Predator can do a lot of damage

One of my friend's Tilapia broodstock  replacement ponds was nearly wiped out by one Mink. It got into the pond shortly after he fed the fish in late afternoon. It chased them around overnight and stressed them to the point of killing them.  Fish with full stomachs can't handle stress. You can see the Mink at the pond wall between the two stalks of Green Foxtail.  

Argh! That's nasty.

I know people who developed a nice pond, worked hard to clarify the water, stocked it with trout. For quite a number of years all was lovely. Then a mink, we think, found its way up from a natural creek. Wiped out the entire population. Attempts to restock with fish, after a year's hiatus, ended the same way. Apparently mink, like bears, never forget a food source. Dammit.

The only sure way to be safe from mink, raccoon, otter etc. is a true pond (no inlets or outlets) enclosed in electric netting.

To protect from flying predation some sort of barrier such as netting or strands of fishing line; OR ensure there is lots of coverage within the pond, be it plants, sunken tubes (clay or plastic piping) or some other form of 'hides' so they can safely hide when airborne predators are about.

I am sorry to hear about your friend's loss.

At one time or another, each of us finds out that wildlife will be wildlife and do what comes naturally for wildlife.

Like Lorinne said, electric netting will help.