Roughly 80 years ago, walleye were first identified in Lake Roosevelt in Washington through illegal introductions. It is likely that other illegal introductions of walleye in the Columbia River basin occurred and led to their spread in the mainstem Columbia and Snake rivers. Walleye are now established in the Columbia and lower Snake rivers and provide a popular fishery.
Now why would this be a bad thing? Walleye are opportunistic, apex predators that will eat any fish it can get its teeth into. These fish can greatly affect native species, including prized anadromous fish.
The Columbia River ecosystem has changed immensely since Bonneville Dam, the first Columbia River dam, was constructed in 1938. Some of what changed are the fish species that now occupy the large rivers. Walleye, smallmouth bass, channel catfish and even Northern pike now occupy the Columbia River basin and prey on native fish.
Salmon and steelhead did not evolve with these non-native predators and can be especially vulnerable to predation. Another recent change is that the big rivers seem to be warming up for longer periods of time, which can favor expansion of walleye into new areas. An average sized female (16 inches or 1.5 pounds) can produce 57,000 eggs in a year, whereas larger females (greater than 8 pounds) are known to produce over 300,000 eggs in a year! This means if conditions are favorable, walleye abundance in areas they are invading can increase quickly.
Several studies have looked at predation rates of salmonids in the Columbia River reservoirs, and many indicate that consumption rates of salmonids by walleye are similar to native Northern pikeminnow, especially for walleye that range in size between 10-16 inches, because small salmonids are the perfect prey size.
As walleye grow, they will include many other fish species on top of salmonids in their diet. Walleye only add to the many difficulties that salmon and steelhead face, and as they expand their range and abundance in large rivers, more hungry mouths will be eating salmon, steelhead and other native species. This is especially important for wild juvenile salmon and steelhead that often overwinter and rear in large rivers in Idaho before making their journey to the ocean.