Many large, deep aquatic systems across the salinity gradient contain a migrating predatory invertebrate. In both northern temperate lakes like the North American Great Lakes and brackish seas like the Baltic Sea, these predators are the 10-20 mm long mysid shrimps. These species have a similar ecology to euphausiids (krill) in marine systems. We compare the biology of mysids in Lake Ontario and the Baltic Sea, including abundance, prey selection, and migration behavior, and discuss their importance for ecosystem function in respective system.

Results/Conclusions

Biomass of mysids is similar to that of krill in marine systems and comparable to the biomass of planktivorous fish in Lake Ontario. Mysids are omnivores, but predation on zooplankton dominates the diets in both systems. Therefore, understanding top-down regulation in these large aquatic systems requires including mysids as zooplankton predators and competitors with planktivorous fish even though they are also prey of the same fish species. Mysids have low reproductive rates (1 – 2 years to maturity, 20-50 eggs per females) indicating that mortality rates have to be low to sustain the populations. Diel vertical migrations minimizes mortality but at a cost of decreased food intake and growth rates. In addition, the availability of a day time benthic refuge appears critical as mysid abundance decline in areas of the with deoxygenated bottom waters.