Can you seine your pond to see what fish you have left? Can you seine the pond and move the fish to a smaller area you might be able to manage?
There are otter repellents you can purchase and place around your property line. Are you able to light off fireworks? Roman candles and bottle rockets move many animals along.
Some folks report success with fencing. I would light that fence up nice so a sharp zap would be enjoyed by all who touch it.
A couple of well placed trail cameras might show you how many and where you need to concentrate your efforts.
Oh, and most cities if they forbid trapping allow landowners to hire a trapper who has permits to trap the pests. This could be an option for you, too.
