I purposely keep my ponds to absolute minimal structure, mostly because it helps me with some management objectives, such as seining.
I have a good friend with a pond that has structure that's probably about 40% of his pond, and that's probably too much. The fishing is extremely difficult because the overall biomass of the pond is so spread out that it actually makes it harder to locate concentrations of fish. He certainly doesn't have any problems with predators of any kind. Also fishing is a pain in the rear, because you lose so much gear.
As frustrating as predators are, imagine how a fish farmer feels who has his livlihood going down the drain when an otter shows up. It would be the equivalent of a hail storm for a row crop farmer. Lots of money going down the drain fast. Honestly, the fish farmer is at an especially high risk becaue of his need to have as little structure as possible.
If you were a fish farmer it would almost HAVE to cross your mind to use any and all means neccessary to get rid of the critters. Fortunately I don't make my living with my fish, so it's not such a monumental deal for me.
I'll defer to those smarter than me, but I'd bet that the 20% is a reasonable number, depending of course on the needs of the particular species of fish you are managing for. Lots of cover for LMB, very little for HSB, etc.
