As recently as twenty years ago, it was only found in the Lower Columbia, below the Deschutes River. Now it can be found as far upstream as the Hell's Canyon complex. The overall impact on these diseases is presently ignored in salmon recovery programs, in that no one has even attempted to estimate the extent to which overall survival has dropped, and whether and to what extent this drop can be offset by any means. It is possible that until salmon evolve with greater resistance to these diseases, salmon populations may be permanently depressed. Efforts have been made to reduce the spread of disease from hatchery operations, but no one has assessed whether there has been any improvement in overall disease levels.
All salmon have to migrate through the Columbia River estuary, which is rife with predators.
Populations of cormorants, Caspian terns, and other fish-eating birds have increased in recent years, but until very recently, no attempts have been made to quantify the impact on Columbia River salmon. 45 Cormorants take an especially heavy toll. A single cormorant can consume up to twenty pounds of smolts in a single prolonged feeding session. In 1997, Oregon State University researchers focused on the Caspian tern population on Rice Island, which was created by the U.S. Army Corps of Engineers from dredged material. No one has prepared any comprehensive review of whether total estuarine bird populations are higher or lower than historical levels and what effect this might have on salmon abundance.