And what is human play? A preparation for future life, maybe in roles that no longer exist as serious needs. It's also skills learning, including interpersonal social skills. And at times, it's creativity. The simple hierarchy view is, in my opinion, very wrong. Fish, like humans, may play because it satisfies the senses. They'll run up a filter current, spar, etc, because tank life is boring and they have pent up energy. They'll sometimes explore. It's play if it gives them pleasure. How can we measure that?
" In the wild the weakest always die first and the strongest get to food and the reproductive rights."
Actually, studies of animal social organization show this to be untrue. The weaker sometimes live longer as the leadership fish are often more exposed to predators. There is a huge element of luck in why any of us are here, and survival can be random, more often than not, I'd argue. There's also an enormous body of research on sneaker males and their contributions to genetic diversity and species survival, as well as on female choices. Sneakers are non dominant males who still father young. It makes sense, because brute strength isn't always what's needed for survival. The species and individuals that survive are the ones with luck who adapt to new situations best. That isn't always a question of physical strength. We're examples of a creature that often has a good brain, but can be beaten up by any adult chimpanzee. Other ape species are bigger and stronger than we are, but we more than get by. We adapt well.
Intelligence is an important survival ability, and it appears to have appeared early in evolution. For sure, it takes many forms, but I wouldn't bet against it showing itself in play in fish. If the ability to play makes a fish better adapted to its world, then it's important.
