The symptoms you have put forth sound like Infectious Salmon Anemia or ISA Disease. This is a disease mostly associated with hatchery raised Atlantic salmon and is fatal. From what I have read, it can show up in wild fish if they are exposed to farm raised fish that have the disease. Do you have any Aquiculture going on near the mouth of this river? Keep in mind this is all speculation by an untrained armature who reads entirely too much. Juro makes a point; you should try to find out if a biologist has done any research on the problem.