why sea otters' population has declined across the Alaskan beach?

The reading is about sea otters reduction and the author considers two probable theories regarding how they have decreased across western coast of North America from California to Alaska which are environmental pollution and predation and mentions three reasons for holding the supposition of pollution as a more credible thesis. However, in the listening the lecturer refuses those reasons according to an ongoing investigation and champions the other one.

The first point that the author poses is about chemical pollution such as oil material and industrial sewage which make sea otters susceptible to dangerous infections and subsequently what they would confront with is death growing trend. Whereas the professor declares that as their death is the result of being infected, the water must wash off their corpse on Alaskan beaches; yet there is no trace of dead sea otters. In addition, that is the affirmation of being eaten by gamers when no one cannot find their carcass.

The other proof mentioned in the passage is that not only such pollution would affect sea otters decline but also diminishes number of other species. Similarly, this phenomenon could happen by huge predators like orcas, while they would rather to hunt larger games. By way of contrast, the lecturer claims that although whales are the most desired prey for orcas, they have been hunted by humans and consequently orcas inevitably shift their prey to smaller sea mammals like sea lion sea otters and so on. So it can drive us to conclude that the reduction is caused by orcas rather than pollution.

The last reason of pollution assumption is propounded due to some locations where the number of sea otters is significantly less than other places and it may have been a result of high level of pollution in some specific locations caused by environmental changes. As regards, the lecturer interprets this event as an effect of orcas hunting. Rocky and shallow location through which orcas cannot enter them and pass, are the places at which the number of sea otters is considerably more than other places where orcas can access sea mammals easily.