I had cause last night to consider the best way to pluralize the possessive form of “fish.” It’s tricky because “fish” is both the singular and the plural, and since our apostrophe rule for plurals is that if there’s an irregular plural form, you add the apostrophe-s as you would for the singular, the possessive for the singular and the plural forms both for “fish” would be “fish’s” (just as for “child” it’s “children’s”). Well that leaves room for ambiguity, doesn’t it? Consider this sentence:
The fish’s lips were beautiful.
Are we talking about one fish here or two (red fish? blue?)?
Of course, sometimes we see the plural form “fishes,” as when we’re talking about loaves and fishes. But it turns out that in current, non-idiomatic English usage, it’s not the preferred form. Usage expert Bryan A. Garner explains it all nicely in his Garner’s Modern American Usage, which is one of my favorite books (not merely one of my favorite reference books):
The Evanses wrote in 1957 that the plural fish is of recent vintage and opined that “the life expectancy of a new irregular plural, such as fish, is not very long.” But the OED cites fish as the plural form as long ago as 1300. Today, fish is the firmly established plural. Fishes appears rarely, at least outside ichthyology. When it does appear, it usually refers to more than one species.
Fish does take the regular -es ending to form the plural possessive — e.g.: “A Yozuri Crystal Minnow seems to be the fishes’ preference.” David Sikes, “Island Time,” Corpus Christi Caller-Times, 4 Aug. 2002, at B14. And the plural form fishes persists in idioms such as The Godfather‘s “Luca Brazi sleeps with the fishes,” as well as the biblical allusion “loaves and fishes.”