The most elegant study of fish pain that I’ve ever seen … was done a few years ago by a biologist named Lynne Sneddon … in the U.K. She used zebra fishes, which are very commonly used in research. And what they did was they put a group of zebra fishes — I don’t remember how many, perhaps 30 — in a complex tank that had two chambers. One chamber was enriched, it had rocks and vegetation, and the other chamber was barren. It was open. You can probably guess which chamber these fishes spent all their time in — it was the enriched one. Fishes like places to hide, they like stimulation in their environments.

And then they injected the fishes either with one of two things. One was with an acid solution, which is known to be caustic and presumably painful to these fishes, if they can feel pain. And then the other … half of the fishes were randomly selected; [they] were injected with saline, which causes just the pierce of the needle and then the pain is not going to be lasting, because it’s not acidic. And then they watched to see how they behaved, and they all remained swimming in the enriched tank. And then they dissolved a painkiller solution in the barren, undesirable chamber of this complex tank. And lo and behold, some of the fishes then started to migrate across and swim and hang out in that normally undesirable tank, and it was only the ones injected with the acid, and not the ones injected with the saline. I find that a pretty convincing demonstration of pain in fishes.