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
Killer whales in the North Pacific usually prey on seals and sea lions but will, when this food source is scarce, seek out other prey.