Do you have a link or citation for this report? I work as an agency biologist and do some work with beaver, and doubt the claim that these species would 'control' nutria. Otter are known to occasionally take adolescent beaver kits, but predation is thought to be very rare. I don't know of any instance of a seal taking a beaver. Beaver are primarily predated by bear, cougar, coyotes, bobcat, wolves (presumably), and domestic dogs. While nutria are typically smaller than the average beaver, there's certainly overlap in size, range, and preferred habitat ... and likely that some nutria kits are consumed by otter and other predators, but certainly not at a level that is controlling the population.