[iainmerrick]

If you don’t slow down when you get to the front, you’ll end up separated from the school and more vulnerable to attack. Likewise if you don’t match the school’s speed when you’re at the back.

Staying in the middle would be advantageous, but when the whole school is cycling slowly, that’s tricky to do. The cheating behavior would need to be a little more complex so that’s an obstacle to its evolution.

Additionally, the more cheaters there are, the slower the school will move overall, which harms everybody (including the cheaters) so that limits the spread of cheating somewhat.

Overall, I’d guess that cheating could evolve, but only in a careful balance with the normal behavior. If the cheating starts to get significantly more complex, policing methods would co-evolve with it.

[bayes]

Interesting. So maybe it is an evolutionarily stable strategy, if you assume it's too hard for the fish to keep track globally of where they are in the shoal (and are therefore limited to strategies where they only react to what's immediately around them). It would be fun to simulate this and see if there are any other strategies that offer an advantage.