Schools of fish can perform complex, coordinated maneuvers without ever colliding with each other. They move in unison, but do not follow a leader. To understand the complexity of collective animal behavior, researchers at Tohoku University have developed a model that simulates the group movement of fish based on visual cues. By accounting for the tendency for fish to concentrate on nearby fast-moving fish, the model reveals the mechanism behind dynamic fish schools.
“Fish have a wide angle of vision and can detect many other fish in a school,” explains Susumu Ito. “However, a recent experimental finding shows that each fish selects a single fish from a pair of targets and tracks its movement. It is a spectacular example of selective decision-making.”
Treating every fish in the school would require an enormous amount of information to process. Just as we can focus on just the words we read on a page of text, fish can focus on the most salient target that determines their next move. Although the fish swimming directly in front of you may seem like the best option, it is actually fish slightly to the side that attract attention.
Ito and his team constructed a model that takes visual attention into account to clarify the role of selective visual interaction in a large group of fish. It contains the property of retinal ganglion cells firing preferentially at targets that are closer and moving faster. Visual attention is then directed in the direction of the strongest signal. Only fish that fall within that focus of visual attention can influence the movement of the individual fish.
Using numerical simulations, the team of researchers found that when a fish follows three targets swimming in tandem, it tends to be attracted to the target to the left or right because they are apparently larger. A slender fish that looks straight ahead when viewed straight from behind will look much smaller than a fish that exposes its longer profile view. These results replicate the selective tracking observed in previous experiments.
Furthermore, the model reproduced several collective patterns of fish schools: a rotating vortex, straight, random and spinning. In the turning pattern, fish repeatedly alternate between straight and rotating movements, so that the school dynamically reshapes itself.
“The selective tracking behavior is also observed in locusts and flies,” Ito adds. “We expect to extend the model to the group movements of different organisms in the future. A three-dimensional version of the model may also be able to explain the formation of a huge school of fish known as the bait ball.”
