The plural of fish is usually fish, but fishes has a few uses. In biology, for instance, fishes is used to refer to multiple species of fish. For example, if you say you saw four fish when scuba diving, that means you saw four individual fish, but if you say you saw four fishes, we might infer that you saw an undetermined number of fish of four different species.
Fishes, with an apostrophe, also serves as the plural possessive of fish—for example, the fishes’ scales were yellow. And of course fishes is the present-progressive verb (e.g., she fishes in the river).
Fishes also appears in the cinematic gangster idiom sleep with the fishes, used to indicate that someone has been whacked (and perhaps given a water burial).
These fish emit distress signals that are picked up on by the mulloway. [Fishing World]
The fish are then sold as salted dry or canned. [Food Science, Sari Edelstein]
No fish were registered on Upriver Lakes, where the season has now closed after the 90 percent harvest cap was reached on Sunday. [Fon du Lac Reporter]
Those most in jeopardy were the smaller fishes with specialised eating and sheltering habits. [U.S. News & World Report]
Chewing aided chemical digestion and improvedthe fishes‘ ability to extract nutrients from their prey. [Life: The Science of Biology, David Sadava, Gordon H. Orians, H. Craig Heller]
He fishes public lakes, many of them smaller city watersheds that most anglers drive past on the way to more popular water. [Norwich Bulletin]