Microhabitat association of Hemimysis anomala on fish spawning reefs in Grand Traverse Bay, Lake Michigan

In 2006 the bloody-red mysid (Hemimysis anomala), a new invasive species to the Great Lakes, was discovered in the Muskegon channel which flows into Lake Michigan. As predicted at the time of introduction, it quickly expanded its range in Lake Michigan and has recently been documented in Grand Traverse Bay near Elk Rapids, Michigan. Its effects on Great Lakes ecosystems, however, remain unclear owing to a lack of information on its abundance. Using a gear intended to sample lake trout and lake whitefish eggs from cobble substrate during spawning, we found variable densities of Hemimysis at three near shore reefs at Elk Rapids, Lake Michigan over a two year period. The highest densities of Hemimysis were found at the reef with the highest quality fish (i.e., lake trout and lake whitefish) spawning habitat based on the proportion of rounded cobble and rubble substrates, and the amount of interstitial space. Abundance of Hemimysis on all reefs was highly seasonally dependent, ranging from 0 to 31.4 per m². The highest numbers were seen during the fall when water temperatures were between 6 and 12 °C. Based on the association of Hemimysis with cobble substrates and the abundance of this type of habitat in northeastern Lake Michigan, we predict substantial expansion of Hemimysis in this area of the lake. Additional evaluation of Hemimysis on fish spawning habitat is needed to determine potential interactions with other Great Lakes biota, particularly larval fish with which they may compete.

► Hemimysis anomala invaded a fish spawning reef complex in Grand Traverse Bay.
► Highest densities of H. anomala were found in the fall and on the highest quality fish spawning habitat.
► H. anomala will likely expand across Great Lakes spawning reefs and shoals.
► Interactions with early life stages of fish and H. anomala should be evaluated.