With statutes, it can be dangerous to rely on plain meaning. Most people, for example, would not consider an amphibian, such as a frog, or a mollusk, such as a snail, to be fish. However, the California Fish & Game Code considers these and crustaceans (including crabs, lobsters, and barnacles) to be fish. Oddly, however, the Code doesn't consider koi, guppies and goldfish to be fish (unless, perhaps, they are particularly unruly):
“Fish” means wild fish, mollusks, crustaceans, invertebrates, or amphibians, including any part, spawn, or ova thereof.
Cal. Fish & Game Code § 45.
In contrast, the Food & Agricultural Code doesn't treat all fish as "fish":
“Fish” means every form of fish, either salt water or fresh water fish, which is used for human consumption, except fish for canning, retorting, reduction, or use in an extraction process.
This definition presents its own problems. Did the actions of three mischievous Harvard undergraduates in 1939 have the unintended consequence of converting goldfish into "fish"? When the craze subsided, did goldfish cease to be "fish"?
Note that the Corporations Code also deals with fish. Part 3, Division 3, Title 1 of the Corporations Code is the "Fish Marketing Act". Cal. Corp. Code § 13200. The purpose of the Fish Marketing Act is "to promote, foster, and encourage the intelligent and orderly marketing of fish and fishery products through cooperation; to eliminate speculation and waste; to make the distribution of fish and fishery products between producer and consumer as direct as can be efficiently done; and to stabilize the marketing of fish and fishery products." Cal. Corp. Code § 13201. The Fish Marketing Act at least seems to recognize that crabs and snails are not "fish". Cal. Corp. Code § 13202(a) ("'Fishery products' includes fish, crustaceans, mollusks, and marine products for human consumption.") However, the reference to "marine products fit for human consumption" suggests that edible seaweeds such as nori (porphyra) are fish. Surprisingly, fish marketing associations are not subject to the Corporate Securities Law of 1968. Cal. Corp. Code § 13208.
Orthographic note: All California code references to mollusks spell the term with a "k" rather than a "c". The word itself has Latin and Greek antecedents. The Romans named a small nut, mollusca, referring to its soft center (mollis means soft). This suggests that mollusc is the correct spelling. The Greek word, μαλακία, which refers to one member of the mollusk family - the octopus. This suggests that mollusk is correct. According to the Corpus of Contemporary American English, mollusk is the much more prevalent spelling.