The debate about fishes’ ability to perceive pain has arisen at various times in past decades, generally prompted by public concerns about angling practices, industrialised fishing, and, more recently, intensive salmon farming.
Advances in neuroscience and behavioural techniques have led to a better understanding of the sophistication of the senses and central nervous system of such lower vertebrates, and there is now more widespread acceptance that fish do feel pain.
Interestingly, jurisdictions following Westminster principles of law have recognised that fish feel pain. In the 19th century, for example, through the 1876 Cruelty to Animals Act, government included fish in the statutory regulation of animal experiments, and then again in the updated legislation of 1986 (later extended to include the octopus and related species).
Since 2005 European legislation has offered some protection to farmed fish, and UK's Defra proposals in response to Brexit may yet define and extend the concept of “sentience” to fish and other animal groups, including certain advanced invertebrates.