Pinnipeds are overwhelmingly cold-water animals, insulated by blubber and adapted to polar and temperate seas. Monk seals broke that pattern, occupying warm waters in the Mediterranean, the Caribbean and Hawai'i. Two of those three lineages are gone or nearly so.
The Caribbean monk seal, Neomonachus tropicalis, was last reliably seen in the 1950s and was formally declared extinct in 2008, making it the only pinniped known to have been driven extinct by humans in modern times. The Hawaiian monk seal survives at a population usually given as roughly 1,500, threatened by entanglement in derelict fishing gear, shark predation on pups, low juvenile survival in the Northwestern Hawaiian Islands and habitat loss as low-lying atolls erode.
The picture is not uniformly bleak. Numbers in the main Hawaiian Islands have grown even as the northwestern subpopulation has struggled, and total numbers have shown modest recent increases. Why juvenile survival differs so sharply between the two regions is not fully explained.