Outline of Salmon Biology

Although much salmon biology is revealed contextually in the various selections, some basic information may be helpful at the start. All six species of Pacific salmons have been reported in California waters, but only three common species are of concern here: chinook (Oncorhynchus tshawytscha ), coho (O. kisutch ), and steel-head (O. mykiss; formerly Salmo gairdneri ). Other species—pink (Oncorhynchus gorbuscha ), chum (O. keta ), and sockeye (O. nerka )—are excluded because they rarely stray as far south as Cali-

fornia. The genus name, Oncorhynchus, refers to a common characteristic of all these salmons: hooked nose, particularly apparent in males during spawning migrations.

Salmon are normally anadromous; that is, they are hatched in freshwater streams, mature in the ocean, and return—commonly to home streams at age three or four—to produce a new generation. Some chinooks and cohos, usually precocious males, return from the ocean at age two. Spawning typically takes place in cool, fast-moving water near riffles that keep eggs oxygenated. Streams with silt-free rocky/gravel substrates are essential for natural reproduction. With minor exceptions, all chinook and coho salmon, and most steelhead, varying from river to river, die soon after spawning.

Chinooks are both the most numerous and the largest California salmon. They are also typically "big river" fish, generally avoiding smaller coastal streams. Within a single river system there may be several distinct spawning runs of chinooks: fall, late fall, winter, and spring. Most California chinooks are fall spawners. Chinooks typically migrate to the ocean a few weeks after emergence from the gravel, while less than four inches long.

Coho salmon spend a year or more in fresh water before migrating to the ocean in the smolt phase. Since they must "summer over" in native streams, it is important that water temperatures not rise above seventy degrees. Therefore, cohos thrive best in cool coastal streams. They mature typically at three years of age and migrate upstream for spawning in fall and winter months.

Steelhead, until very recently, were considered a "cousin" of salmon. They are anadromous. They look like salmon. They spawn like salmon. Like cohos, they thrive in cooler coastal streams. But they exhibit greater variations in individual behavior than do salmon. For example, some steelhead are able to survive several spawning migrations, a trait of Atlantic, not Pacific, salmon. Some steelhead remain in fresh water for three or more seasons before migrating seaward, and a few steelhead offspring mature without ever migrating to sea—becoming, in effect, resident rainbow trout. Steelhead up to nine years old are known.

Spawning migrations of steelhead extend over longer periods than other salmon, sometimes ranging from May until the following spring. Because upstream migrations merge and the numbers wax and wane, almost never stopping completely, it is unrealistic to

divide the runs into distinct seasonal events. This remarkable genetically endowed variability underscores the difficulties of attempting to reproduce viable runs of these fishes by artificial means.