Shorty -- I know of no situations where diploid grass carp were able to spawn in ponds. As you indicated, they need current. Unlike its cousin the common carp, grass carp are a big river fish. Grass carp eggs are nearly neutral buoyancy, and need to float downstream as they develop prior to hatching. Common carp, which certainly can spawn in ponds, have heavy and sticky eggs. So, if the grass carp are eating "holes" in your vegetation, it probably is just from feeding. They certainly are not nest spawners. Hope this helps -- I think you knew most of this already.
