Evidence for a size-structured explanation of partial diel vertical migration in mysids

Mysids are known for benthic-pelagic diel vertical migration (DVM), where the population is benthic by day and pelagic by night. However, historical and recent observations in members of the Mysis relicta complex suggests populations exhibit partial DVM, with some remaining benthic at night. We used pelagic net and benthic sled tows to assess diel habitat use by Mysis diluviana at two stations (60 and 100 m deep) in Lake Champlain, USA, during June-November 2015. At both stations, mysids were on the bottom both day and night, but the extent of pelagic habitat use by Mysis varied by site depth. At 60-m, pelagic densities were an order of magnitude lower during the day compared to at night, indicative of benthic-pelagic DVM. Contrary to expectations, we found no diel difference between pelagic and benthic sled density estimates at 100-m, suggesting an equal number of Mysis are benthic day and night, and an equal number are pelagic-day and night at deeper sites. Mean body length of benthic-caught mysids was greater than pelagic-caught individuals, a pattern that was evident both day and night at 100-m. Our findings indicate Mysis partial DVM is common across seasons and influenced by body size and depth.