Die and let live — Cycle of salmon spawning serves valuable purpose on Kenai River

One of the mysteries that has vexed stream ecologists for years involves large Pacific salmon that return from the oceans to spawn in headwater streams and promptly die. Very few fish grow this large and only reproduce once during their lifetime. As an example, halibut can reproduce for decades in the ocean, and many freshwater trout species will reproduce each year for dozens of years.

Evolutionary theories predict that for such an unusual behavior to exist, there must be a benefit to the species when the adults die. Wouldn’t it be better for the adults to survive and reproduce multiple times? The mystery is becoming clearer these days, thanks to stream and fisheries researchers from all over the country and throughout the world.

First, it must be understood that most streams along the Pacific Coast are nutrient-poor. This means that there are not high concentrations of minerals and essential elements in the waters that would support plant and algal growth. In turn, there is only marginal plant growth along and within the stream. The in-stream and riparian (located along the stream banks) plant growth can normally support only small populations of invertebrates that then become food for stream fish. Because of this nutrient-poor stream water situation, salmon have evolved an anadromous life cycle.

Anadromous fish lay their eggs in a stream that cannot actually provide adequate sustenance for their offspring, so the young soon migrate into the oceans to complete their growth. The surrounding oceans are nutrient-rich and provide great opportunities for young fish to find food. As an example, many Alaska silver salmon stay in headwater streams for three years and grow to only 6 inches in length.

They then head to the ocean and return to spawn one year later weighing 10 to 16 pounds. They obviously found a lot of food out in the ocean that they could not have found in the stream where they were hatched.

Since the streams are so nutrient-poor, it now appears that the dying adult salmon carcasses release substantial amounts of elements into the stream. Those nutrients, like the fertilizer we use on our gardens, enable in-stream vegetation and riparian plants to thrive. Those plants then deposit their leaves, twigs, bud scales, pollen, etc., back into the stream. In turn the leaves are used as food sources by various aquatic invertebrates — mostly aquatic insects. The insect populations are then able to grow large enough to support the resident fish and the newly hatched salmon fry.

There has been considerable research on these nutrients using what are called stable isotopes. We know the oceans provide higher concentrations of Nitrogen-15 while atmospheric-captured nitrogen is mostly Nitrogen-14. By looking at the nitrogen isotopes found in riparian vegetation and plants near Alaska salmon streams, we now know that a considerable amount of nutrients in these plants came from the ocean and arrived there as part of salmon tissues.

So, the dead salmon along a stream are providing nutrients for plants that will in turn provide nutrition for the food of the young salmon — a complicated but effective circle of nutrients from adult salmon back to juvenile salmon.

The next time you see a grotesque fish carcass along a stream, remember that it is providing food for its young just as any good parent would do.

David Wartinbee, Ph.D, J.D., is a biology professor at Kenai Peninsula College’s Kenai River Campus. E-mail your science questions to redoubtreporter @alaska.net.