Low densities of Diaptomus ashlandi, Diacyclops thomasi, and Daphnia galeata mendotae were measured at depths where Mysis relicta formed nighttime aggregations . Calculations suggest that mysid predation can not account for the rarity of prey animals at these depths, which further suggests that the prey avoided the mysids . Unlike D. galeata mendotae, Daphnia pulicaria was common in mysid aggregations . The somewhat larger size of D. pulicaria may reduce its vulnerability to mysid predation, and consequently may explain the vertical distribution differences between the two congeners . Vertical distributions of Limnocalanus macrurus and copepod nauplii showed no obvious relationships to the mysid distributions . These were the only two taxa with distributions that were correlated with chlorophyll a concentrations . All crustacean taxa were rare in the epilimnion at night when sonar recorded a dense fish school .