River of salmon flows with nutrients

Posted: Sunday, August 12, 2007

There is a 'river' that flows upstream in salmon-spawning season-a 'river' of fish is headed upstream to the spawning area.

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The returning fish have grown large during their time at sea, feeding on marine organisms. More than 95 percent of their body mass has been accumulated at sea. Now they are bringing to the streams all that body mass constructed from marine prey. The river of fish is like a conveyer belt, bringing marine-derived energy and nutrients and some contaminants into freshwater streams and associated lakes.

Some of the returning salmon complete their life cycle, spawning and then dying a few days later. Floods may carry carcasses onto the floodplain of the stream, and underground waterflow also may carry the nutrients many meters from the stream itself. The depleted carcasses may wash down into the estuary, feeding flatfish, crabs, and snails. But carcasses often lie in the stream, caught up in pools and eddies, where they are colonized by decomposers such as algae, fungi, and bacteria, and the decaying flesh is eaten by invertebrates and fish.

Both resident fish and stream-rearing juvenile salmon grow better if they feed on salmon carcasses, and large juvenile salmon usually survive better than small ones. Thus, salmon production tends to be better when our nutrient-poor streams and lakes receive high levels of salmon-derived nutrients. In effect, fertilizer from the dead fish can enhance fish growth, survival, and population size.

More nutrients are added to the system when salmon eggs drift away before being buried in the gravels by the spawning females. Furthermore, when the density of spawning females is high, late-arriving females may dig up the nests of earlier females, making more drift eggs. Drift eggs are colonized by decomposers, and they are avidly consumed by fish and birds. In some streams in Alaska, hordes of sculpins follow the salmon into the spawning rivers to feast on eggs. Dolly Varden sometimes congregate by the dozen just downstream of a spawning female, waiting for spilled eggs to drift out of the nest. Bonaparte's gulls hover above spawning females and squabble over loose eggs. American dippers often feed on drift eggs, which may be especially important to juvenile birds just learning how to forage for themselves - eggs are easy to catch and provide lots of energy per catch. Drift eggs will die, so predation on these eggs has a negligible effect on recruitment to the salmon population.

Some salmon do not complete their life cycle but are captured by predators before they spawn. Black bears, brown bears, and wolves are among the more important predators of spawning salmon. Bears sometimes consume salmon right in the stream, discarding the unwanted body parts into the water and contributing to fertilization of the stream. More often, bears and wolves carry their prey ashore, sometimes over a hundred yards into the forest, and consume the favored body parts there. Sometimes dozens of partly eaten carcasses lie scattered on the ground.

Locally, the easiest place to observe these 'bear kitchens' is at Steep Creek, near the Mendenhall Visitor Center. This is a popular evening entertainment for many local folks, who can observe quietly from the raised walkway without disturbing the bears.

Partially eaten carcasses and the urine and feces from the predators' digestive processes contribute nutrients to the forest soil. An individual bear may deposit 20 to 60 kilograms of nitrogen on the soil via digested remains of salmon. Most of this is within 500 yards of the stream, but some is deposited at least three kilometers away. Because there are several, and sometimes many, bears at each salmon stream, the total deposition of nitrogen and other nutrients can be very high. The greater the density of salmon, the greater the potential effects on soil and other organisms.

The dead, spawned-out salmon and uneaten remains attract many kinds of scavengers. Eagles, gulls, ravens and crows, voles, sparrows, thrushes, deer, mink, bears, wolves, and many other animals are known to forage on carcasses and thus help spread the nutrients around the landscape.

Thousands upon thousands of fly maggots commonly infest carcasses that are exposed to air; they can reduce a carcass to a skeleton in less than five days. Ground-dwelling insects such as beetles also benefit from decomposing salmon.

Several studies have now documented the passage of salmon-derived nutrients from the soil into the vegetation around a salmon stream. Nitrogen of marine origin is found in the foliage of many kinds of plants; depending on the plant species and the density of salmon, in some cases as much as 20 to 60 percent of the nitrogen in leaves may be of marine origin. Phosphorus is an important supplement too, at least in some situations, but less is known about it. Thus, the trees, shrubs, and herbs growing near a salmon stream are enriched, and in turn, they are likely to attract more herbivorous insects, which often prefer high-nutrient plants.

The effect of salmon spawning extends even to the bird community. The density of nesting songbirds in spring appears to be greater near streams that have salmon runs than near streams that lack salmon. Thus, the legacy of the salmon is seen the following spring, long after the salmon are gone. The response of breeding songbirds presumably reflects not only an increase of terrestrial, herbivorous insects from the streamside vegetation but also an increase of insects, such as midges, mayflies, caddisflies, stoneflies, whose aquatic larvae fed at salmon carcasses but whose adults are airborne.

Thus, the marine, freshwater, and terrestrial systems are intricately and intimately tied together by the conveyor belt of upstream-migrating salmon. There are more than 5,000 salmon streams in Southeast, and more than 90 percent of the forested landscape is estimated to lie within a few kilometers of a salmon stream. That means that a sizable portion of Southeast and a good deal of its wildlife are affected, directly or indirectly, by the 'river' of salmon.

• Mary F. Willson is a retired professor of ecology and a Trail Mix board member.

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