Pond Covers or Pond Netting?
We have 2 large ponds in the gardens surrounding the Kitchen Garden and have also had problems with otters in the past.
Our ponds are now fully netted to prevent any invader from stealing the fish. We use heavy duty pond netting on a raised steel pond cover and find this very effective. This would avoid the use of galvanised netting and any worries that it may be harmful to your fish. We have also had success catching otters using humane rabbit traps set with a tin of pilchards as bait.
I would suggest that the netting would be your best long term option, but it might be worth trying to catch your current visitor as well.
Good luck. Please let me know if I can be of any further help.