Information Cascades in Schools of Fish

https://www.princeton.edu/news/2019/09/30/get-moving-mystery-animal-group-behavior

Nature has a lot of bewildering phenomena, and near-instantaneous decisions of animal crowds are one of them. These “behavioral cascades” are not too different from the information cascades found in human behavior. One animal being skittish in response to a threat can lead to another and yet another responding in the same way. However, the behavior of one or two animals is not nearly guaranteed to set off a behavioral cascade.

As stated in the attached article, a fish suddenly darting one direction does not automatically lead the school to hold the same response. This differentiation in responses has been somewhat of a mystery–while it is intuitive that there will be some tipping point that leads to group behavior, it is less intuitive to pin down how this tipping point is reached. Golden shiners are a particularly skittish schooling fish. An individual fish will sometimes exhibit darting behavior without any actual threat. It would not be too beneficial for a school to suddenly change directions for every singular fluke.

Golden shiners rely on their own kind of “private information” to determine how credible a threat is based on the behaviors of other fish. There is a chemical mixture called “schreckstoff” (this translates literally into “scary stuff” from German) that is released into water through injuries in the fish’s skin. The presence of such a compound generally causes fear responses in fish by signaling there is a potential threat that harmed a nearby fish. Golden shiners tend to bunch closer together when there is schreckstoff present. This response indicates to individual fish that the fright responses of other fish are more credible. Thus, even if a fish cannot see what is making one or two other fish dart away quickly, it is much likelier that they will follow the same behavior. This information travels incredibly quickly to move away from a threat, making it seem as if schools of fish are moving simultaneously.

On the other hand, the lack of schreckstoff prevents shiners from bunching together. This leads to each individual fish’s “private information” indicating that there is no threat present even if one or two other fish are exhibiting frightened behavior. Under these conditions, it is far more unlikely for a behavioral cascade to form.