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Pink Salmon
credit: Timothy Knepp

Salmon: The Amazing Nutrient Machine

In any part of the world where wild salmon still return to freshwater to spawn, the arrival of these fish surging up from the sea serves as a rallying cry to the wildlife for miles around. The congregation of Grizzly Bears and Bald Eagles around this annual glut of protein is one of nature's most impressive spectacles. But perhaps even more impressive are the hidden and yet profound and far-reaching consequences of this annual event for animals and plants from near and far. Recent field studies have begun to shed light on some of the unseen effects that spawning salmon have on the world around them. This story starts where the life of the spawned-out salmon ends.

The Cycle Begins
After mating and producing eggs, adult salmon die. They usually remain in the area of their nest as they expire, eventually drifting with the current and coming to rest among dozens of other fish carcasses. As they wash ashore or sink to the bottom, their bodies are immediately set upon by hordes of organisms, from microbes to crustaceans to insects. Invertebrate scavengers such as beetle and fly larvae swarm over the fish, and predatory species, including stoneflies and dobsonflies move in to devour the attendees at this feast. In a few days the flesh of the fish is gone, and only bones and cartilage remain. The protein from the fish is now distributed among a million representatives of the lower links in the food chain. Many of these insects and their offspring will be eaten by the next generation of young salmon as they make their way downstream to the sea.

Into the Wild
The nutrients from the adult salmon also find their way into many other parts of the inland ecosystem, and, in ways that biologists are just beginning to understand, affect life far away from the river. Black and Grizzly Bears easily capture the dying salmon and drag their bodies away from the stream. They feed on only part of each carcass, leaving the rest to be consumed by other animals, including birds, mammals, and insects. The feces and urine of the bears further distributes the biomass of the salmon, in some cases well beyond the drainage in which the fish spawned and died. The insects that have scavenged their share of the salmon remains, both in the stream and in the surrounding forest, are themselves fed upon by birds and bats, which in turn further spread this bounty.

From Sea to Stream
Scientists are able to evaluate the influence of salmon-derived nutrients by tracking certain substances as they pass through the food chain. One of these is a naturally occurring isotope of nitrogen that is found primarily in marine algae. Occurring in plankton of the open ocean, this isotope is further concentrated in the animals that feed upon it. Many of these plankton-feeding organisms eventually become prey to salmon during their period of growth in the sea. When these salmon return to freshwater to spawn, they carry this very stable isotope with them in all of the tissues of their bodies. Biologists have learned to read the "signature" of this compound and have been finding this nutrient of oceanic origin in places far from the sea.

As young salmon work their way downstream, they seek out cool, shaded pools. Alders, maples, and spruce trees growing along the stream banks spread their branches over the water, providing this shade. Studies have detected, within the tissues of the trees' leaves and needles, molecules of nitrogen bearing the telltale signature of the open ocean, delivered upriver by salmon and now helping to provide cool shelter for the next generation.

New Discoveries
Clearly, salmon are keystone species that influence the flow of nutrients far beyond the oceans and streams in which they live and die. These discoveries have led researchers to question how plant and animal communities have been affected along rivers where salmon populations have been eliminated or decimated in the last century. Researchers in Olympic National Park have a unique opportunity to probe such questions in the next few decades, as one of the rivers that flows through the park is about to be restored as a salmon spawning habitat. Dam removal and other restoration efforts should result in a slow return of four species of salmon to the upper 70 miles of the Elwha River, which has been essentially salmon-free since 1913. Biologists are already collecting data on the plant and animal communities in the area so they can monitor changes as the fish rediscover this river.

 

 

 

 

 

2007 eNature.com