How to surprise a copepod: Strike kinematics reduce hydrodynamic disturbance and increase stealth of suction‐feeding fish

Abstract

To capture prey with suction, fish must get sufficiently close to their prey to allow the suction flow to overwhelm the prey and draw it into the mouth. Both swimming towards the prey and suction flow create a hydrodynamic disturbance, which can elicit an escape response by the prey. Using particle image velocimetry, we measured flow speeds and derived fluid deformation rates at the location of the prey as bluegill sunfish fed. In front of the mouth, flows had a composite time‐dependent nature. First, the bow wave pushed water away from the fish, but when the mouth opened and suction commenced, flow reversed and water deformation rates increased rapidly. Our inferences indicate that, at the prey, the approaching bluegill is detected primarily based on its suction‐induced disturbance, rather than its bow wave‐induced disturbance. A comparison of suction‐induced disturbance with the signal produced by active suspension feeders indicated that fish are able to produce a more subtle disturbance than expected based on their flow speeds and mouth size alone. Jaw protrusion and the rapid opening of the mouth during the strike both help to minimize the signal available to the prey. We propose that the temporally quick strikes and high jaw protrusion that are seen in many zooplanktivorous teleosts represent adaptations that minimize the time available to prey for executing an escape response.