@Micecream420They can only be caught in Sulani. But I get around it by only keeping the endangered fish if nobody is around. My Sims often fish overnight when it's more deserted.
Edit: to mitigate the negative effects on reputation I only keep 1 and catch and release several. Throwing a lot of them back boosts reputation and makes up for any losses.
Actually they can be stocked in fishing ponds if you manage to catch one! They'll still be considered endangered even if you stock multiple fishing ponds with them! Personally I think it should only effect you in Sulani, but not if they are caught in fishing ponds or they should add the option to breed them to increase their numbers to improve your reputation maybe as part of the Conservation career or a related activity!
I also wonder if they'll stop if you improve your reputation (like using the PR Agency to publish a good story)!
