Fish Irregular Plural Forms

Solution 1:

I expect an answer to your question will be difficult to come by. Many fish names form regular plurals (Bluegills, guppies, sardines), and many of the irregular plurals are fairly modern usages, so someone will have to account for the regular plural disappearing.

As You Like It, Act II, Scene IV. Touchstone says:

... and I remember the wooing of a peascod instead of her, from whom I took two cods and, giving her them again...

From The Sportsman's Dictionary: Or The Gentleman's Companion: for Town and Country. (1800) in a review of various rivers, one in particular:

well stored with gudgeons, dace, flounders, perch, pike, and some carp and trouts.

The American Fisheries Society has helpfully compiled a list of the proper plurals of fish names in A Guide to AFS Publication Style. Go here and look for Appendix C.

Solution 2:

As I mentioned late last month in answer to the identical question, I have long suspected that with a few exceptions, species of fish (and sometimes fowl) destined for human consumption are often treated as mass nouns in Modern English, mostly because they are seen as commodities like flour or sugar.

bass, halibut, herring, carp, perch, trout, cod, salmon, tuna, pike, mackerel, flounder

One sardine won't make a meal even for a kitten, so it's hardly a surprise sardine has a plural, but krill are so small they don't even rate a singular.

Tilapia is a species relatively recent to the seafood counter, likely the reason the style guide of the American Fisheries Society (scroll down to the appendix) lists it among those that can form an s-plural or not.

Most crustaceans form s-plurals even though delicious — lobsters, scallops, clams, mussels — but while crab and shrimp may be plural crustaceans, i.e., doing their natural thing in the ocean, they are almost always mass noun shellfish, i.e., playing a crucial role in gumbo.

Any fish whose name is a compound ending in -fish stays -fish in the plural whether you eat it or not.

The men in my family of my father's generation enjoyed dove and quail hunting, so these two birds were always mass nouns at home, but I noticed that classmates who never ate them would use s-plurals. My brother's friends, at least those who got BB guns for Christmas, hunted squirrel, though since the Depression I don't think they landed on anyone's table. When one heard from old-timers that back then armadillos were called "Hoover hogs," then the question arose, "Can you eat armadillo?" Ask or answer the question often enough in the affirmative, and I can easily see how this mass noun usage could in some cases transfer to any mention of the creature in question.