Otters are appealing creatures and some species are critically endangered. The most common otter in North America, the North American River Otter (Lontra canadensis), is no longer in serious danger but its population plummeted in the past and protection measures were introduced in some states. If you find otters in your pond, you might see them as an asset. However, otters eat fish and a committed koi or other fish enthusiast might be less than pleased to see them. They aren't welcome in angling or aquaculture ponds either. To keep them from eating most of your fish, you need a physical barrier.
Provide places for the fish to hide from predators first. Crannies formed with rocks and a lot of aquatic plants make it much harder for otters to catch their prey.
Measure the circumference of your pond. Add a couple of feet to this measurement.
Buy 3-inch stiff wire mesh to the length calculated above.
Bend into the shape of your pond and connect the ends by weaving the wire through them.
Place the fence around your pond and secure in place by hammering in the tent pegs
If the otters are persistent, you may need to install an electric fence instead.