Some cats can get away with eating fish, others can't. I don't know anyone who would want to be in the percentage who feed their cats fish and UTIs and blockages are the result. Fish, it doesn't matter if it's fresh or canned , for human consumption or in pet food, can cause some cats to block in less than 24 hours. I guess that's why in nature, it's a rare cat who can catch a fish. Birds are the equivalent of chicken, and rodents are the equivalent of beef but fish are in water and cats have an aversion to water, maybe for good reason. It works to keep feline and fish that much farther apart.