Diel feeding behavior in a partially migrant Mysis population: A benthic-pelagic comparison

Populations that exhibit partial migration include migrants and non-migrants. For benthic-pelagic organisms that exhibit partial diel vertical migration (PDVM), migrants and non-migrants spend different amounts of time in benthic and pelagic foraging arenas over a diel cycle. For example, mysids exhibit PDVM and can feed on benthic and pelagic resources. Migratory individuals are assumed to undergo vertical migration at night to access pelagic food when predation risk is low. However, feeding behavior of non-migrant benthic individuals is not well understood. One hypothesis to explain individual variability in diel vertical migration (DVM) behavior is the hunger-satiation state of individuals (hunger-satiation (HS) hypothesis), which predicts that migration is driven by hunger and non-migration is a response to satiation. We assessed diel feeding patterns of benthic- and pelagic-caught Mysis in Lake Champlain to evaluate if PDVM was consistent with predictions of the HS hypothesis. Stomach fullness and diet composition revealed little diel difference in stomach contents between time of day or between benthic and pelagic individuals at night. Pelagic individuals had consistently higher stomach fullness shortly after sunset compared to near midnight. Non-migrant benthic individuals at night and benthic-caught individuals during the day had similar amounts of detritus in stomachs. High stomach fullness and levels of zooplankton in benthic-caught stomachs indicate Mysis actively feed when benthic, regardless of time of day. Our results suggest variation in Mysis migration behavior is not likely due to hunger-satiation, and highlights the importance of variable behavior in determining Mysis effects on food web interactions in deep lakes.