This sign is located at Fisherman's Park on Brennen Road off of Judd Road in Brackendale. It is nowhere near the Mamquam River - Squamish River - CN Rail Bridge. The sign is on the Dyke just north of Fisherman's Park and is indeed on IR # 15. They do have the right to restrict access as this is not public property. That being said, Reserve property only extends to the high water mark as defined by the "Regina vrs Lewis " case of a few years ago. You just may not access that land by crossing Band property.
Why this sigh has appeared is unknown. It could have been the result of all the people on that area of the Squamish last summer and fall that angled at one of the traditional netting sites or it could have been caused by all of the garbage that was left in theses areas by anglers or it may be a preemptive strike before this years Live at Squamish campers start looking for a free camping place for the festival.