The other night, a friend was over and browsing the books on my shelf. When she came across one of my textbooks, titled 'The Diversity of Fishes', she laughed and mentioned that one of her professors would cringe at seeing the word "fishes". This comment kind of annoyed the biologist in me (more specifically, the marine scientist in me). I understand that people who are not familiar with the use of the word (which I think would constitute most people) would consider it poor grammar or a grammatical error. But trust me, when used in the right context, "fishes" really is a word! Note the following Wikipedia article:
Note on usage: "fish" vs. "fishes"
"Fishes" is the proper English plural form of "fish" that biologists use when speaking about two or more fish species, as in "There are over 25,000 fishes in the world" (meaning that there are over 25,000 fish species in the world). When speaking of two or more individual fish organisms, then the word "fish" is used, as in "There are several million fish of the species Gadus morhua" (meaning that G. morhua comprises several million individuals). To see both in action, consider the statement "There are twelve fish in this aquarium, representing five fishes" (meaning that the aquarium contains twelve individuals, some of the same species and some of different species, for a total of five species). The usage of the two words is similar to that of the words "people" and "peoples".