| Nice pond Dimas. I do not pretend to be in touch with mud pond management in Japan, but some things remain the same around the world. It would be hard to compare your pond to a koi mud pond in Japan though. The Japanese ponds are seasonally drained and dried. This, along with the colder winter temperatures, makes a big difference in how the pond behaves.
Is there is a lot of spring water running through your pond? A lot of water exchange in a mud pond is good and bad. It is good because it flushes away nutrients which helps prevent oxygen depletion. It is bad, because it flushes away nutrients and keeps natural productivity (development of natural food items) low. The best growth is usually in a pond with high natural productivity plus high oxygen from supplemental aeration.
If the water flow is not great, you should consider adding some agricultural lime to stabilize the pH. I would add 5 kg, check the pH after a week and add another 5 kg if it does not stay at 7.0 or above. Make sure is it agricultural lime (calcium carbonate), not quick lime (also called caustic lime, burnt lime, or calcium oxide).
The shrimp make great forage when the fish can catch them. If there are many koi, they will usually wipe out the shrimp. I doubt that seasonally-drained Japanese mud koi ponds ever have many shrimp. Chye sim and other leafy green vegetable are always good for koi. In a mud pond, the koi can often find other vegetable matter to eat. But, if they take the chye sim then it is good for them.
A muddy bottom is a nuisance when you need to work in the pond and it impacts water quality. 10 to 15 cm is not too bad though. The fish will root around in the bottom (like pigs) looking for food. When the fish disturb the bottom sediment, the sediment pulls oxygen out of the water above. The more organic matter there is in the sediment, the higher the oxygen demand. Bioturbation (fish rooting around in the mud) erodes the banks and keeps the bottom soft and muddy.
For get the maximum growth in a scavenger like koi/carp, the amount of fish should be less than about 1000 kilos per hectare. That would be only 12.5 kg in your pond, or about a half-dozen fish at 50-60 cm. However, you can get growth that is almost at it's maximum with two to three times that number. You could use the shrimp as a guide. When you have so many fish that the shrimp disappear, then you know you are not getting close to the maximum growth. Only you can decide which is more important - maximum growth or having a lot of fish.
I hate to keep harping about oxygen, but that is always the first thing to become a problem when you increase the number of fish and amount of feed in a mud pond. If electricity is available, aeration is the best thing you could do for your pond and your fish. If supplemental aeration is not possible, then try to keep the feed rate below 350 grams (dry weight) per day. 350 grams per day can maintain about a dozen fish.
-steve |