In a good year in Bristol Bay, a push of over 100 million pounds of sockeye salmon make their way upstream, spawn and die. In Bristol Bay and elsewhere in Alaska, this incredible pulse of salmon carcasses is enriching streams and rivers and making young salmon more resilient.
It’s the discovery of scientists studying the streams and rivers of Alaska that teem with salmon. Aquatic ecologist Mark Wipfli of the Institute of Arctic Biology at the University of Alaska Fairbanks is one of those scientists putting on rubber boots to find out how salmon improve the waters of their birth and the surrounding forests.
The process begins with millions of salmon returning to Alaska’s rivers and streams. Moving upstream, salmon is a bundle of proteins, fats, and nutrients like nitrogen and phosphorus.
Bears are among the first to intercept them, carrying the salmon out of the water and sometimes eating only part of the fish, such as the brains of male salmon and the eggs of females. Once nibbled by a bear, a carcass on land is fair game for flies and other insects, which lay eggs that quickly turn into larvae. Heavy rains can bring the larvae back into streams, where young salmon and other fish catch them. Terrestrial carcasses also provide food for other animals and fertilize riparian plants as they decompose.
Salmon that escape bears and other dangers continue to lay eggs – rich in protein, fat and nutrients – which are perhaps the best food in any stream. Once the salmon dies and begins to decay, the algae and bacteria absorb the nutrients from the salmon, and aquatic insects in turn eat the thriving algae and bacteria.
Aquatic insects also feed on rotting pieces of salmon, and fish and birds reap the benefits of more insects. Nitrogen and phosphorus from the “salmon tea” that rivers become can soak into the ground up to about 200 feet from a stream, and scientists have found traces of ocean-derived nitrogen in shrubs. and trees over 1,500 feet from streams in Southeast Alaska.
“These salmon literally bring tons of fertilizer back to these systems,” Wipfli said.
Curious about how salmon carcasses help young salmon, Wipfli and his colleagues created “artificial streams” in Southeast Alaska by diverting small portions of existing streams through artificial canals in the forest.
Scientists enhanced the water in each man-made stream with different doses of salmon carcasses to see how juvenile coho salmon reacted. Young salmon exposed to two, three, or four carcasses per square meter of stream bottom grew larger than salmon in habitats without carcasses.
Juvenile fish exposed to salmon were higher in omega-3 fatty acids than fish in carcass-free water and had much higher levels of lipids, fats that serve as energy stores during lean times.
Wipfli and his colleagues also compared natural streams with and without salmon, and found that salmon streams were a better place to develop young fish and other animals.
“We are now learning that salmon not only make food webs more productive, but improve the health of fish and other creatures that live there,” Wipfli said.
The lack of returning salmon in the Columbia and other Lower 48 rivers that had good populations before the advent of dams could be a reason salmon restoration hasn’t worked there, Wipfli said. . The effect of millions of bodies fighting gravity and returning nutrients upstream is profound. Wipfli and other Alaskan scientists think what’s good for salmon is good for flying squirrels, black-tailed deer, Sitka spruce and arctic terns.