In the late 1980' s, the population of sea otters in the North Pacific Ocean began to decline  Of the two plausible explanations for the decline-increased predation by killer whales or disease-disease is the more likely
After all, a concurrent sharp decline in the populations of seals and sea lions was almost certainly caused by a pollution-related disease, which could have spread to sea otters, whereas the population of killer whales did not change noticeably
Along the Pacific coast of North America in the 1980's, sea otters were absent from many locations where they had been relatively common in former times.