salmon populations

As Vancouver, B.C., watches Fraser River stocks of sockeye fail, the count of steelhead passing the Bonneville Dam in Vancouver, Wash., is soaring. And while low Alaskan Yukon runs of king and chum salmon predict a devastating winter for subsistence fishermen, salmon are even making a comeback in the Seine, as InvestigateWest reported last week. What differences could account for these drastic population changes?

Multiple environmental factors could be affecting populations. Warmer weather can heat up rivers, especially those overdrawn by humans, and discourage the cold-water-loving fish from heading upstream. Shifting ocean currents or other predator influences could be altering food sources. Pollutants from stormwater can accumulate in the fish. Overfishing can deplete numbers. Sea lice from farmed salmon could be transferring to wild salmon, weakening them and increasing the likelihood of succumbing to disease or predators. Even superb returns from previous years could be problematic, as too many fish spawning and then decomposing could produce excess bacteria, possibly resulting in disease.