Bluegills: Biology and Behavior

1: Movement


Fishes move about by bending their bodies and using their fins. Depending on the situation the fins can act as thrusters, steering devices, or brakes. To a casual observer a swimming fish appears to leave no trace of its movements, but in fact it disrupts the section of water through which it swims, temporarily altering its shape and leaving behind a wake, or area of turbulence.

Some standard features of hydrodynamics require explanation before being applicable to living bluegills. The starting vortex, or detached eddy, has a counter clockwise rotation (+ K). The bound vortex, or bound circulation, which has a clockwise (- K) rotation, remains attached until the starting vortex has been shed. Kelvin’s theorem requires that the total vorticity, or circulation, must have a neutral charge balance over time. Thus any variation in the bound vorticity necessitates a vortex having the opposite sign being released into the wake. If the bound vorticity varies with time, Γ( t), then in the brief interval δ t the total vorticity released into the wake will be (∂Γ/∂t)δ t, and once separated from the fish it assumes the form of a continuous vortex sheet.8 Because relevant motions made by a fish are periodic, the resultant variations in vorticity are periodic too.

Vortex sheets are dynamic, containing places of positive, negative, and coalesced vortices. A von Kármán vortex street describes two infinitely long rows of vortices with alternating signs (Figure 1). In the figure, flow is left to right. The wake indicates a momentum deficit compared to the incoming flow, and the subsequent wake creates drag on the organism producing it. Figure 2 shows a reverse von Kármán vortex street resulting in a wake, or jet, having an excess of momentum and generating thrust instead of drag. Drag and thrust increase in direct proportion to the vertical distance between rows, and either is eliminated if this distance is reduced to zero. The fins of swimming bluegills produce wakes of reverse von Kármán vortex streets.

A swimming fish exerts some control over incoming vortices, whether self-generated or extraneous in origin, by altering their locations and strength. Three situations are possible.9 All can result in either drag or thrust that is always maximized. In vortex annihilation a vortex generated by a fin or body interacts destructively with an incoming vortex, weakening the downstream vortex street. Constructive interference is a situation in which the generated and incoming vortices produce an additive effect, strengthening the vortex street downstream. During vortex pairing the vortex that is generated interacts with another of opposite sign and results in pairs, or four per cycle.