| Water walkers (water striders) which walk on top of the water are not a big problem. They mainly eat insects on the surface, but might grab a fry immediately below the surface. A larger problem are water boatmen, back-swimmers and diving beetles, which stay under the water, and definitely do eat fish fry. These can be killed by putting some diesel fuel or vegetable oil on the water surface several days before adding the fish. When the insects come to the surface to breath they are suffocated.
Dragon fly larvae (nymphs) are more difficult as they do not have to come to the surface to breath. They can be killed with organophosphate pesticides, but the residual may cause kidney and liver problems in older fish and can cause death or deformity in developing fry. I have only found two ways to deal with dragon fly larvae:
1) Drain the pond and let the bottom dry to kill nymphs. Fill the pond 10 to 20 days before the fish spawn and the fry are stocked. The dragon flys will begin to lay their eggs in the pond again, but the fish fry will typically be able to stay ahead of them enough to avoid serious losses.
2) Keep the pond covered with 1-inch netting. The netting cannot touch the water of the dragon flys will lay their eggs there. Some think the dragon flys will drop their eggs through the netting, but in my experience this does not happen.
These insects are not a problem for fish larger than about 3/4-inch. Fish that are 3-inches or more are insect predators rather than the prey.
For fish between 3/4-inch and 2-inches, you have to worry about bull frogs and leopard frogs. I know they have bull frogs in Japan (imported them from N. America) but am not sure about leopard frogs or native frogs. Of course, herons and egrets can be a problem if the fish are less than 8 inches.
-steve |