I've spent some time on the St. Mary's so I'll begin the discussion. I'm not sure what type of information you're looking for, but let me just make several comments. The Atlantics arrive in the river in mid-to-late June, just about the time the spring steelhead drop back to the lake (Huron.) Once, there, they are available through the rest of the summer. Wading here is serious, especially if you venture beyond the berm (sp?) as it is necessary to do most years in order to reach the fish. Double-handed rods are becoming quite popular as the river is about a mile wide. Access for that part of the river, usually referred to locally as the St. Mary's rapids, is available only from the Canadian side so you'll need an Ontario license. Though fish can be caught by swinging traditional salmon wet flies, most fish are hooked using trout sized nymphs. Every other year a large run of pink salmon make getting your fly to the Atlantics a real challenge. I strongly encourage you to engage a guide the first time out as you'll learn a lot about the fishing, but will also learn about surviving out there. It's been ten years since I used a guide, but he was very inexpensive and well worth it in my opinion. There are two or three very well thought of fly fishing guides in the area. I hope this will get you started.
I agree on the wading ... very tough ... get and use a CO2 fired vest or belt ... if you fall there you are in big trouble ... the rocks are cover in algee and the water is cold and fast ... there are deep holes below you you could hide a bus in.
As for the fishing ... I went there 6 years and it was fun, but I never hooked an Atlantic Salmon ... drys, speys, classic atlantic, indicator fishing with nymphs ... eveything but dynomite ... go to Gaspe' ... longer drive but worth it.
In other words, try the Ste. Jean---not the St. Mary's. You have a point, though the Gaspé is a lot more expensive, too. Even the rivers at Land's End can get fairly costly if you're fishing the best zones. And that's another place where one should probably book a guide for at least the first day or two. They may be some of the most beautiful rivers on the planet, though.
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