Patch exploitation by planktivorous fish and the concept of aggregation as an antipredation defense in zooplankton
Limnol. Oceanogr., 58(5), 2013, 1621-1639 | DOI: 10.4319/lo.2013.58.5.1621
ABSTRACT: Contrary to the suggestion that swarming can be used as an antipredation defense in zooplankton, the individual risk of a planktonic prey (Daphnia) increased rather than decreased with prey density up to 20–30 individuals L−1, suggesting that planktivorous fish (roach [Rutilus rutilus L.]) preferentially feed where prey is more abundant, and confirming that planktonic prey may use both high- and low-density antipredation refuges at densities greater than 30 and lower than 20 individuals L−1, respectively. This was revealed in experiments conducted in a system of interconnected 1 m3 tanks with patchy Daphnia distribution, where fish were allowed to feed from dusk to dawn. The decline in Daphnia density was most dramatic in the tank with the highest prey abundance (reduction of up to 90%) as a result of a sigmoidal functional response (in capture rate of individual fish) combined with the rapid relocation of fish to where Daphnia were most plentiful. Adult Daphnia were depleted more rapidly than juveniles, this being most apparent in tanks with a low prey density, where the fish cruising speed doubled, hence the energy gain from small-bodied prey was probably insufficient to cover the cost of post-capture accelerations. A compartmental size-structured population model, which accounts for a predator's numerical response in space and optimization of foraging strategy, predicts less-selective low-speed harvesting at higher prey abundance, and high-speed searching at lower prey abundance, where the predator moves more rapidly in search of aggregations, but the increased cost of post-capture accelerations leads to lower value juveniles being ignored.