Changes in Mysis diluviana abundance and life history patterns following a shift toward oligotrophy in Lake Michigan

The abundance, biomass, and life history characteristics of Mysis diluviana were evaluated at a 110-m (offshore) and 45-m (nearshore) deep station in southeast Lake Michigan during two time periods, 1995-2002 and 2007-2015. Rapid changes in the lake's productivity were noted between the two time periods, including declines in chlorophyll, the spring phytoplankton bloom, the size of the deep chlorophyll layer, spring/summer zooplankton biomass, and the benthic amphipod, Diporeia spp., which historically served as a major source of food for fish along with Mysis. Mysis were more abundant during 1995-2002 than 2007-2015, with average declines of 82 % at the nearshore station and 54 % at the offshore station. One factor that may have led to declines in M. diluviana between the two time periods was altered reproductive characteristics due to decreases in food availability. Changes in reproductive characteristics included a shorter period of brood release during the spring and early summer and declines in brood sizes. Other characteristics such as growth rate and generation time did not appear to change between periods, however. Predation from fish appears to be focused on newly emerged juveniles and small adults based on the size of mysids found in stomachs of the predominant planktivorous fish in the lake. Despite declining planktivorous fish abundance during the 2007-2015 time period, M diluviana populations have not rebounded as in the past, indicating that there has been a decoupling between predator-prey in the lake following declines in system productivity and the loss of the alternative prey for fish, Diporeia spp.