Penns maybe a limestoner, but it gets awfully warm in the summer. Trout are very fragile in these conditions. They do not recover well from the stress of a fight. Even if they swim off fine, they are frequently dead within a day or two.
It's also a very heavily fished stream. It only survives as a good fishery due to anglers doing what they can to prevent fish mortality. One of the things they can do is NOT FISH when water temperatures are high. To do otherwise would be disrespectful of other anglers.