Temporal trends in Mysis relicta abundance, production, and life-history characteristics in southeastern Lake Michigan

The density and life history characteristics of Mysis relicta were evaluated at a 110-m and 45-m station in southeast Lake Michigan during spring, summer, and fall for two time periods, 1995–2002 and 2007–2008. Mysis were more abundant during 1995–2002 than 2007–2008 for all seasons and depths, with average declines across seasons of 81% at 45 m and 70% at 110 m. Offshore densities of Mysis in 2007–2008 were similar to published densities within the same region during 1985–1989, but under differing ecosystem conditions (e.g. higher fish biomass and primary production in the 1980 s). Growth averaged 0.032 ± 0.002 mm/day in both 1995–2002 and 2007–2008, and the proportion of females with broods (overall 7%) did not differ between time periods. Mean brood size adjusted for length did not differ between 1995–2002 (17 ± 0.6) and 2007–2008 (15 ± 1.3). The mean length of reproductive females was higher in 2007–2008 (17 ± 0.2) than during 1995–2002 (16 ± 0.1). New recruits (≤ 4 mm) were collected during each season for each time period, but were lower in abundance in 2007–2008 than in 1995–2002. During spring, there was a significant relationship between offshore water column chlorophyll concentration and Mysis recruit abundance, but not during summer/fall. Declining spring chlorophyll levels may be altering food availability for small mysids in spring, and the decline of the benthic macroinvertebrate Diporeia may be increasing fish predation pressure on Mysis despite declining planktivore abundance.