Cecil, I think that people could get by with higher water temps than 65°F if they could keep their O2 levels at or near saturation. I fyke netted Golden Rainbows from a clients pond with water temps in the mid 70's and the pond at saturation level with regards to O2. Pond is only 7' deep max. 3 diffuser stations, each with 2 Vertex diffuser discs, and a Kasco surface agitator were running 24/7. No well water was being pumped into the pond. As far as I know, the last trout was caught with water temps approaching 80°F. Pond is slightly larger thean 3/4 ac.
No doubt but that seems like a lot trouble and at some point once that water remains well above optimum you will run into problems from stress and bacterial infections. The immune system of a trout has not evolved to work optimally at those temps. * If it was as easy as cranking up the air, trout farms would be doing it to save on water use.
My first year when I had planted trout into the big pond where there was no incoming well water, by May they had stationed themselves next to the diffuser bubbles but at some point that just wasn't enough and they croaked.
*I did read about a strain of rainbow in Nevada that has evolved to handle 80 degree water.
