The Bonaventure in early September can be grand. It's a good idea to prepare for a wide range of water conditions and salmon taking behavior.
For medium to low water, bring the usual complement of small (sz. #6-10) wet flies in the usual patterns: Ruelland Special, Green Stonefly, Pompier, Black Bear Green Butt, Black Ghost, Undertaker.
Don't forget to also bring some big stuff: #2 Muddlers, Popsicle-style flies, large streamers (you can get some high water in September). The salmon also begin to respond well to larger flies again about this time.
Try the "E" Sector, limited to 4 rods. I've had great fishing there in September. Tight lines, TB