‘[f]ish’ means a wild fish, mollusk, crustacean, invertebrate,
amphibian, or part, spawn, or ovum of any of those animals.”
It's just a technicality. A statute was written using the term "fish" in a bunch of places, and rather than replace "fish" with "fish, mollusk, invertebrate, &c" in every place where that term was used, they just changed the definition. Every statute you'll ever read has explicit definitions, presumably for this exact reason.Fun fact: for a time, according to Dave Arnold, the Catholic church defined beavers as meat, but beaver tails as fish. You could eat the tail on a Friday, but not the rest of the beaver.