Crayfish In Koi Pond?

[SherLar]

   We have a fairly large koi pond- 6,500+ gallons (about 24,000L) -with several koi and a few small (12") catfish that can potentially grow large. I would like to throw some large, local crayfish into it it help eat the detritus that the fish won't eat. As it is, we do monthly water changes and sludge vacuums. I'd like to only have to clean it out 3 times per year: Spring, immediately after thaw; mid-summer; and fall, before freeze.
   Having never kept crayfish as pets before, my question is 2-fold. When the pond freezes (we use a stock tank heater to keep a small section open to allow gas to escape), the crayfish will want to hibernate; will they be able to dig through my pond lining, causing holes and therefore leaks, or is the few inches if sand at the bottom sufficient cover for them? Also, in a fairly well-planted pond, with numerous rocks and decor, will they be able to keep hidden from the catfish, and still be able to eat enough to do me any good?
   OK, 3 parts, sorry: how many would be needed to make any impact on the detritus, which turns to sludge with decay, in a pond my size (our local crayfish grow to about 6")?
 
 
 
 
 
 
 
 
 
 

 

[Baccus]

Depending on which species of crayfish you wanted to try really depends on if they are suitable to keep with any fish no matter the size of the fish. Many crayfish while being mostly vegan wont hesitate to start eating a fish they stumble across wether its alive or dead. Also some crayfish will not leave a plant alive in your tank/ pond.
As for them digging out through your pond liner, I think that will depend on the age and strength of the pond liner, to stop them digging you could lay tyres in the pond (many commerical crayfish farms use tyres exactly for this purpose), or coconuts cut in half or any other thing like PVC pipe that you can think of to give the crayfish shelter. Just remeber to have more shelters than crayfish (I would say at least double) to reduce fighting and you will need different sizes to accomdate them as they grow.
If you are hoping that the crayfish will eat the sludgy mulm that accumilates in any water that is not filtered, dont be surprised when they dont reduce the gunk and in fact add to it. I would place crayfish right up with goldfish in the mess maker category of aquatic animals The only way to do minimal maintenance on a pond is to set it up like a true natural little pool, where there is substrate  such as clay and sand, various plant life and even bog or edge plants. All the plants will be feeding off various nutrients that the fish and other water creatures produce. Then living in the substrate in a natural pond/ pool there will be snails, possibly muscles (but I would never add muscles to any ponds or tanks), worms such as blood and tubifex, free swimming there will be daphnia and other small inverts. Basically the pond needs to replicate a complete food cycle/ web as possible.
I am reminded of a true story about a Ranch in America called Don Coyote by Dayton O. Hyde in which the ranch had heaps of channels that every year his Uncle used to flush out and keep clean never allowing the water to become a viable habitat. Once the author Dayton took over the ranch and he started taking more interest in the wild things and stopped cleaning the water channels he made fantastic discoveries. Like by allowing the water to age and develop plant life and mulm the water stayed warmer in winter and created a warm pocket over his ranch enabling him to plant sooner than his neighbours without fear of frosts. The same effect can happen in your pond, perhaps to the extent that you no longer need to struggle to keep a section of water ice free.
I know traditionally Koi ponds are pretty clear of plant life apart from water lilies (partly because the koi eat everything) but a complete ecosystem set up in a pond makes life easier for you with less cleaning and even feeding once the other pond life starts establishing and seasonal invaders like mosquitos join the food chain.