>>212174952>>212174953It's not only because of that. Cascades of hydroelectric power plants prevented them from going up into the rivers. So there isn't much place for them to spawn.
Soviets tried to build some "fish lifts", but from what I've read it didn't help beluga. There should be something for such large fish probably.
We also have a relative of beluga. It's called kaluga, lives in Amur river. It's a bit smaller but also according to the evidence can grow up to 1.5 tons and 5-6 metres long. They're also endangered.
The species are protected, but still getting hunted by some poachers. Also this fish has very long life cycle, which is why it's hard to find big ones nowadays. For example wiki says that kaluga can reach its peak length and weight at 55 years old. There were also some evidences from China where kaluga was caught and those fish were like 350 kg and 53 years old, and 514 kg and 100 years old.