In Japan there is a lake where the sockeye either stay (kokanee) or smolt depending on lake productivity. Studies in Idaho have shown that kokanee retain the ability to smolt after 10,000 years of being lake residents.

Those fish can, with proper management of the lake productivity, become anadromous.

The presence of beaver dams does not guarantee blockage to migration but if "they" are serious about sockeye recovery then they will need to be dealt with each fall.

There are lakes in WA where the kokanee population produces anadromous sockeye without direct intervention. I think, if memory serves (Smalma may know) that the sockeye and kokanee in Baker Lake are genetically the ame fish.

It will probably take more than merely closing the lake, but that would be part of a holistic solution.