sea otters TPO10

There are two theory about population decrease of the sea otter, which are environmental pollution or predators. The author claims that main reason of this declining is pollution and suggests three reasons for supporting that. In contrast, the professor explains that the main reason is predation and refutes every reason of the author.
First, there are pollution sources and chemicals in water sample. The speaker explains that if the ottera was killed by pollution, there must be their corpses on shore or water, while there are not. Because of that, they must have been eaten by orcas.
Second, the population of other sea mammals such as seals and sea lions were also declining. The professor explains that because of hunting small animals by human, orcas must have changed their diet to big mammals such as seals and sea lions. And this is the reason of these animals population decrease.
Third, the uneven pattern of otter decline. The lecturer tells us that this pattern is a sign of predation. She claims that in shallow and rocky regions the population hasn't declined, because orcas don't have access to this parts.