Japanese for Busy People 1 (lesson 11) mentions when talking about [キス]{kisu} (whiting) at a [てんぷらや]{tempura-ya}:
NOTE: Names of fish, fruits and vegetables etc. are sometimes written in katakana.
Does using katakana only apply when talking about the animal, as opposed to talking about the food? (For an English equivalent, people refer to the animal as "shark" but sometimes refer to the food as "flake")
Assume that it's not gairaigo and therefore would be in katakana anyway.