Trends in Mysis diluviana abundance in the Great Lakes, 2006-2016

With the large Diporeia declines in lakes Michigan, Huron, and Ontario, there is concern that a similar decline of Mysis diluviana related to oligotrophication and increased fish predation may occur. Mysis density and biomass were assessed from 2006 to 2016 using samples collected by the Great Lakes National Program Office's biomonitoring program in April and August in all five Great Lakes. Summer densities and biomasses were generally greater than spring values and both increased with bottom depth. There were no significant time trends during these 10-11 years in lakes Ontario, Michigan, or Huron, but there was a significant increase in Lake Superior. Density and biomass were highest in lakes Ontario and Superior, somewhat lower in Lake Michigan, and substantially lower in Lake Huron. A few Mysis were collected in eastern Lake Erie, indicating a small population in the deep basin of that lake. On average, mysids contributed 12-18% (spring-summer, Michigan), 18-14% (spring-summer, Superior), 30-13% (spring-summer, Ontario), and 3% (Huron) of the total open-water crustacean biomass. Size distributions consisted of two peaks, indicating a 2-year life cycle in all four of the deep lakes. Mysis were larger in Lake Ontario than in lakes Michigan, Superior, and Huron. Comparisons with available historic data indicated that mysid densities were higher in the 1960s-1990s (5 times higher in Huron, 2 times higher in Ontario, and around 40% higher in Michigan and Superior) than in 2006-2016.