While there are many fish words that are pluralized by adding an -s (sharks, minnows, guppies, etc.), there are also many which do not inflect in the plural (trout, cod, bass, salmon, etc.)
This contrasts strongly with the other animal classes. A few mammal words do not inflect (e.g., sheep, moose, deer), but the vast majority do. And I am not aware of the non-inflection of bird, insect, reptile, or amphibian words (although there may be some):
? a sky full of crow
? a field full of butterfly
? a pit full of snake
? a stream full of newt
There are some questions about the pluralization of fish elsewhere on this site, (https://english.stackexchange.com/search?q=fish+plural) but none that I can find answers my question:
Why is it typically only fish words that do not inflect in the plural?