Temporal trends in condition and reproduction of quagga mussels (Dreissena rostriformis bugensis) in southern Lake Michigan

Currently little is known about the condition and spawning patterns of quagga mussels (Dreissena rostriformis bugensis) in deeper, consistently cold regions of the Great Lakes. This lack of information limits our ability to predict the future expansion of D. r. bugensis in the offshore regions of the Great Lakes and other large, deep lakes. We collected quagga mussels on a monthly basis in 2013 (April–September) at three established sites along a depth transect (25, 45, and 93m) in southern Lake Michigan and calculated a condition index (CI), a ratio of dry soft tissue weight to internal shell capacity, to assess condition, and a gametogenic index to assess reproductive activity. We also measured size–frequency, density, and biomass in March, July, and October. Condition was consistently highest at 25m, population biomass and average shell length were highest at 45m, and density was highest at 93m. At all three depths, CI decreased with increasing shell length suggesting that food availability may be limiting for larger individuals. Mussels at 45m spawned earliest with over 50% spent by July while mussels at 93m began spawning in August. Mussels at 25m had not yet spawned by September. Long-term trends in density indicate that D. r. bugensis populations continued to expand at the 93m site but may be stabilizing at 25m and 45m. Consistent with density trends, CI at 25m and 45m was lower than in 2004 and 2008.