Fish do indeed feel pain from hooks.
Although it was once thought that fish didn't feel pain from hooks new research has suggested that fish are able to feel pain.
So when you hook a fish or remove the hook from the fish it can feel pain and it can look in distress.
They just can't scream or holler or tell us they are feeling pain but fish are able to feel the pain.
Fish do feel pain. It’s likely different from what humans feel, but it is still a kind of pain.”
At the anatomical level, fish have neurons known as nociceptors, which detect potential harm, such as high temperatures, intense pressure, and caustic chemicals.
Fish produce the same opioids—the body’s innate painkillers—that mammals do.
And their brain activity during injury is analogous to that in terrestrial vertebrates: sticking a pin into goldfish or rainbow trout, just behind their gills, stimulates nociceptors and a cascade of electrical activity that surges toward brain regions essential for conscious sensory perceptions (such as the cerebellum, tectum, and telencephalon), not just the hindbrain and brainstem, which are responsible for reflexes and impulses.
Fish also behave in ways that indicate they consciously experience pain.
In one study, researchers dropped clusters of brightly colored Lego blocks into tanks containing rainbow trout.
Trout typically avoid an unfamiliar object suddenly introduced to their environment in case it’s dangerous.
But when scientists gave the rainbow trout a painful injection of acetic acid, they were much less likely to exhibit these defensive behaviors, presumably because they were distracted by their own suffering.