Blacknad wrote:
"The reason that a school of fish 'ball'
Is enrirely due to a group of Dolphin that swim very fast around the fish in ever tighter circles."

Read my example again and you will see that my reference was to tuna: "When a tuna approaches." Tuna don't strategize.

But you do bring up one fascinating aspect of evolution. If the smaller fish didn't form a ball the dolphins would use a different strategy. The reason they do it, and the reason it works, is that the dolphin's understand, and can predict, the behaviour of schooling fishes.

DA Morgan