You may not ever see them, but there does exist a happy population of river otters here in our area of New Jersey. They are clever and cautious creatures, and are quite adept at staying hidden. Otters avoid contact with people, and quickly dive out of sight when they see us, but they aren't shy about leaving behind signs of their presence if you know what to look for.
In recent years, we've heard anecdotes of otter sightings becoming much more frequent in places where they hadn't been seen before. We've also been seeing much greater numbers of Alewife Floater Mussels and Asian Clams in our local waters, especially in waters that connect to branches of the Rancocas and Mullica Rivers. It is possible that the growth of this food supply is what is allowing the otters to expand their territories and spread into areas where we are more likely to encounter each other.
If you would like to see an otter in person here in South Jersey, the best time to go is right at dusk as they embark on their nightly hunts. Once you've scouted an area that has the telltale signs of otter activity, find a position where you can comfortably sit very still and wait for them to swim along. You'll see a wake that looks like it's made by a large fish, and it may or may not have its little face above the water, depending if it's looking for food underwater, or watching for danger above the surface. They will dive under and hide there for several minutes if they see you; it may not be an easy task to see one in person. If the search to see an otter becomes too much trouble, you can use a well-placed game camera aimed at the waterline where you find the empty mussel shells. This can allow you to get a glimpse into their lives without intruding.
Fun Fact: River otters in New Jersey are 75% cuter than their relatives elsewhere!
Electric fencing seems to be the favored precaution to defend a koi pond against determined otters. More typical predation-avoidance measures, such as small garden fencing, fishing wire, predator decoys, and motion-activated noise deterrents or sprinklers, are not reported to be very effective against a determined otter. These are extremely clever and cautious animals with a reputation for being elusive and difficult to trap. They can climb a fence, dig under walls, and dodge the trip lines that befuddle avian predators. The steep sides of a pond that would discourage a raccoon or cat from fishing for koi mean absolutely nothing to the otter. The fish caves we build into every fish pond that provide such valuable security for pond fish only serve to corral the otters' meal into a tighter area. Contending with such a formidable adversary tends to be either frustrating or expensive. After all, who actually wants an electric fence in their yard obscuring the view of their lovely pond? It's just a better compromise when compared to losing cherished koi.
There are other options that will offer some protection, and maybe that's all you need. Maybe you're pretty low on the risk scale, but you live a mile or two from the Mullica River and want to be on the safe side. A fenced-in yard will help block view of your pond and prevent otter from accidentally discovering your pond, and it will offer the added benefit of keeping away animals like fox and coyote. Another benefit of a fenced-in yard is the ability to have a dog outside. We know otters are afraid of coyotes, so it makes sense that they would avoid a place smelling of dog. Some people have even suggested using bear urine as a repellent; we have no research on the efficacy of that. It might keep wild animals from hunting your koi, or you might just have a yard smelling like a bear den! Decoys of larger predators such as bobcat, coyote, fox, or eagle are going to scare off an otter or mink for a while, probably. You'll want to move the decoy around the yard to keep up the illusion. Decoys are most successful when you have limited the vantage point. If they can see the decoy from any angle they're more likely to test it, but if they are peeking under a fence and see what could be something that wants to eat them standing there, they're more likely to just move along.
Motion-activated noise-makers or sprayers are fairly effective against heron and cats, so they may work on otter, too. There isn't enough data for us to say exactly how effective they'll be, it depends how familiar the otter are with the property, and how strongly are they attracted to the pond. If the smell of fish is strong and it smells to them like the best meal ever, they'll be more determined to test things out and take risks. I wouldn't rely on a sprinkler as my main line of defense, but it's an relatively noninvasive precaution that can offer additional peace of mind.
The strongest piece of advice I can provide with regards to otter and koi ponds is to know the animals you share space with, and to adapt accordingly. Most of us will never see an otter in the wild, let alone in our backyard, and unless you're within the fringes of otter habitat, you don't have to take any special precautions on behalf of your koi. This article is geared towards people who live very close to streams and rivers. If there's a stream, branch or tributary leading to a major river having known otter activity within 500' of your pond, then it would be wise to safeguard your pond against otters. If you know you have these creatures in your neighborhood and are thinking of building a fish pond, there's a good chance your fish will be at risk. Keep that in mind when designing and building your pond, or eventually we may be asking you for advice on how to keep koi safe from otters!