ZEBRA MUSSELS (Dreissena polymorpha) IN NORTHERN MICHIGAN LAKES: A CALAMITY FOR NATIVE CLAM HEALTH

Abstract

Zebra mussel (Dreissena polymorpha) infestation has expanded since the 1980’s into inland lakes and rivers by contamination from boats and larval dispersal through streams. Consequently, costs of boat and water treatment plant repairs have increased, and the aquatic food web has been altered by the excess of zebra mussels. Native clam species are being colonized and wiped out in infested lakes. Inland lakes near the northern lower peninsula in Pellston, Michigan were surveyed for clam health on transects until one meter of depth. Clams were collected and surveyed for health, which was measured by shell width (mm) across the longest part of the shell, and clam weight (g). Douglas Lake, Burt Lake and Round Lake have zebra mussel infestations, and Round Lake was treated with Zequanox. Wycamp Lake and Lark’s Lake are free of zebra mussels. We hypothesized that there would be an inverse linear relationship between the presence of zebra mussels and clam health, and we would see greater clam weights and shell widths in lakes with less or no zebra mussel infestation. Additionally, we hypothesized that the mean of the smallest clam shell widths would be similar across lakes, but the mean of the largest shells differs, then lakes with infestations are losing their largest clams. We found the healthiest clams based on weight to width ratio in Lark’s Lake, followed by Wycamp, Douglas then Round. No clams were found in Burt Lake. We saw significant differences in the mean shell widths of the five smallest and five largest clams between lakes. The largest clams in each lake had significantly larger mean shell widths in lakes without D. polymorpha infestation, while we found significant but not notable trends in the mean shell width of the smallest clams from each lake.