Fish have feelings and intelligence on a par with other animals and deserve better consideration of their welfare, according to a behavioural biologist at Australia's Macquarie University.
Dr Culum Brown came to the conclusion after reviewing the scientific evidence on fish capabilities.
He found that fish have good memories, lived in social communities, co-operated, and learned from one another.
They displayed behaviours normally seen in primates and were even able to build complex structures and use tools.
While their brains differed from those of other vertebrates, they contained structures that performed similar functions seen in other animals.
There was also mounting evidence that they felt pain in the same way humans do.
Brown believes fish are just as likely to be sentient as other animals.
He wrote in the journal Animal Cognition: "Although scientists cannot provide a definitive answer on the level of consciousness for any non-human vertebrate, the extensive evidence of fish behavioural and cognitive sophistication and pain perception suggests that best practice would be to lend fish the same level of protection as any other vertebrate.
"We should therefore include fish in our 'moral circle' and afford them the protection they deserve."
People rarely thought about fish other than as food or pets, said Brown.
He pointed out that fish were second only to mice in terms of the numbers used in scientific experiments.
With more than 32,000 known species, fish far outweighed the diversity of all other vertebrates combined, he added.