Georg Wilhelm Steller described Hydrodamalis gigas while stranded on Bering Island in 1741 after the wreck of the Sankt Peter. The animal was around eight to nine metres long, slow, buoyant and unable to submerge fully, which made it trivially easy for fur hunters to kill from small boats.
What Steller encountered was already a remnant. Fossil and subfossil material shows the species once ranged widely around the North Pacific rim, and the Commander Islands population Steller met, often estimated at around 1,500 to 2,000 animals, was what survived earlier human coastal hunting. Russian sealing and fur-trading crews then used the islands as a provisioning stop. The last generally accepted record is 1768, twenty-seven years after the description.
The standard telling gives all the blame to direct hunting. Some researchers argue that pressure on sea otters, and the resulting sea urchin increase and kelp decline, removed the sea cow's food supply as well. Both mechanisms have supporters and the relative contribution cannot now be tested.