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A group of fish of the same species are called 'fish'. Two or more species of fish are called 'fishes'.

For example, a number of Eastern Australian Salmon swimming together can be called a school of fish. But if one Tarwhine starts swimming with the Eastern Australian Salmon they are called fishes.

<i>Arripis trutta</i>
Part of a school of several hundred Eastern Australian Salmon at a depth of 5m, Shellharbour, Bass Point, New South Wales, March 2004. Image: Sascha Schulz
© Sascha Schulz

Books such as the Sea Fishes of Southern Australia and Coastal Fishes of South-Eastern Australia use the term 'fishes', because more than one species of fish are included in the book.

If you don't believe me, believe Shakespeare. In Pericles Act 1 scene 1 the Third Fisherman says "Master, I marvel how the fishes live in the sea".

For more information, view the Wikipedia comment on Fish or Fishes?