The relationship between coastal Mysis diluviana abundance and spring thermal bar dynamics

We studied the density and age structure of Mysis diluviana in Lake Michigan with respect to spatial structure relating to spring thermal bar dynamics. The thermal bar is a zone of sinking 4 °C water that separates warmer inshore water from colder offshore water. The thermal bar migrates offshore as inshore water warms. The density of Mysis did not differ statistically between inshore (about 6 °C, as shallow as 17 m bottom depth) and offshore of the spring thermal bar, but the percentage of Mysis that were juveniles (< 10 mm length) was significantly higher inshore (P = 0.011). Our data suggested that inshore Mysis may have an advantage in growth, but an impact on the entire population is unlikely. This study has important implications for both the predators and competitors of Mysis. First, the thermal bar period may be the most extended time that M. diluviana and the invasive Hemimysis anomala overlap spatially. Second, at the only well-studied Great Lakes lake trout nursery, a Lake Superior shallow reef, juvenile Mysis are important prey for lake trout fry (Salvelinus namaycush) which emerge and begin feeding in spring. Our study shows that Mysis are often abundant in coastal Lake Michigan water during the period when the lake begins to warm. Hence, lake trout restoration efforts for coastal spawning areas of the other Great Lakes may have potential Mysis-based nursery grounds essentially “on site,” at or adjacent to spawning reefs.

► Chlorophyll was more concentrated inshore of the thermal bar than offshore.
► Mysis densities did not differ significantly on both sides of the thermal bar.
► The percentage of Mysis newborns was significantly higher inshore (P = 0.007).