Of course there is nothing other than location that defines western stellers from eastern stellers.  The dividing line created by NMFS in 1997 is not recognized by the animals.  Perhaps the western group is following the food source and moving east.  Figure out what they eat and how it changes over the decades and you will go a long way toward figuring out why their numbers wax and wane.  But asserting federal control over Alaskan waters via the Endangered Species Act is a hammer that is too sexy not to use.  Cheers -