The northern fur seal's pelt has two layers: coarse guard hairs and, beneath them, an extremely fine underfur. Densities frequently quoted run to roughly 300,000 hairs per square inch in the thickest areas, among the densest fur of any mammal, rivalled mainly by the sea otter. The underfur traps a layer of air that keeps the skin dry while the animal swims.
That pelt made the species commercially valuable and drove hunting on the Pribilof Islands to the point of crisis. The North Pacific Fur Seal Convention of 1911, signed by the United States, Britain for Canada, Japan and Russia, banned open-water sealing and is generally described as the first international treaty to address wildlife conservation.
The fur strategy has an exposed flank. Because air-trapping insulation depends on the fur being clean and intact, fur seals are unusually vulnerable to oil contamination. The species is currently listed as Vulnerable, and the Pribilof population has continued to decline for reasons that are not fully explained.