Lake-wide, annual status of the Mysis diluviana population in Lake Michigan in 2015

Mysis diluviana is one of the most abundant zooplankton by biomass in the Laurentian Great Lakes of North America, a predator of other zooplankton and an important prey for fishes. Studies of long-term trends in Lake Michigan have shown 2005-2016 densities to be 50-80% lower than 1990s densities, but these observations have been based on annual monitoring that is either spatially or seasonally limited. We combined Lake Michigan Mysis data from three annual programs and the 2015 Cooperative Science and Monitoring Initiative to achieve broad spatial coverage during spring, summer, and fall of 2015 and broad depth coverage during spring 2016. Lake-wide, annual density and biomass were 82 (SE: 10) Mysis/m(2) and 200 (SE: 36) mg dry mass/m(2). Density and biomass estimates were highest offshore, generally higher in the north basin, and seasonally highest in summer. Annual lake-wide averages for depths >30 m were better captured by seasonally-extensive annual programs than spatially-extensive annual programs, although spring sampling may bias annual values low. Mysis cohorts grew 0.026 mm/d (age-0) and 0.007 to 0.027 mm/d (age-1). Annual mortality was 81-98%. Reproduction was fall-spring and seasonal lake-wide estimates ranged from 0.6 to 19.1% females brooding, 13-20 embryos/brood, and 3-46 embryos/m(2). Annual production (423 mg dry mass/m(2)/yr, SE: 31) was lower than all but one previous estimate from lakes Michigan, Huron, and Ontario. While Mysis tend to persist, low Mysis production may be a concern for prey fishes that feed on Mysis. (C) 2020 the US Geological Survey and International Association for Great Lakes Research. Published by Elsevier B.V. All rights reserved.

Automated summary

The population density and biomass of Mysis diluviana in Lake Michigan have shown a decreasing trend over the years, with the highest estimates found in deep water and the northern basin, peaking during the summer season. Seasonal monitoring may affect the estimates, and the declining productivity of Mysis could pose a threat to fish species that rely on them as a food source.