Steller sea lions move to take advantage of high-energy seasonally available prey resources such as salmon and small schooling fish. In the attached photo, which was taken during an aerial survey by NPS wildlife biologist Jamie Womble on 25 August, Steller sea lions are aggregated ashore at Point Carolus, a sandy reef at the mouth of Glacier Bay. Jamie estimated ~500+ sea lions at the site, substantially more than she observed on 8/16/15. The image below only shows a portion of the sea lions, as there was a much larger group scattered along the NW beach of Point Carolus. Steller sea lions typically move to Point Carolus during late summer and early autumn in response to Pacific salmon which are migrating through Cross Sound, a primary migratory corridor for salmon entering northern Southeast Alaska from the Gulf of Alaska.
More details on Steller sea lions and seasonal prey resources in southeastern Alaska can be found in the following publication.
http://doc.nprb.org/web/publication/project_0406_Womble_etal2009JBI.pdf