I have no question as to whether animals feel pain, but I do question whether fish experience pain on any level comparable to mammals. Fish lack the very nerves that carry pain in mammals and completely lack the part of the brain that registers mammalian pain. (Yes, they have nociceptors, I get that.)

Read a great article on the subject where scientists gathered from around the globe, like a Michael Crichton novel, to try and answer the question, as it was relevant to a new law in Germany.

They did not conclude, one way or the other, but one item caught my attention.

Ever kept fish? They will swim around, fight, fuck and eat, even with the most grievous of injuries. Mammals OTOH display every symptom of depression. They don't move around, eat, pursue sex, etc.

suffering of animals is measurable in a quantifiable way

Was hoping to see how they measured that. Amoeba react to negative stimuli, all life does so, or it won't be alive for long.

These estimates are based on a comprehensive review of existing research to assess the intensity and duration of pain and distress experienced by the fish.

And I learned nothing.