At my favorite little Brook Trout lake, the surface temp (ST) is already 75F (I like Imperial measures....more human, I leave CGS to laboratories). According to Crossman's Fish of Canada, the textbook most biologists are forced to buy at $120 states that 75F is fatal to Rainbows. Browns can take a bit higher but same for Brookies. Our technical advisers from Carleton U. say that 75F is not actually fatal. The fish can take a bit of time of it but they will avoid water at that temperature if they can. We have had fish go belly up being brought in at ST70F. The surface temperature usually goes down 5-10 ft before slowly getting cooler. At 20ft down, it can be dramatically cooler.
The ideal temperature for trout feeding is 55F-65F. Many experienced trout fishers who practice C&R, move off trout waters at 68F, Bill Spicer for one. Three of my Rainbow lakes are already at 66F.
The issue is the trout's ability to get O2 out of the water at these temperatures. Try struggling in the water with a hook in your mouth while holding your breath and you get an idea of what the trout is going through.
So to avoid this area of the lake, the trout will hang out in 25-35 feet of water for most of the day time. 25 ft is the deepest plants survive and that is where the trout food is likely to be. According to the information I've received from these advisers, the next issue is baratrauma. Trout coming up quickly from 30 ft will suffer the human equivalent of Bends. Some believe that Lake Trout have a release valve for this, other experts are not so sure. But our sport fish of Rainbows, Brook Trout and Browns do not have this ability. So when we do our summer fishing with our full sink lines down at 20 to 30 feet we could be subjecting our trout to this who will come up pretty fast to shake that hook (which they can only do by jumping in the air), then hit that oxygen depleted water, well I'm sure you get the picture.
In the Ottawa area, we don't have cold mountain trout lakes but not to despair. Pike and walleye are open and Bass will soon be and so will Musky, the new target of flycasters. I can't wait to get my winter made 10wt rod into action.
If you still want to catch the remaining couple of weeks for trout, get him in real quick, don't net, just run your hand down the line and flick that hook out. No time for pictures. Otherwise, you may be taking something home. Our lakes aren't well enough stocked for too much harvest which unfortunately is the case.