bobby my experience is that you will lose fish, especially small forage as they seemed to follow the current.
Rigid fencing, available at most box stores, zip tied to t posts "should" keep them in the pond, but be careful if you have floating debris, as it can clog up. I would put the fence on the pond side of the rocks to keep the fish out of the rocks.
Also, using the t posts lets you easily remove it when not needed.
How wide is the high water spillway, and how high is the water going over it?
