IMO, it should be added as a separate precept. Maybe also add a precept for animal products like eggs and milk. So your pawns can be pescetarians, vegetarians, or vegans.

As a real life vegetarian who hates fish I am ok with my Naturalists murdering some digital river demons. I like this for two reasons: 1. It still treats fish as meat so if I want to go vegetarian they won't eat it. 2. They can fish their hearts away without getting mood debuffs from killing or butchering animals. This is important because I use the meat to make kibble and pemmican early on. Otherwise it would be very hard to have any non-veggie eating pets early to mid game.

Well, for consistency they should have a mood debuff for killing fish (a new precept basically).
The solution for having carnivorous pets is to have a stock of veg animals that can prey upon... ethical? well, at least it's natural

I agree with you OP.