COLONIZATION PATTERNS AND DENSITIES OF ZEBRA MUSSEL DREISSENA IN MUDDY OFFSHORE SEDIMENTS OF WESTERN LAKE-ERIE, CANADA

Zebra mussels (Dreissena) have expanded rapidly throughout most of the Laurentian Great Lakes since their inadvertent release in 1986. These exotic molluscs now occur in great numbers on the bottom of western L ake Erie where they are found increasingly in deeper areas of the basi n (average depth: 10 m), on soft, muddy substrates. This study is aime d at quantifying the density and the distribution patterns of mussel c olonization in the basin as a first step in investigating the effect o n sediment properties of such an abrupt change in benthic community st ructure. Underwater video imagery and diver-collected samples taken fr om representative offshore areas (seven sites) in western Lake Erie sh owed colonization levels of up to 20,000 live mussels per m(2) in soft sediments (adults with shells >10 mm comprised 47%). Digital side-sca n sonar records confirmed that colonization patterns were not random b ut showed distinctive spatial signatures ranging from 30-m-long parall el stripes, to large ovate masses. Broad irregular mats were found in association with hard bottoms (bedrock, boulders, or wrecks and large debris). Mussel densities were averaged from the sites, assuming consi stent relationships with substrate type and were combined with digitiz ed percentage of areal coverage of major bottom types in western Lake Erie. This resulted in the first population figure of 10(13) in the ba sin. This figure includes molluscs of all sizes > 0.84 mm.