Mysis relicta in Lake Michigan: Abundances and Relationships with their Potential Prey, Daphnia

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Abstract
Mysis relicta were sampled at a 100-m reference staton in southeastern Lake Michigan (43°N 86°40′W) from 1985 to 1989, to quantify nighttime water column abundances and to compare vertical distributions with those of Daphnia. Diel vertical migration produced maximum concentrations in the thermocline at night of 1 to 10 mysids∙m−3. Variation among replicates averaged 28% with a 3-net Tucker trawl and 19% with Puget Sound vertical closing nets. Mean areal abundances over 5 yr averaged 110 mysids∙m−2 (SE = 20; n = 30; range = 25 to 645) based on nighttime vertical and oblique net tows at 1–3-wk intervals during summers at the reference station. Synoptic cruises from 43°N to 45°N during August indicated that densities were considerably greater offshore than inshore, and greater in the north than in the south. Hypothesized long-term changes in mysid abundances were not detected. Although Mysis is potentially an important predator on Daphnia, differences in nighttime vertical distributions reduce encounters between Mysis and Daphnia during summer in Lake Michigan, such that Mysis exert mortality rates on Daphnia of < 1.5% per day; the latter are in general less than 10% of the birth rates of Daphnia populations, estimated from fecundities.