Perhaps that's what they thought ...
Not true though - certainly not in British English, and not according to the dictionary. Plenty of fishes all around this island, and in the textbooks of Oxford and Cambridge :-)
British English will generally use 'fish' for a large (uncounted) number, but will often use 'fishes' for small and easily counted numbers, as (very probably) in this example.
In the story of the loaves and fishes, for example, the KJV (King James Version) of the bible uses "fishes" repeatedly.