Impact of round goby predation on zebra mussel size distribution at Calumet Harbor, Lake Michigan

The size distribution of zebra mussels atop (exposed to predation) and beneath (protected from predation) rocks were compared at a round goby infested site (Calumet Harbor, IL/IN) and round goby-free site (Evanston, IL) in southern Lake Michigan. The largest zebra mussels were atop rocks at both sites and those from Calumet Harbor were significantly larger than those from Evanston. The smallest zebra mussels were beneath the rocks at both sites and those from Calumet Harbor were smaller than those from Evanston. Nearly all zebra mussels from Calumet Harbor rock tops were larger than the size range preferred by round gobies in published laboratory experiments. Conversely, most of the zebra mussels from rock tops from Evanston were within the size range preferred by round gobies. In afield experiment, glass sheets colonized with zebra mussels from Evanston rock tops were exposed to round goby predation. Predation was videotaped to determine what size zebra mussels were eaten by round gobies. Smaller zebra mussels were consumed more frequently and surviving zebra mussels were either very large or had refuge in the space between larger zebra mussels. The study showed that more effort is required to remove larger zebra mussels than smaller zebra mussels. The results indicate that, while round gobies prefer smaller zebra mussels, the underside of rocks, as well as proximity to larger zebra mussels, can provide refuge for the smaller zebra mussels. Hence round gobies are unlikely to totally remove zebra mussels from a habitat and their impact will likely vary between habitats.