Lake-wide distribution of Dreissena in Lake Michigan, 1999

The Great Lakes Science Center has conducted lake-wide bottom trawl surveys of the fish community in Lake Michigan each fall since 1973. These systema tic surveys are performed at depths of 9 to 110 m at each of seven index si tes around Lake Michigan. Zebra mussel (Dreissena polymorpha) popu lations have expanded to all survey locations and at a level to sufficiently contri bute to the bottom trawl catches. The quagga (Dreissena bugensis), recently reported in Lake Michigan, was likely in the catches though not recognized . Dreissena spp. biomass ranged from about 0.6 to 15 kg/ha at the various s ites in 1999. Dreissenid mussels were found at depths of 9 to 82 m, with th eir peak biomass at 27 to 46 m. The colonization of these exotic mussels ha s ecological implications as well as potential ramifications on the ability to sample fish consistently and effectively with bottom trawls in Lake Mic higan.