We do know that there are about 30 major dams on the Columbia system, some blocking salmon runs. Most Pacific Salmonids must swim upstream from the sea to where they were hatched, in order to lay eggs and complete the cycle that perpetuates the species. We know that they are vulnerable not only to dams and reservoirs, but also from changes in siltation loads, which, in turn, increase in response to forestry and agricultural activities that enhance erosion, augmented by increased numbers of land slides, forest fires, droughts, and floods over recent years.
Aquaculture, the penning of vast numbers of Salmonids in coastal waters, has led to concerns about the spread of diseases (see this). A small number of sea lice on wild salmon is normal, but the research indicates that the amount has increased due to the presence of salmon farms that, by creating artificially dense populations of salmon, increase the overall biomass of sea lice (a tiny parasite), to the detriment of wild fish.
While a host of human activities are demonstrably damaging to Salmonid survival, including pollution, mega-fish farms, acidification, competitive exotic species, and over-fishing, cormorants and Salmonids have co-existed for millions of years. We know that, in naturally evolved predator-prey relationships, the population size of the prey determines the population size of the predator. We also know that apex predators serve various natural functions in the maintenance of biodiversity (for example, by eating diseased prey and competing species).