Adapted from Salmon Nation: People, Fish, and Our Common Home
Although the six species of Pacific salmon (like their distant relatives the Atlantic salmon) spend most of their lives at sea, they spawn in freshwater, homing in on the rivers of their birth. They swim upstream, often traversing hundreds of miles en route to small waterways where their size as full-grown returning adults makes them seem out of proportion to their surroundings. The female uses her body to dig a depression in the gravel, where she lays her eggs while a male hovers at her side to fertilize them. She then moves upstream and flaps her tail against the stream bottom, covering the eggs with a protective layer of gravel. The eggs incubate in this nest, known as a "redd," where they depend on the flow of water through the spaces between the rocks to carry vital oxygen to the developing embryos.
After a couple of months, the fry swim up through the gravel and begin to feed on small aquatic insects. Salmon are most at home in water colder than 60° F. Depending on the species and race of fish, temperatures of 65 to 70° can be stressful or even lethal at this age. After a period ranging from a few days (in the case of pink and chum salmon) to as much as a year and a half (in the case of steelhead or coho), the fingerlings swim downstream to the ocean, where they spend between one and five years migrating and feeding across thousands of miles of open water before returning to their home rivers to spawn. The salmon's ability to find their natal streams was a mystery for many years. Although their ability to navigate to the mouths of the rivers is still not perfectly understood, we know that their sense of smell guides them once they enter fresh water.
Because salmon return to their native streams to reproduce, they divide naturally into distinct populations that rarely interbreed with their neighbors. Each population, or "stock," adapts to the conditions of its home river.
The salmon's sense of direction is not perfect, however, and some fish do stray from one river system to the next, enabling surrounding healthy populations to recolonize streams where the salmon runs have been extinguished. This straying tends to take place within certain bounds. Straying behavior, plus other factors including life history, geography, the geology of home streams, and genetics, is involved in the designation of "evolutionarily significant units" by regulators applying the Endangered Species Act. For instance, the coastal rivers from the mouth of the Columbia River south to Santa Cruz, California, are divided into four such units and the rivers of the Columbia Basin into five.
Jim Lichatowich is a biologist who has spent twenty-nine years in salmon management and research. His recent book Salmon Without Rivers describes the roots and evolution of the Pacific salmon crisis.
Seth Zuckerman serves as Ecotrust's circuit rider, reporting on issues of community, economy, and environment throughout Salmon Nation, and is active in watershed restoration on the Mattole River in northwestern California.