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Spatial variability and drivers of Mysis partial diel vertical migration

Jason Stockwell and Rosaura Chapina (U. Vermont), Brian O’Malley and Brian Weidel (USGS), Lars Rudstam (Cornell), Sture Hansson (U Stockholm). (Funded by Great Lakes Fisheries Commission)

 Diel vertical migration (DVM) is an important component of food web interactions and population assessments in the Great Lakes. In the case of Mysis, a key prey species for Great Lakes fishes, the population is assumed to undergo DVM and migrate into the pelagia at night. Consequently, Mysis assessment protocols rely on nocturnal pelagic sampling. Mysis, however, exhibit partial DVM (PDVM) – Mysis have been observed on the bottom at night in lakes Ontario, Superior, and elsewhere. The proportion of Mysis populations which exhibit PDVM is unknown, as are the factors which cause some individuals to remain benthic at night. Recent data from Lake Champlain challenge conventional assumptions about Mysis DVM – a substantial portion of the population does not migrate and is thus unavailable to pelagic gear. Furthermore, differences in body size and condition were evident between migrant and non-migrant Mysis (e.g., larger individuals were benthic at night). If similar patterns exist in the Great Lakes, where food web models indicate Mysis is a key player, then previous biomass estimates based on pelagic sampling underestimate lake wide abundance. In 2019 we started to assess variability in benthic-pelagic distributions of Mysis in Lake Ontario to test if PDVM is a ubiquitous behavior of Mysis in this system, what PDVM may mean for population assessments and food web modeling, and what mechanisms may be driving PDVM. If the night benthic component of Mysis populations is high, then a re-evaluation of profundal food web structure/function and outcomes of management actions would be necessary to include more realistic estimates of Mysis biomass and distribution. Moreover, if PDVM is related to seasonal patterns in reproductive status, energy reserves, body sizes or environmental conditions, then sampling protocols can be modified to account for these drivers of Mysis behavior when determining lake-wide biomass estimates. A review paper on partial diel migration in mysids is in preparation.