Shoal Sense

Eastern Mosquitofish           creative Commons Credit : Osado

Soon after I started out on my mile and a half trek to get coffee, a moderately sized dog fell into step alongside. He kept his station at two and a half feet away regardless of whether I slowed, accelerated, or weaved a little side to side. I stress it was only a little, and that it was due to a shortage of caffeine and not a surplus of ethanol. His owner was at the end of a long lead some way behind and he too kept station, so we were like a mini-shoal of queer fish, albeit 2-dimensional, progressing smartly along the sidewalk.

Animal synchronized motion as we see with flocks of starlings or shoals of mackerel are compelling to watch as suddenly the group seems to fold and move in a different direction. The reaction speed is amazing. The whole thing seems to be democratic with no evidence of an alpha-mackerel leading the pack. As mackerel are rumored to have a very short attention span, (rather like most politicians, if Republican Party debates are any indication,) it would have to be democratic as no one would be able to remember where they were going in the first place.

However, science is rather more interesting than politicsand Herbert-Read et al have been studying the shoaling behavior of a mini-shoal in a tank (1). The chosen fish were Eastern mosquito fish which move about in small shoals eating algae and plankton with a taste for mosquito larvae should the opportunity arise.

After a lot of photography, plotting trajectories and speeds, the results were that the shoaling had only three simple rules.

Firstly, the fish liked to be close to their neighbors, so that they liked other fish to be within their social space. 6 cm is a good social distance. Secondly, they didn’t want their personal space invaded so they accelerated away if a neighbor was tailgating. Thirdly, they seemed to be only able to think about one neighbor at a time and so were actually responding to their nearest neighbor interaction, so trusting that everything else would take care of itself.

Whenever I drive on a busy freeway, I see a lot of drivers who appear to be obeying the same three rules as they accelerate up close and then brake and dodge to one side or another if there is a space. It seems to be only the verges which prevent the traffic all changing direction like a shoal of mackerel, so that if that predatory semi came rushing in from a rest stop, the whole column would take a sharp left to avoid the large teeth on the front of the Mack truck.


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