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Part II-Life History and Biology of Steelhead

February 17, 2005

Steelhead or rainbow trout (Oncorhynchus mykiss) are in the Pacific salmon genus, with the Chinook, coho, chum, sockeye and pink salmon. They were placed here by scientists who determined they were taxonomically (naturally related) most like the Pacific salmon, over ten years ago. Prior to that, they were in the genus "Salmo", along with the brown trout and Atlantic salmon. Factually, they belong there, being native to the same waters as the Pacific salmon, evolving along with them.

Steelhead are spring spawners, unlike their salmon cousins, which spawn in the fall. They begin their spawning migration in October and continue all winter, with the peak being in April. The steelhead run is in two stages, the late fall/winter migration and the late winter/spring run. These fall fish enter coastal streams in large numbers only when there is a high water event and are the first to spawn in March or April, when stream temperatures, reach 40ºF. This water temperature also triggers the main spring run, with the peak of spawning being in mid-April. Steelhead continue to spawn into May and I have observed spawning fish in mid-June in the Pere Marquette River. Once spawning is complete, not all steelhead die, unlike their salmon relatives, but rather return to Lake Michigan, regain energy and do spawn multiple times. Steelhead also continue to feed while in streams, another difference from their salmon relatives, making them an easier target for anglers.

The average female will deposit 4,000 eggs in gravel areas. The eggs lie in the gravel for 30 - 40 days incubating and hatch during May and June. Once hatched, the young steelhead begin feeding on zooplankton (microscopic animal life) and aquatic insects (mayflys, stoneflys, caddis flys, etc.). The young steelhead are called "parr" and remain in their natal streams for one, two or three years. Most remain two years (84%), fifteen percent stay one year, while one percent stay three years. The length of their stream residency is dependent on how fast they grow. Steelhead parr must stay in streams until they reach the magical size of eight inches or more, which ensues good survival in the fish-eat-fish world of Lake Michigan. Once they reach size, they "smolt" or out migrate to Lake Michigan. The parr under go a physical change, turning a bright silver, which is their protective coloration for life in Lake Michigan. The peak of the steelhead smolt migration is mid-May and generally associated with a rain (high water) event. In Lake Michigan they feed on terrestrial insects and other fish, growing and gaining energy for the spawning run. Steelhead remain in Lake Michigan from one to four years before returning to their birth streams for the first time. Males tend to return at an earlier age, while females stay longer, as it takes more time and energy to produce eggs. A "typical run" is made up of 5% Age I; 24% Age II; 58% of Age III; and 13% Age IV fish. All of the Age I steelhead are precocious males, which steelheaders call "skippers". This is because of their tendency to jump, jump, jump (or skip) across the top of the water when hooked.

Previously, I stated steelhead can spawn multiple times. How do we determine this fact? Through reading the scales, fish lay down annuli or rings on their scales, much like a tree, as they grow. A skilled scale reader can gain a wealth of information about a steelhead's life by simply "reading" the scale. They can tell if it is a wild or hatchery fish, how many years it spent in Lake Michigan before spawning the first time, and how many times it spawned! The number of repeat spawners ranges from 30 to 40 percent, so if you do release them, they can live to spawn again. The oldest steelhead I ever observed was a female from the Pere Marquette River. She was a wild fish that spent two years in the river before smolting. She then spent three years in Lake Michigan before coming back to spawn for the first time. Finally, when we scale sampled her and "read" her life history, she was coming back to spawn for the fifth time, making her ten years old. Wow! Those are the kind of genes that are most desirable in our wild steelhead populations.

So, if you choose to harvest a steelhead for the table, a male would be the better choice. Go Fish and Bon appetit!


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