Both my ponds have been stripped multiple times by river otters. Discouraging, to say the least. There is a city of Fayetteville, Arkansas, owned protected otter haven, called Lake Sequoyah, just a few miles downstream from my ponds, plus the ponds are only about 1/4 mile from the White River, which feeds that lake. Not much I can do. I cannot keep any large fish, especially CC, because of the otter problem. Other neighbors have the same problem. The otters have a huge daily range.
For the above reason, I have not been very active on the threads. To make an otter fence around the ponds would be prohibitively expensive, plus it would interfere greatly with mowing, making a lot more maintenance necessary. I have decided that what will be, will be.
Call the local DNR officer and explain the problems you are having and the amount of $$$ that it is costing you. Ask them if there is a trapper in the area that could do nuisance trapping for you.
A fence won't be that expensive, here's a list:
Solar 2 mile fence charger Tractor Supply $139.00
Poly wire @ TSC 1312 feet $29.99
Grounding kit @ Lowes $39.99
Fiberglass Posts @ TSC $1.69 each.
Run two wire around the pond. one 6" above the ground, another 12" above the ground. That way you can weed whack under the lowest wire easily and that will keep the otters out. It will also be low enough that you can easily step over it to get to the pond. If you want to make the pond more easily accessible, spend a little bit more for the electric fence gate hardware and you can make a gate that you can open and close, or put a disconnect near the solar charger to turn the juice off easily.
I'm guessing for less than $300 you could protect your pond.