The naturalist Georg Wilhelm Steller encountered the animal during the Bering expedition's forced overwintering on what is now Bering Island. His account remains the only scientific description made from living animals: a slow, tame grazer eight to nine metres long, unable to submerge fully, feeding on kelp at the surface.
The population Steller met was a remnant. Fossil and subfossil evidence shows the genus once ranged widely around the North Pacific rim, and the Commander Islands group survived largely because no humans lived there. Estimates of that final population are typically in the low thousands. Once Steller's shipmates demonstrated that the meat kept well at sea, fur-hunting crews used the islands as a provisioning stop.
The conventional extinction date of about 1768 rests on the absence of later reliable reports rather than on a recorded final kill, so it should be read as an approximation. Occasional claims of nineteenth-century sightings have never been substantiated.