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

Which of the following, if true, most seriously weakens the reasoning?