Many states have spent significant resources trying to reintroduce river otters to areas in which they've been historically extirpated. Prior to owning a pond and managing fisheries, I would have probably reacted to trapping otters in a negative way. However, since I've educated myself on the topic and researched their negative impact on fisheries, I can't make ethical judgments based on my ignorance of the topic. Otters are intelligent and I think are very cool animals, however, once they find a fishery they simply have to be dealt with. If they can be live trapped and relocated, I'd recommend considering that first. State laws may treat killing otters rudely without a permit. Bottom line - you're likely going to feel the impact of their presence on your fishery and will likely need to address the situation.