Plural
Invariant nouns: The common names of animals is often used as both singular and plural. One can say "We shot grouse today" correctly no matter what number were killed. With "fish" there is a choice. The traditional English usage is that the word is used for both singular and plural, but American usage seems to prefer fishes as the plural. With groups of animals, one uses the singular, as in a herd of bison or a shoal of herring. However, if the animals are known as individuals then, for example, we feed the ducks, or stroke our cats. There are other cases where there is no plural at all, as with sheep, salmon, deer. Other general words which add no suffix in plural are aircraft and offspring. Some look like singular but are always plural, such as vermin, livestock, cattle, people.