Above 70 °F can be particularly challenging. Fish are cold-blooded animals, and the concentration of oxygen in water decreases as temperature increases.
Of course, there can be exceptions. Statistically, there always can be an outlier. But, it is really tough to prove that, unless you put RFID tags on fish and track their mortality. And, it is a logic fallacy to say the outlier is the norm.
Personally, I choose to play it safe, as very few freestones seem to have high holdover rates. I do this because a fish can swim away upon release, only to die later from a buildup of lactic acid, unbeknownst to the angler.
Unless there is a period of cold rain, I target only tailwaters in the summer and also bring with me a stream thermometer.
I know other anglers who do the same or shift to saltwater starting in July. Some also head north to the colder mountains and its headwaters.
Of course, fishing warm water is perfectly legal in many states. Anglers are free to do what they want. Some states have a different POV, however.
For example, Montana will declare “hoot owl” restrictions, and Connecticut will prohibit fishing where rivers and tributaries converge. Hopefully, other states will be more mindful, too.
It is amazing how quickly rivers can heat up. The Millers at high noon on May 19 already was at 70 °F (post here). Below, is a USGS temperature gauge for the Quinapoxet.
Thank you for listening!