Spawning salmon engineer landscapes

by Rachel Crowell
Friday, February 23, 2018

Spawning sockeye salmon swim in a stream in Alaska. Credit: Katrina Mueller, U.S. Fish and Wildlife Service.

All animals depend on their ecosystems for habitat. And, in turn, many animals impact their ecosystems by engineering the landscape to suit their needs. Beavers provide an iconic example of ecosystem engineering when they build dams, which influence streams and wetlands. The engineering efforts of salmon, meanwhile, can even shape the bedrock of the watersheds in which they live, according to a recent study that modeled the evolution of those watersheds over several million years.

The study is one of the first to quantify the fish’s ability to sculpt the landscape. Female salmon use their tail fins to move streambed sediment to create nests, called redds, for their eggs. This nest-building disturbs the gravels sitting atop bedrock in mixed alluvial-bedrock streams, says Alexander Fremier, an ecologist at Washington State University and lead author of the study, published in Geomorphology. By moving gravels, “salmon make erosion of the streambed more efficient,” he says.

“When there is a lot of alluvium [on streambeds], the bedrock doesn’t erode much,” Fremier says. But if enough alluvium is removed, the bedrock is exposed and erosion can occur. During floods, high waters that spilled over outside stream channels eventually drain back into the streams. “The weight and the movement of [this] water squeeze [streambed] gravels in place, making them harder to move,” Fremier says, so “the top part of the stream gravels are packed, while below they are looser.” Spawning salmon unpack that upper layer, making it easier for the looser sediments to be moved during the next flood.

Results from previous research show that “chum [salmon] spawn in small gravel, Chinook in large, and sockeye and Atlantic [salmon] in between,” Fremier says. “Think Goldilocks,” he says. If the gravel is too small, “the eggs they bury, which need oxygen, will suffocate; too large, and the adult salmon cannot move them,” he says. Individuals of the same species may also spawn in different gravel sizes depending on their location. Sockeye in British Columbia, for example, have been observed to spawn in larger gravels on average compared to sockeye in Washington state.

Fremier and his colleagues modeled the effects of four salmon species on river-bed profiles over 5 million years, considering gravel sizes ranging from roughly half-centimeter pebbles up to cobbles 10 centimeters or more across. The modeling showed that each species of salmon can influence the elevation and slope of alluvial-bedrock stream watersheds over geologic time, and that these influences vary depending on spawning location, which the fish select based on the availability of suitably-sized gravels. For example, increased bedrock erosion due to pink salmon nesting can “lower the slope at the bottom of the watershed,” where gravels are smaller, while erosion due to Chinook salmon can “steepen [slopes] higher in the watershed,” Fremier says. And, overall, landscapes where salmon spawn can be as much as 30 percent lower than they would otherwise be without salmon. “The 30 percent relief change [is] the maximum observed effect,” he notes.

“It’s an interesting modeling experiment,” says David R. Montgomery, a geomorphologist at the University of Washington and author of “King of Fish: The Thousand-Year Run of Salmon.”

“Most people don’t think of fish [playing a role in] shaping rivers — as ecosystem engineers,” he says. “We think of rivers as places where fish live.”

This work builds on previous studies that show that salmon behavior is the main way in which gravel is transported in some rivers, Montgomery says. However, he says, it doesn’t factor in some other processes that can also shape such watersheds, including log jams, which trap sediment and have “a competing effect on river profiles.”

Because the researchers were looking at landscape evolution over geologic timescales, they didn’t quantify the effects of dams or dam removals on watersheds or on salmon. Since bedrock erosion “occurs very slowly, our models run over millions of years to see an effect,” Fremier says. “Dams are a recent blip. Of course, they impact salmon by influencing their populations, but only over the last 80 years in North America.”

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