Carson Jeffres, Senior Research Associate, Center for Watershed Sciences, University of California – Davis
Migration to and from the sea (anadromy) is the iconic pattern we associate with Pacific salmon. They spend most of their life in the ocean, taking advantage of its productivity to grow and mature. These adults return upstream to spawn in freshwater and die. Their offspring encounter fewer predators in freshwater allowing more time grow to the point they are ready to migrate to the ocean. This freshwater to ocean to freshwater life cycle is documented in myriad scientific articles, books and films. But some Chinook salmon are not sticking to this simple script, as seen on the Shasta River in northern California.
Pacific salmon life history strategies—the script Pacific salmon follow during their life–can vary. This variability allows salmon to adapt to a wide range of conditions and disperse broadly in the northern Pacific. Factors such as water temperature, parental spawning timing, nutrients, and food sources can shape these life history strategies. A large part of the “decision” as to what strategy to pursue likely happens annually in response to environmental conditions: fish “decide” to either remain in freshwater, migrate to the ocean, return to freshwater to spawn or remain in the ocean for another year.
Chinook salmon fry (i.e., the freshwater young) generally use one of two juvenile life history strategies: migrate to the ocean in their first year (ocean-type), or remain in freshwater and migrate during the second year or later (stream-type). In rare cases, stream-type Chinook fry may also choose a third strategy: mature enough to reproduce during their first or second summer in fresh water and then spawn with adults returning from the ocean. This happens exclusively in males. Descriptions of Chinook male parr (i.e., juvenile salmon rearing in freshwater) that mature to reproduce in their first summer in the wild are uncommon (Rutter 1903, Rich 1920, Gebhards 1960, Flain 1970, Mullan et al. 1992).
For wild Chinook salmon on the Shasta River, however, mature parr seem common. The video below shows parr spawning with an adult female. When spawning begins, you will see mature parr sneak between the males and a female to fertilize the eggs. At the end of the clip, when all the adult males leave, you will see three mature parr immediately below the female, a location where fertilization success could be very high.
Video: Salmon spawning Big Springs Creek, a tributary to the Shasta River. Mature male parr are highlighted during the spawning activities.
The mature parr life strategy in the Shasta River may be an important evolutionary adaptation. In systems where migrations are long and migratory conditions are poor due to water quality, disease, and barriers (e.g., dams) the mature parr life strategy may be a form of reproductive bet hedging. If parr remain in the Shasta River, where habitat conditions are suitable for over-summer rearing, and then spawn with fish returning the following fall, they avoid the risk of high mortality during the difficult migration to the ocean down the Klamath River, with its degraded habitat. This strategy also increases the genetic diversity between cohorts (i.e., groups of fish spawned in particular year) because the mature parr were spawned in a different year than the adults returning from the ocean. Intriguingly, if these mature parr survive spawning, as sometimes seen in hatcheries (Robertson 1957, Unwin et al. 1999), they may continue to grow and spawn again in a subsequent year, unlike the typical spawn-once-and-die approach, further hedging their genetic bets.
The simplistic picture of the salmon life cycle—hatch in freshwater, migrate to the ocean, grow, return to freshwater, spawn and die—under-represents their remarkable flexibility. Variations on this theme, such as juveniles that mature and spawn before going to sea, are adaptations that allow salmon to persist under changing environmental conditions. If mature parr are contributing significantly to the population, then this life history strategy may have some role in stabilizing declining populations and promoting persistence of Pacific salmon in streams where they are in trouble.
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Gebhards, S. V. 1960. Biological notes on precocious male Chinook salmon parr in the Salmon River Drainage, Idaho. Progressive Fish Culturist 22:121-123.
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Robertson, O. H. 1957. Survival of precociously mature king salmon male parr (Oncorhynchus tshawytscha Juv.) after spawning. California Fish and Game 43:119-130.
Rutter, C. 1903. Natural fistory of the Quinnat salmon. Bulletin of the United States Fish commission.
Unwin, M. J., M. T. Kinnison, and T. P. Quinn. 1999. Exceptions to semelparity: postmaturation survival, morphology, and energetics of male chinook salmon (Oncorhynchus tshawytscha). Canadian Journal of Fisheries and Aquatic Sciences 56:1172-1181.