INITIAL COLONIZATION OF THE ZEBRA MUSSEL (DREISSENA-POLYMORPHA) IN SAGINAW BAY, LAKE HURON - POPULATION RECRUITMENT, DENSITY, AND SIZE STRUCTURE

The various life stages of the zebra mussel (Dreissena polymorpha) wer e examined during the initial years (1991-93) of the mussel's invasion into Saginaw Bay, Lake Huron. Yearly trends in densities of larvae, n ewly-settled juveniles, and adults were poorly related. Larval densiti es were lowest in 1991 and increased each year, but the number of sett led juveniles was highest in 1991. Adults increased between 1991 and 1 992 and then declined in 1993. Mean adult densities at sites with hard substrates were 11,700, 33,200, and 4,100/m(2) in each of the 3 years , respectively. Year-to-year variation at individual sites was high an d likely a result of recruitment dynamics and spatial patchiness of av ailable substrate. By 1993, densities on hard substrates were generall y similar throughout the bay, but length-frequency distributions in th e inner and outer bay were quite different. The 1991-cohort was not di stinguishable in the inner bay in 1993 either because of poor growth o r a limited life span, but this cohort was readily distinguishable in the outer bay. In addition, ash-free dry weight of a standard 15-mm mu ssel in the inner bay declined 65% between 1991 and 1993. Although foo d concentrations (chlorophyll and particulate organic carbon) declined to low levels in 1993 and both densities and soft-tissue weight of Dr eissena declined, it is not clear whether populations in the bay have peaked and are now at equilibrium with the surrounding environment.