Fish and fishes

Why do I see the word ''fishes'' written in biology books and fishing books. Isn't the plural ''fish''.

The plural for fish of the same species is "fish", but the plural for fish of more than one species is "fishes". For example, two Barramundi are two fish, but one Barramundi and one Australian Bass are two fishes.

Source:
http://www.amonline.net.au/webinabox/fish/about/introduction.htm