July 10, 2017
By Liz Clift
One of the definitions of ecology is the study of how organisms relate to one another and their environment. Think back to your childhood classrooms when you considered food chains. For the most part, they were structurally pretty simple.
Slightly more complex are food webs, which showed interrelated dietary preferences. Animals with diversified sources of food are generally seen as better able to tolerate disruptions to the food web. For instance, a grizzly bear (Ursus arctos ssp.) eats salmon (Oncorhynchus sp.) in the food chain above. But as this is expanded into a food web, we also see that grizzlies eat berries, roots and tubers, small rodents, and even carrion or human food when it’s available.
If diet alone determines a grizzly bear’s survival, and say, chinook salmon (Oncorhynchus tshawytscha), are eliminated from the food web due to changes in how a river flows (perhaps because of a dam), the grizzly would, theoretically, simply rely more on other sources of food. Case closed, right?
An ecologist—by training or by curiosity about the world—knows it isn’t this simple.
If that same river is dammed, the grizzly might be able to find other sources of food, and do well enough, but the health of the river is impacted, as is the health of other apex predators, like orcas (Orcinus orca). Some orcas have diversified diets and will also prey on seals and other marine mammals or fish and so would feel less impact from the extinction of salmon species.
However, according to an article from American Rivers, published in June of this year, a distinct population of fish-eating orcas, called the Southern Resident Killer Whales (SRKW) waits for a salmon feast at the mouth of the Columbia River each spring. Although SRKW mostly eats chinook (Oncorhynchus tshawytscha, which make up 80% of their diet), they will also eat other salmon—and each whale eats 18-25 of these 30+ pound salmon every day. For that population to remain at its current level, they need at least a half million salmon a year—and if we want them to reach their recovery level (140 individuals), they would need a million salmon a year. It should be noted that since this group of orcas was first counted in 1974, the population has not been higher than 98 individuals (1995). This population lives in the Salish Sea, and Granny (J2), the oldest living whale until her presumed death in 2016, used to be part of their population.
SRKWs rely on salmon populations, and although they will travel through the Salish Sea, and down through the Haro Strait and Strait of Juan de Fuca, and are often spotted near the San Juan Islands, salmon populations in these areas are decreasing—and have been for a long time.
Unfortunately, structures we use for hydroelectric power or river control (like dams) can impede fish passage, and by extension, fish reproduction. Overfishing, algal blooms, non-point source pollution, and the disconnection of floodplains can all also impact river health, and by extension the ability to reproduce and the health of fish who use these rivers. (See our blog on healthy and connected floodplains!)
To give you an understanding of how these things have impacted salmon populations, more than 800,000 salmon used to return to the Yakima River every year to spawn. However, increases in agriculture and large reservoirs that were built without fish passage systems have impeded this population. In 1990, only 3,000 to 4,000 salmon returned to this river system each year. A variety of groups, including government agencies, tribal entities, recreation groups, non-governmental organizations, and others have been working to restore salmon populations. This includes reconnecting floodplains, and restoring instream habitat, as well the establishment of local hatcheries where supplemental stock is raised.
These are important steps to restoring salmon populations, as are dam removal projects, like the Elwha River Restoration Project. This is the largest dam removal in US history, and now the river is thriving—tons of sediment have been pushed out, salmon and steelhead are running the river, small squid are making their homes within the estuaries, and other animals including elk and shorebirds, appear to be thriving in the regenerating ecosystems (including young forests) that are moving into former reservoir areas.
The role of salmon in riverine ecosystems isn’t just limited to their benefits to bears and orcas though. They play an important cultural role for many Native peoples, a large economic role for people in the Northwest, and act as “pumps” that move ocean nutrients into lower productivity rivers. In fact, salmon can support the growth of forests as their remains are dispersed by predators, which in turn, provides more shading of the rivers, increased bank stabilization, and improved water quality before it ever hits the river (among other ecosystem services). In fact, salmon populations in Alaska contribute up to 25% of nitrogen in foliage!
Restoring and protecting salmon habitat—and salmon themselves—is critical because so much evidence points to wild pacific salmon as a keystone species for the northern Pacific. So, as you think about those food webs you once worked on, think about all the lines you might need to erase if salmon were to disappear.
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