Koi fish invaded my pond?

I have a 1/2 acre natural pond, fed by a creek. I just purchased this property in December '18, and the seller said that his brother-in-law gave him some koi which he threw in the pond. He also said that their was catfish, bass and bluegill. I have about 30 ducks and coots on any given day and egrets frequently stop by to fish. There's a ton of frogs so they could be interested in that as well. I've also seen some turtle families here. I've tried fishing the pond and I've pulled up nothing. Also, the water is constantly merky with almost no visibility. My friend suggested that the koi have reverted to carpe, and that they've eaten the other fish and are rooting around the pond heavily.

If carpe have taken over my natural pond, what can I do about it? With permaculture principles in mind I'd rather not have someone come in and shock the pond and pull out a bunch of dead fish. Not to mention that would be highly inconsiderate to my neighbors who are fed from the same stream.

Is there a trick out there to find out if carp have indeed taken over?

Thanks all!
Byron

Perhaps a fish trap of some sort would work.  You could leave it in place for a day or so and then check it.  If there's anything in the pond, you're bound to catch at least a little one.  Just make the entrance small enough that a turtle can't fit into it.  If one gets in, it'll drown if it can't get back out in time for a breath.  Plus, it'll likely eat any fish that you do catch.

Koi are a Carp and they will root around, it would take a lot of Koi to murk up a 1/2 acre pond. They will eat fish eggs and frog eggs but they won't catch and eat many fish or frogs. True Koi will retain their color and not revert to common Carp.
I don't know where you are, but many ponds are murky in the Spring with runoff and turnover. With a stream fed pond there may be more types of fish involved than what you've mentioned. Ask the previous owner if the pond is usually murky this time of year. Catfish, Bass, Bluegill, and Carp may be more inclined to feed (take bait or lures) when the water has warmed up more. Many types of ducks will stir up sediment in a pond, especially the shallower areas. Take some time to figure out your pond before taking any remedial action.

Great response. I appreciate the insight. I suppose I should yield to patience and observation. Through the cold seasons the pond was completely covered by duck weed. We got some heavy rains this year in San Diego, and it washed away. Whatever was left fed the ducks.

We had a friend who would feed his fish dry dog food.  He threw out a hand full and the fish would come to the surface to eat it.  Maybe this would be worth a try?

That's exactly what a friend had suggested. I think I'll start doing that this afternoon. Hopefully, before too long they'll start showing up. Hopefully, the ducks don't get it all before the fish do.