Grayling tend not to frequent waters that’s are prone to warming in Summer and tend to be laying low and less likely to be caught in warmer conditions. Our local rivers are spring fed, so water temperatures don’t fluctuate that much at all, from season to season.I’m more interested in the considerations around heart-failure of fish than hook punctures or tearing. I guess because I don’t have a zoological background I struggle with questions of fish-mouths healing, pain-receptors and so on. I can grasp the idea of a fish becoming exhausted and overheating in warmer water though.
The Chalkstream that runs across the bottom of our garden, will have water in it now, that fell between 5 months and 20 years ago, as it’s nearly all aquifer groundwater fed . These streams and rivers are called Winterbournes as they are often quite high in Summer and low in Winter, due to the delay in water percolating through the bed rock.
This keeps water temperature in a very narrow band- ideal for Wild Trout and Grayling. I have carried a water thermometer on my vest for decades and it’s interesting how cold the water is, even after a very hot prolonged period.
In July and August your legs can get really cold still after prolonged deep wading and daft as this sounds some members wear thermal leg warmers under their chest waders even in mid summer.
Small shallow stillwaters tend to suffer more, but Grayling don’t frequent them.
My local Stillwater is chalk stream fed, and has some Klonking Grayling in it as a result, They are hard to catch but always recover well as the water is naturally cold and well oxygenated.
It’s not the temperature persay that harms them, but the lack of oxygen dissolved in it, as warm water carries much less oxygen than cold can.