Is the plural of "fish" "fish" or "fishes" — it depends.
The noun "fish" has two different, completely acceptable plurals—"fish" and "fishes"—but "fish" is by far the most common plural. It's what you usually use to refer to a group or collection of fish. For example, if Squiggly brought home a big bag of goldfish from the pet store, Aardvark might ask, "Do you have a bowl for those fish? Do you have food for those fish? What were you thinking buying all those fish?"
"Fishes" tends to be used in more specialized areas and in some well-known sayings.
Scientists use 'fishes'
For example, scientists who study fish (they're called "ichthyologists")— "Ichthys" is Greek for "fish" — often refer to different species as fishes. A quick Google Books search returns a bunch of scientific tomes talking about the different kinds of fishes that researchers have found in various regions, such as thirty kinds of fishes off the coast of California and 149 kinds of fishes in Minnesota.
The majority of quotations using the word "fishes" in the Corpus of Contemporary American English are also from scientific publications, like this one from The Fisheries Blog at blogspot.com: "Some deep living fishes like the orange roughy don't reach maturity until nearly 30 and can live to 125 years." Those are some old fish!
Aside from scientists, people also use the word "fishes" in certain sayings.
'Sleep with the fishes'
The movie "The Godfather" popularized the saying that someone who has been killed and dumped in the ocean "sleeps with the fishes" when the Corleone mob family wondered why they couldn't reach their enforcer, Luca Brasi. While they were wondering, a package arrived with fish wrapped in brown paper and a bullet-proof vest, and the character Peter Clemenza explains, "It's a Sicilian message. It means Luca Brasi sleeps with the fishes."
'Loaves and fishes'
Moving to a less gruesome saying, in the Bible, one of Jesus' miracles is to feed thousands of followers who had gathered to hear him in a remote location with a small number of loaves and fishes.
"Loaves and fishes" is how I learned it, but when I checked the reference, I did find that the New International Version of the Bible had changed it to simply "fish." The line in Matthew 14:18 reads, "He directed the people to sit down on the grass. Taking the five loaves and the two fish and looking up to heaven, he gave thanks and broke the loaves." But I still saw it referred to more generally in many places online as the "story of the loaves and fishes."
'If wishes were fishes'
The final common saying begins with "If wishes were fishes," which works because of the rhyme. Today you can find multiple endings, but it seems that Frank Herbert, author the book "Dune," was probably the first person to write it down, and he phrased it "If wishes were fishes, we’d all cast nets."
The origin of 'fish' as a noun and verb
It's a very old word, going back to Old English, and it was a noun before it was a verb. In other words, we talked about fish before we used the word "fish" to describe catching them. Verbing nouns goes way back; it's a common way to get new words in English.
'A lot of other fish in the sea'
If all this fish talk is making you think about the time you told a friend who had just been dumped that she should cheer up and move on because "there are a lot of other fish in the sea," it looks like that use of fish — refer to a desirous person you want to catch as a fish — was first used in 1723 by Daniel Defoe, according to the Oxford English Dictionary.
It was a little later that people started to refer to others as "fish" in a less than desirous way. For example, in 1790, Benjamin Franklin, in his autobiography, referred to another man as an "odd fish."
Finally, there are so many "fish" compounds in the Oxford English Dictionary "fish" entry that I stopped counting at 100. You really get a sense of how important fishing is or has been to daily life for English speaking people: "Fish-blooded," "fish-farming," "fish stick," "fish-way," it goes on and on.
To sum up, your Quick and Dirty Tip is that the plural of "fish" is "fish," unless you’re writing about a group of different species of fish, are a mobster, are talking about a Bible story, or are trying to make a rhyme.
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