What large sirenian from the North Pacific was hunted to extinction in 1768?

The large sirenian from the North Pacific hunted to extinction in 1768 was Steller's sea cow.

Steller's sea cow (Hydrodamalis gigas) was described by naturalist Georg Wilhelm Steller in 1741 after Vitus Bering's expedition became stranded on Bering Island. By then, the species survived only around Russia's Commander Islands, although its ancestors had once ranged more widely across the North Pacific.

This enormous marine mammal could grow to about 9 meters long and weigh several tonnes. It fed mainly on kelp in shallow coastal waters and had a thick layer of blubber, adaptations suited to the cold Bering Sea. Unlike manatees, it belonged to the dugong family and was more closely related to the living dugong.

The animal was slow-moving, approachable, and valuable for its meat, fat, and hide. Fur hunters and sailors rapidly exploited its tiny remaining population, and researchers generally place its extinction around 1768—only 27 years after European discovery. Occasional later sightings have been reported, but none has been confirmed.