Patch exploitation by planktivorous fish and the concept of aggregation as an antipredation defense in zooplankton

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 21 , 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 21 , respectively. This was revealed in experiments conducted in a system of interconnected 1 m 3 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.

Interference competition in a planktivorous fish (Rutilus rutilus) at different prey densities and temperatures

“…The findings of this study support the notion that capture rates in planktivorous fish increase at higher prey densities (e.g., Eggers 1977;Gliwicz et al 2013). The results did not reveal any difference in the strength of interference at different prey density levels, even though it might be expected that interference would be stronger at low prey densities when fish swim faster in an attempt to compensate for a lower prey encounter rate (Munk andKiørboe 1985, Maszczyk andGliwicz 2014).…”

“…The experiments were performed during the summer (August 2009) and autumn (October 2009) periods at the Hydrobiological Station in Pilchy, Poland, within a field enclosure with mosquito netting walls and a transparent roof (Gliwicz et al 2013). This setup ensured semi-natural conditions, including similar light intensities (1.7-3.8 lm m -2 s -1 30 min before dusk), with temperature being the only factor that differed significantly between the summer (19.4-20.2°C) and autumn (11.9-14.4°C) experiments.…”

Warming increases the number of apparent prey in reaction field volume of zooplanktivorous fish

“…We cannot preclude the (rather unlikely) possibility that the difference in RD between the morning and the afternoon estimates could be also partly attributed to the diel rhythm or slightly greater satiation of the fish feeding in the afternoon in the presence of predator‐risk information. Each fish used in the afternoon session was allowed to capture up to 50 prey items during the morning session, which is a small proportion of the number of zooplankton prey a small rudd (or danio) can capture during a day (over a thousand according to Gliwicz et al ). Note that food was withheld from the fish for a minimum of 24 h before the next experiment started.…”

“…This then leaves it to be more successful in the next prey encounter several seconds later. The velocity may become greater when the prey has been overexploited and the foraging fish speeds up to find another prey aggregation (Gliwicz et al ). This may be why the body speed of planktivorous fish remains lower than expected, with the ability of much greater body velocities likely reserved for a parry when evading danger from a piscivore (O'Brien ).…”

“…Foraging planktivores face the alternative of either feeding on tiny indiscernible prey at reduced speed, filter‐feeding, or undertaking long‐distance forays in search of prey that would be worth stopping for. But each stop is followed by a costly post‐capture acceleration (Domenici and Blake ; Johnson et al ; Gliwicz et al ). In the wild, most of these events happen within a brief “antipredation window,” at the twilight of dusk or dawn, when the light is strong enough to allow planktivorous fish to feed on zooplankton prey but limits the visual detection of prey by piscivores (Clark and Levy ).…”