They are looking at median swimming speed and how fish swim. Calling this "behavior" is misleading. It could just as likely be developmental physical differences.
They watched fish swim and saw that they are more correlated with themselves than others. Everything else is a very fishy narrative. Many people in this thread judge it by empathy with the story instead of what can be reasonably done.
The more we learn about animals (see the article on HN ~2 days ago that bees engage in play) the more it seems like they're a lot more like us than we'd like to think. We're smart, and we can try to explain every thing away (like you do in your comment), but there's a pattern that's emerging. It's an uncomfortable pattern and I get why we try to convince ourselves that animals are fundamentally different than we are, but I'm more and more believing that we're all actually a lot more similar than we'd like to think.
A lot of my current thinking is from the book How to be Animal by Melanie Challenger. I recommend it!
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