Predators concentrate on current-concentrated prey

Posted on December 13, 2016

In marine environments, recirculating eddies, upwellings and downwellings play an important role in concentrating plankton in small areas, a process that attracts predators such as fish, mammals and birds and boosts ecological productivity significantly above background levels. However, much less is known about the ways in which physical processes may concentrate organisms and intensify food-web interactions in freshwater systems. Working in Lake Opeongo, Ontario, Canadian scientists observed the effects of wind-induced currents on plankton and fish using current profiling, plankton counting, hydroacoustics, telemetry and netting. They tested the idea that wind-forced aggregation of plankton improves the foraging efficiency, growth and productivity of their consumers. In stratified lakes, wind forces the thermocline down and creates a recirculating current that dives at the downwind end of the basin and returns as a “conveyor belt” current running back upwind along the top of the thermocline. The researchers found that zooplankton were concentrated into the “conveyor belt” above the thermocline, as well as in surface waters at the downwind end of the basin. In windy conditions, the induced water currents were stronger than the swimming speeds of small (355 – 399 µm) zooplankton, but not of larger zooplankton or fish. Large animal plankton and plankton-feeding fish aggregated at the thermocline at the downwind end of the basin, and larger piscivorous fish (lake trout) foraged in wind-exposed areas, where they appeared to feed more easily (by using fast swimming less often) than in calm conditions. Taken together with the findings from marine environments, these results show that current-assisted predation occurs across a large range of spatial scales in aquatic ecosystems.

Reference: Kerckhove, D.T. et al. 2015. Wind on lakes brings predator and prey together in the pelagic zone. Canadian Journal of Fisheries & Aquatic Sciences 72, 1652–1662.