Predation and the Salmon Food Web
Healthy rivers and oceans are supported by complex food webs that represent a network of individual relationships between plants, animals, the foods they eat, and their environments. Like a balancing act, food webs continually shift to adjust to changes in the environment. Too big of a shift in the food web and individual relationships or food chains may be broken. When this occurs animals must adjust or perish.
Salmon are a keystone species and play a critical role in a food chain that stretches from the upper basins of the highest mountains to the open waters of the Pacific Ocean. Salmon are food for other animals at every stage of their lives from when they are eggs in the gravel to when they return back to the streams as adults. Salmon are critical for the survival or livelihood of other fish, small mammals, birds, marine mammals, and humans. At the end of their lives, when they have spawned and died in streams, their final contribution is to return an abundance of nutrients from the deepest oceans back to the riverbanks to nurture old-growth forests. As humans have modified the land, they have upset the food webs and have made it more accommodating to predators and more hostile to salmon.
This is seen in two ways. First, native species (sea lions, cormorants) that eat salmon can benefit from a changing food web and grow in numbers.
Scientists estimate that birds eat as much as 35 percent of the juvenile spring Chinook salmon in the upper Columbia River as the salmon head to the ocean.46 Northern pikeminnows also eat millions of young salmon and steelhead in the reservoirs behind dams on the Columbia and Snake Rivers.47
Second, a changing food web may benefit non-native, invading species. Northern pike, which is a non-native species introduced illegally, has established populations in eastern Washington. They are concerning because of their ability to eat large numbers of young salmon.
Managing predators is a very real and complicated issue, confounded by scientific uncertainty and ethical issues. Consider sea lions, which are protected under federal law and have grown in large numbers under that protection. Now, they are eating endangered salmon by the tons. This issue is exacerbated by decreased habitat for salmon and potentially could nullify the ongoing work to recover salmon.
How salmon, steelhead, and the habitats upon which they rely are managed must be addressed. Care must be taken not to upset this delicate balance.
Between 1970 and 2015, scientists estimate that seals and other pinnipeds increased the amount of Chinook salmon they ate from 75 tons to 718 tons-double that of resident killer whales and six times more than the combined commercial and recreational catches.48