During the day Mysis relicta could be taken only near the bottom of Lakes Huron and Michigan. In the evening they migrated into the overlying strata as the surface light intensity decreased from 15 to 1 foot-candles. Frequently the mysids migrated through the metalimnion when first ascending, but later in the night the majority occurred in or immediately below this layer. As the length of day decreased following the summer solstice, the mysids ascended progressively earlier each evening and descended later each morning (they descended when the surface light intensity was increasing from 10−3 to 10−2 foot-candle). Moonlight and fog influenced the extent and time of vertical migration.The optimum light intensity for mysids is so low that they are concentrated near the bottom during the day. As the light decreases at sunset, they move upward rapidly; possibly because of a positive reaction to decreasing light. Their night-time distribution is determined by their response to the existing light and thermal conditions. Their dawn descent may be due to a photo-negative condition which originated during the night.
