known with certainty—of the routine intentional discharge of oil by tankers at sea. Populations of several species face local extinction in the northeastern Atlantic, and one penguin species is imperiled with global extinction.

The effects of oil on marine mammals are much less well known than for birds, nonetheless they do not seem nearly as catastrophic. Marine mammals are not particularly abundant along most coasts of the world, but concern was expressed for the sea lions and elephant seals of Santa Barbara's Channel Islands, and for the migrating endangered grey whales. Despite confusing public reports, there is no conclusive evidence that any of these mammals were killed by the Santa Barbara blowout (). This was in part fortuitous, since the elephant seal pups had been recently weaned and were not feeding. Had the spill occurred earlier or later, there is a greater probability that they might have ingested oil and so increased the chance of injury.

Mammals have been affected by some spills (), however. Possible effects include suffocation by oil, disruption of insulation abilities of fur coats, and poisoning from ingested oil. In addition to pinnipeds (seals, walruses, etc.) and cetaceans (whales and dolphins), other mammals occupy coastal marine habitats. Sea otters are recovering from near extinction along the West Coast of the U.S. Commercially valuable fur bearers, mainly muskrats, inhabit coastal wetlands in Louisiana and are susceptible to oil pollution from the production fields, although fur production has not declined there ().