I realize my situation is not exactly the same as you describe. I have a 2.5 to 3 surface acre pond that is the only water supply for the 30 cow/calf pairs(7.5 acres/pair) that occupy the pasture. The pond is about 14' deep when at spillway level. The pond has never been dry since it was built. I have an excellent fish population mix of LMB, channel cats, and bluegill. The cattle are in the pasture May 1 - Nov. 1 each year. The pond is very clear for a Kansas farm pond. It has produced an 8# LMB that is on my wall. I caught and released a 6+ LMB in July. The largest channel cat caught weighed 14.5# (15 yrs. ago), & there are 10+# in it now. The area fish and game fish biologist has sampled the fish population, and said they are doing well. I realize you are in another state, but I would not be overly conerned in Kansas of cattle being grazed in an area that drained into my pond.