They're not taxonomically fish (even if you want a monophyletic category & thus count mammals as fish), they're not colloquially fish, they're legally fish under California Fish & Game Code § 45, but they're not necessarily legally fish in other jurisdictions or other subsections of the California Fish & Game Code. Because laws also define words to create jargon, and thereby new meanings dissociated from their common use. The extra fun bit is that other parts of the law can choose to define "fish" differently, for other purposes. Jargon has scope, and allows overloading.