Salmon life cycle enriches Southeast

Nutrients from fish carcasses help nourish plants, insects, birds

Posted: Sunday, October 16, 2005

When the salmon come back to the local streams to spawn, we all know it. Their annual migrations and deaths fill the air with the lovely (to some of us!) aroma of decaying fish carcasses. It's one of our seasonal pastimes to watch the eagles gathering at a salmon run to eat their fill and the bears near the visitor center teaching their cubs to fish. By leaning over the railings of the observation platforms near the glacier or the bridges on Sheep or Salmon creeks, we can watch toothy male salmon fighting over females or female salmon fighting over nest sites. There are, however, some aspects of this seasonal miracle that are less obvious but no less interesting and thought-provoking to a curious naturalist.

For example, Dolly Varden and sculpins often congregate at salmon runs to feast on drifting or poorly buried eggs (which would probably die anyhow). These predators can root through the gravels and exhume eggs that were buried too close to the surface or that were disturbed by a flood or a late-spawning female that tore up the nest of a previous female. Gulls can bring eggs to the surface by moving their feet up and down on the gravels, and dippers can probe the gravels with their bills. All these predators make it advantageous for female salmon to bury their eggs as deep as possible and to guard them well.

Salmon use up all their energy in their homeward migration and the spawning effort, and (if not eaten first by a predator) they die soon after spawning. The nutrients in their bodies enrich the streams, feeding hordes of insects, bacteria, and other small things that are later eaten by other consumers. Some of these valuable nutrients get into the terrestrial streamside system as well, either by subsurface water flow or being carried to land by bears or wolves - Go to any of our good small salmon streams and look for carcasses and bone piles along the shore and even many yards inland. The nutrients from salmon are known to enter streamside vegetation, where they nourish leaf-eating insects and, eventually, the birds that eat those insects. When the enriched leaves fall into the stream, they in turn feed aquatic organisms that are eaten by young salmon after they hatch the next year. Directly and indirectly, the salmon feed multitudes of other organisms.

Bears are a major agent of nutrient transport. Research on Chichagof Island has shown that there may be as many as 15-25 bears per mile of salmon stream in a period of a few days, but they are not all present at the same time. This density of touchy, irritable critters in a relatively small space can create problems, especially for young bears or females with cubs. Adult males get first choice of feeding stations and subordinate bears, such as females with cubs, try to avoid the big aggressive males, so they often settle for poorer foraging areas or forage at night but retreat far up the hillsides during the day - and some never come down to the streams at all, missing out on the salmon bonanza altogether. Complex social adjustments among the bears thus allow a tentative coexistence, but some bears clearly get "the short end of the stick." Social interactions have a major role in determining how much salmon each bear gets to eat.

The salmon feast feeds the multitudes in various ways. For example, gulls of several species all eat eggs, but they differ in their ability to tackle the fish themselves. The smallest species (Bonaparte's gull) often hovers and dives for drifting eggs. The biggest gulls (mostly glaucous-winged and herring gulls) often stab at the vents of female salmon to force the extrusion of eggs. The big gulls also tear up carcasses, even showing interest in carcasses so old they are nothing more than rags of skin and bones. Medium-sized gulls (mew gulls) get to feed on carcasses chiefly when the big gulls (and the eagles) are busy elsewhere.

The upstream spawning runs of adult salmon in late summer and fall gets most of our attention, but in spring there are also downstream migrations of young salmon. The tiny fry of pinks and chum usually go to sea right after hatching, but young coho and sockeye stay in fresh water for a year or more and go to sea a bit larger. Although they often go down at night, they run a gauntlet of predators, not only birds (gulls, dippers, herons, mergansers) but other fishes, including herring, pollock, other salmon - it's a tough, often short life!

That's just a sampling of all the goings-on in and around our salmon streams. Southeast Alaska would be a rather dull and dismal place without the richness that comes with the salmon.

Where to go to see some of these things? Besides the walkways and overlooks on Steep Creek near the visitor center, one can stand on the bridge over Sheep Creek and study the delta with binoculars (to avoid disturbing all the interactions). Try hiking the lower parts of the Fish Creek trail, go along Peterson Creek on Douglas, or check out Switzer Creek - but keep an eye out for large furry beasts!

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

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