A Man’s Quest to Save the Most Colossal Fishes on Earth

The word "fishes" is commonly used to refer to multiple species of fish instead of multiple individuals

http://grammarist.com/usage/fish-fishes/:

"In biology, for instance, fishes is used to refer to multiple species of fish. For example, if you say you saw four fish when scuba diving, that means you saw four individual fish, but if you say you saw four fishes, we might infer that you saw an undetermined number of fish of four different species."