Status and Condition of Mysis diluviana in Lakes Michigan and Huron, 2007-2008

Abstract

The opossum shrimp (Mysis diluviana) is an important part of Great Lakes ecosystems, but its role may be changing as invasive species alter the lower food web. I investigated the new role of Mysis in food webs in Lakes Michigan and Huron in spring and summer 2007-08, with a focus on describing and comparing the abundance and condition of Mysis in the two lakes. I also investigated the importance of Mysis in late summer/early fall diets of pelagic forage fishes from both lakes. Mysis abundance and population characteristics were determined by counting and measuring samples taken from vertical tows in the two lakes. Mysis condition was assessed by analyzing total lipid content and fatty acid profiles of samples of Mysis collected from the study areas. Fishes for diet studies were obtained using midwater trawls performed during acoustic surveys. Mysis were more abundant in Lake Michigan than in Lake Huron, and comparisons with past studies suggested that densities might have declined in Lake Huron over time. Population dynamics revealed that Mysis had similar reproductive patterns in the two lakes, with some recruitment occurring year-round, but gravid female Mysis in Lake Huron were smaller than in Lake Michigan and had smaller brood sizes. Large bloaters (Coregonus hoyi) and rainbow smelt (Osmerus mordax) were the most effective pelagic predators of Mysis in 2007 and 2008, but in general, occurrence of Mysis in fish diets was low. On average, Mysis in Lake Huron had lower total lipids and elevated concentrations of the fatty acid docosahexaenoic acid (DHA) in comparison with Lake Michigan mysids, particularly in spring, suggesting that they may have experienced food limitation in Lake Huron. These data suggest that declines in phytoplankton and zooplankton production in Lake Huron in recent years, linked to dreissenid mussel expansion, are resulting in starvation of Mysis during at least part of the year.