True. I don't know if dolphins are that fussy. :-) As you said, the English plural of fish is usually fish and fishes refers to several different species of fish. I think you have a good point. Try and report it to see if it gets picked up as an acceptable answer.
For the benefit of others: http://grammarist.com/usage/fish-fishes/
des is not a partitive article - it is the plural indefinite article which does not exist in English. un / une = "a / an" while des = "some". French nouns, as a general rule, require and article → mange des poissons however the English translation does not → it is simply eats fish.