detection. It is because of the extremely fast growth rates that such frequent sampling is recommended. Continue to sample twice weekly even after early veliger detection is first noted to determine the peak in veliger density. It is usually between 3 and 5 weeks after this peak that settlement will begin to occur. At temperatures below 10-12 °C for zebra mussels and 8-10 °C for quagga mussels, veliger sampling would likely yield no results and may pose an added and unnecessary expense to the facility. However, in high-density areas, veligers have been known to persist throughout the winter (Cameron Lange, personal communication). Although veligers may be found in winter, little or no settlement will occur.
It is important to remember that zebra mussel adults that have colonized in shallow bays, in holding tanks, and in forebays may spawn earlier than expected. Even though the temperature of the main source water is not yet viable for spawning to occur, the warmer water temperature of more shallow areas or areas closer to the facility may initiate a spawning event. These veligers may then be transported out into the main source water, where temperatures are well below 12 °C.
Importance of Maintaining Long-Term Records of Monitoring Data