10. Nouns with identical singular and plural forms

aircraft, hovercraft, spacecraft, and other “-craft” vehicles
Ex.
There are two aircraft prepared for landing.

bison: The bison were grazing in the distance.

cod: The cod are known to migrate in large numbers.

deer: Deer are easy prey for wolves.

fish: Three fish swim in the fish tank.

However "fishes" is the proper English plural form of fish that biologists use when speaking about two or more fish species, as in "There are over 25,000 fishes in the world" (meaning that there are over 25,000 fish species in the world). When speaking of many fish that all are part of the same species, then the word "fish" is used, as in "There are several million fish in the species Gadus morhua."

moose: Moose actually belong to the deer family.

offspring: The fox gave birth to five offspring.

pike: The pike are big freshwater predatory fish.

salmon: Salmon are often seen jumping over dangerous waterfalls.

sheep: One sheep, two sheep, three sheep…

shrimp: Shrimp are among the most commonly eaten animals.

swine: Swine are reared extensively in Europe.

trout: The trout are fish related to the salmon.



www.antimoon.com/forum/2004/5588.htm
jakubmarian.com/nouns-with-identical-singular-and-plural-forms/