It is increasingly clear to scientists that other marine mammals, such as dolphins that live in the upper layers of the seas, are also to some extent vulnerable to oil pollution, including floating oil slicks.
Firstly, oil spills can also kill off smaller animals (such as shrimp-like creatures called krill) that are eaten by whales and the fish that dolphins eat.
Secondly, scientists have observed that dolphins at sea do not necessarily avoid floating oil slicks, sometimes actually swimming into them. Once dolphins are surrounded by an oil slick, there is a risk that the toxic oil will enter their blowholes and mouths and, perhaps more importantly, that they will inhale the volatile toxic chemicals that evaporate from the surface of the oil.
Few scientific studies have been made of how the chemicals from oil spills affect the bodies of wild dolphins and whales. However, two orca populations have not recovered from the Exxon Valdex oil spill in Prince William Sound, Alaska, in 1989. In the year and a half after the Exxon Valdez spill, both groups of killer whales swimming through Prince William Sound at the time experienced an unprecedented high number of deaths. The pod of resident killer whales lost 33% and the pod of transients 41% of their populations. One pod of resident killer whales still hasn't reached its pre-spill numbers.
Ongoing studies are looking at the effects of the huge 2011 oil spill in the Gulf of Mexico, and the findings from these studies are expected to greatly improve our understanding of the potential effects of spilt oil on whales and dolphins. Early signs are not good for the Louisiana bottlenose dolphin population, who are showing signs of severe ill health.