Larry --
I hear you; I'm entering my 62 year of fly fishing in recent years have had trouble with my wrist, elbow, and thumb.
I agree with some of the others about using ibuprofen as needed. However I also found that if I stop and thinking about what is going on with my casting/fishing play that was causing the stress and resting pain. Once I forced myself to look closely at what was causing the stress I found that I could control it somewhat (though not eliminate) by modifying my techniques. One obvious step is take short breaks in the fishing; say every 15 to 30 minutes.
For example went through a period where I was having trouble with my elbow after a day of hard fishing (especially when play numbers of large fish). Found that by modifying the way I held my rod provided relief; in this case I pull my elbow in to my belt for some support and subjecting to less torque on the joint. When ever I feel the slightest tinge of pain forcing myself alter the rod position helped a lot. With my thumb I typically had problems when I was for extended periods casting at near my casting limit. I found that I could get relief by modify my thumb positon on the rod several times a hour and/or take a fishing break by moving or looking for a situation where I wasn't casting as far or frequently.
Another adjustment I have made is by eliminating weighted flies and paying close detail to fly constructions I can fish with lighter (easier on the body) rods than what might be considered normal. For example in your tiger musky fishing by tying without weight, using water shedding materials, lighter wire hooks, etc. you maybe able to fish flies with similar profiles and be as effectively with a lighter weight rod (say an 8 weight). After all with the fly rod chasing muskies, steelhead, etc. the playing of the fish (usually not more than a few times a day) is much less stressful than the repeating casting and one can usually learn to successful handle the fish with a rod that is couple weights lighter.
Making any or all of the "adjustments" isn't easy but beats the heck out of not fishing.
Good luck
Curt