Complex sentence: Prior to the 2011--12 season the WSHL and all its team members, including the Gulls, became Amateur Athletic Union sanctioned instead of USA Hockey, the first Junior A hockey league to make that transition.
Split into simpler sentences: Prior to the 2011-12 season the WSHL and all its team members, including the Gulls, became AAU sanctioned. The first Junior A hockey league to make that transition.