Fish acting together as one is not the only example, we see this all the time in nature. A flock of birds, freakin\' ants work together to build gigantic structures.
Basically their more primitive brains are more in-tune to instinct. Instinct is a genetic imprint of all the lives of lineage. It\'s not perfect, so it takes hundreds to thousands of years to evolve certain instincts, but fish have been here for a really long time. Their instinct replaces individual "thoughts." (I put thoughts in quotations because we can\'t confirm that fish "think" the way we do because the only way of communication is body language, and that\'s not a great form of communication.)
Just a thought: Militia groups have intensive "Boot Camp" programs to de-program humans into thinking like fish.
Instincts play a powerful role in nature.