Singular and Plural Nouns

Names of certain creatures do not change in the plural. fish is normally unchanged, fishes exists but is uncommon.

Some types of fish do not normally change in the plural: carp, pike, salmon, trout,cod, plaice, squid, turbot,mackerel

but if used in a plural sense they would take a plural verb. Others add s: crabs herrings sardines eels lobsters sharks

Deer and sheep do not change: one sheep, two sheep.

Sportsmen who shoot duck, partridge, pheasant etc. use the same form for singular and plural.

But other people normally add s for the plural: ducks, partridges, pheasants.

A few other words do not change: aircraft, craft (boat/boats) quid (slang for £1} counsel (barristers working in court)