Schooling Behavior in Fishes
Fish schooling and aggregation behaviors are some of the most prominent social and group activities exhibited by fishes. Fish may school or form aggregations for many reasons, including foraging, reproduction, and defense from predators. One of the most enduring hypotheses regarding fish schooling is that fish in schools can obtain a hydrodynamic advantage, thus reducing the cost of locomotion, by taking advantage of the wakes shed by neighbors within the school.
We have been studying the kinematics and hydrodynamics of schooling fishes by using 3D kinematic techniques to track the positions of fishes within small (3 – 10 individual) schools in our laboratory flow tanks, and by using a novel custom laser-scanning device to image flow within schooling groups of fishes. Our laser-scanning system involves using two lasers simultaneously and two rotating mirror drums to sweep laser beams through the school. This results in a near-3D image of the flow within the school, and also eliminates shadows cast by individual fish, allowing detailed imaging of flow in between adjacent fish within the school.
Basic questions addressed in our schooling research derive from the theory of Weihs (1973), who suggested that specific spacing patterns among members within the school would offer a substantial hydrodynamic advantage and reduce the cost of locomotion.
Our laser scanning system, developed by Dr. Wolf Hanke, consists of two rotating drums with spirally staggered arrays of mirrors, triggered to high-speed cameras. These rotating drums generate a flashing array of light sheets that allow visualization of flow patterns within a small school of fish.
The array of scanned light sheets looks something like the above image when operating. Here, three bluegill sunfish are swimming in a flow tank, and the laser scanning system is generating an array of horizontal light sheets.
In the above image, flow within a school of 8 fish is visualized using laser scanning. The wake of fish 1 can be clearly seen, while the wakes of the other fishes are visible in light sheets at other heights not shown here. Note how compact the wake is, and the near-linear array of vortex centers.
We are also studying schooling behavior by swimming fish behind two robotically controlled flapping foils that can be used to generate wakes of known spacing and hydrodynamic character. In the above image, a bluegill sunfish is swimming behind two black flapping foils, generating a “schooling” wake.
Image of a trout swimming between the vortex wakes generated by two flapping foils at the top of the figure.