I know I lose fish through the regular and emergency overflows on my ponds. I've posted about picking fish out of the bushes when we've had very heavy spring-time rains.
I'm not worried about losing a few fish -- and I believe that I only lose few fish.
I'm far more concerned about overflowing the dams, and somewhat concerned about species from my upper ponds getting into my lower ponds. I have a number of settlement ponds in between my ponds, to hopefully catch most of anything swept from the upper pond to the bottom pond.
I think we can be fairly sure that fish can go from an upper pond to a lower pond. At one of the very early Pond Boss conferences we had a presentation where a fisheries biologist from out west (sorry - I forgot his name, but I believe he was somewhere in Montana, Wyoming, etc., ...)
He had a series of stair-stepped ponds that fed each other. He presented results that his fish were escaping the upper ponds into the lower ponds.
I have a 1/2" wire barrier across the outlet where it is about 6ft wide.
I'd be far more concerned about the fence barrier than escaping fish. When we first bought our present place, the only pond on the property had a wire fence about 20 feet down the 150 foot long emergency spillway. It was made from chicken wire fencing, which probably has about 1-1/2 inch openings.
Because of it, I thought we were going to lose our dam. We got a true gully washer. The debris that floated into the pond during the storm jammed up against that emergency overflow fencing, causing the pond water to back up rapidly, and started overflowing the dam.
I took the dangerous chance of going down to the emergency overflow, getting as much debris out from behind the fence, and then ripping the fence out. I believe that saved the pond.
It has been my learning that most fish swim into the current, so they are averse to going out an overflow, unless overpowered.
IMHO I'm OK with losing a few fish as compared to the possibility of losing my dam.
Hopefully, one of the others can correct or add to this. Without aeration, a pond of your size will develop a very low oxygen thermocline, probably somewhere below about a 5-7 foot depth, depending on its location with regard to sun, wind, water inflow, etc. Most fish cannot live below that thermocline due to the lack of oxygen.
Depending on you pond edge slopes, you might consider lowering your main drain to fit your worst-case thermocline depth, if it doesn't significantly impact the size and shoreline of your pond.
Just a thought ...
Ken
