My understanding is that the waters have changed over time. The changing water temperatures can affect the travel of the fish. Again, it's not my area of expertise; we're just going by what we've read and seen through various studies. The change in water temperatures could be making the fish look at different spawning areas. It could also be bringing in disease that is not natural to the colder water.
On the predation side, for the tiny fish that are running up and down the Yukon River to try to make it to the ocean so that they can survive for five, seven or eight years and come back, there are many factors there, from otters to bigger fish, such as northern pike, and various things like that.
As I mentioned earlier, the larger, older fish used to be the ones that made it back up. In those cases, I would have to assume that an 80-pound or 90-pound salmon would have fewer predators than a 10-pound or a 20-pound salmon.