I'm planning on turning a ravine into a 6-8 foot deep 1/4 to 1/3 acre-sized pond, and I'd like it to serve many purposes in the permaculture design. I'd like it to reflect sunlight towards the food forest on a hill adjacent to it, and keep the water table high. I'd like to grow aquatic plants like Sweet Flag and Wapato. And I'd like to stock it with a functional mini-ecosystem of snails, crayfish, frogs, minnows, bluegills, and finally some carp. And because everybody knows they're great for slug control, I'd like to keep some ducks as well, giving them somewhere to forage for the majority of the year when they aren't needed for slug patrols, and then harvesting a few for meat in the autumn.
Here's what I need to figure out, and I'd love to hear from those with firsthand knowledge:
-Ducks are well known for their ability to fill up a pond with feces. How might their presence ruin things for the minnows, bluegills, and the carp? Would they benefit from each others presence, or would the fish simply die out in murky water?
-What permie-friendly, closed-loop options are available to keep the water clean? I've heard barley straw is used to kill pond algae, are there any plant-based methods for dealing with the duck muck? I know it's supposed to make excellent fertilizer, what would you use to collect it? Are there any good hand skimming tools you use every couple days to collect it, or is that just an exercise in futility? I'd hate to simply have to pump the water and keep new water flowing in, that's very un-permie, and I'd mostly like to keep the pond full by collecting snowmelt and rain runoff via swales.
-For a 1/4 or 1/3 acre pond, how many adult ducks would make for a stable population in balance with a fish population? How many ducklings? I plan on establishing a rich and diverse polyculture of aquatic plants around the edges and growing in the pond itself, and I'd like it to be enough to sustain them without them destroying everything. The problem is unlike rotational chicken paddocks, you can't really subdivide a pond.
-I plan on letting the carp reach a good size before getting any ducks. When it comes to the subsequent generations, however, will the ducks pretty much massacre them, or will they let enough of the fry grow to appreciable size?
In my opinion for a pond that size. ONE carp will be to much. A carp will eat every plant in your pond faster than it can regrow. Also your carp and ducks will eat a majority of the same thing and would compete for food.
I would go with bream, bluegill, or perch whatever you want to call them. They breed like crazy, the fry will eat mysquitos and any other bugs. And a plus side is you can eat them. Grass carp on the other hand are a rather poor quality food.
If I were building a pond 12' or deeper I would say go with crappie and shiner minnows. I don't know of any freshwater fish that could beat the quality of a crappie filet. You would just have to have a minnow area in the pond. A shallow area and areas with rather thick brush to keep out the bigger fish.
I wouldn't put a carp in a pond unless it's been completely over run with aquatic plants. Milfoil, hydrila, duckweed, or whatever. Then I would only leave the carp in there until it has cleared out the majority of the vegetation. Then shoot the carp and only put another one in as needed. Around here in Alabama the fish truck comes to the farmers co-op twice a year and you can get a good sized 12-14" grass carp for around $3.00.
A great resource for all things related to ponds and aquaculture is forums.pondboss.com I wouldnt do anything until you get the consensus blessings of the regular contributors over there. Very friendly forum, very knowledgable contributors.
I think that ducks and carp would pretty much ensure than your pond is barren. Carp they are voracious consumers of all vegetation, and dificult to get rid of once introduced. Ducks will dabble to death all the shorline plants, particularly if you have any significant numbers of them. I had 30 ducks in a 100 yard long section of creek, and they removed every scrap of vegetation, down to the roots, over a summer. It has taken 3 years to recover. In concert, ducks and carp wouldnt fight or even excessivley compete with one another, but they would be one serious team of vegetation destruction.
There are two major considerations to putting a pond in a ravine. First, if the ravine is considered a watercourse of any type by the Dept of Making you Sad, you will need permits through Army Corp of Engineers. Not easy. Second, if you aviod problem number one, the big issue will be runoff and dam damage. I have a 'dry' ravine on my property that occasionally flows a trickle of water in the spring. But about once a decade, following heavy summer thunderstorms, the ravine floods catastrophically. Previous owners tried to have a pond in the ravine, but the flooding made quick work of that. Dont underestimate the power of flooding, it is nearly impossible to mitigate on a small scale.
Lots of planning to consider, let us know how your research progresses. good luck!