Well, ongoing investigations have revealed that predation is the most likely cause of sea otter decline after all.
First the pollution theory is weakened by the fact that no one can really find any dead sea otters washing off on the Alaskan beaches. That's not what you would expect if infections caused by pollution started killing a lot otters. On the other hand the fact that it’s so hard to find dead otters is consistent with the predator hypothesis. If an otter is killed by a predator, it’s eaten immediately can't wash on shore.
Second, although otters may prefer to hunt whales. Whales have essentially disappeared from the area. Because of human hunters. That means that otters have had to change their diet to survive and since only smaller sea mammals are now available otters have probably hunting those. So it probably is the otters that are causing the decline of all the smaller sea mammals mentioned in the passage. The seals the sea lions and the sea otters.
And third, the uneven pattern of otter decline is better explained by the otter predation theory than by the pollution theory what happens to otters seems to depend on whether the location where they live is accessible to otter or not. In those locations that otters can access easily the number of sea otters has declined greatly. However, because otters are so large they can't access shallow or rocky locations. And shallow and rocky locations are precisely the type of locations where sea otter populations have not declined.