individual populations. However, the genetic structuring has no clear geographic pattern that 

 would allow identification of major subgroups within this area. 



In addition to summer- and winter-run steelhead. there are populations sometimes referred to as 

 fall-run steelhead in the Klamath River Basin. Disagreement exists as to whether these fall-run 

 steelhead should be considered summer-run. winter-run. or a separate entity. During this status 

 review. NMFS considered fall-run steelhead from the Klamath River Basin to be part of the 

 summer run. 



Because most summer-run steelhead populations in the Klamath Mountains Province are 

 substantially depressed and difficult to sample, genetic studies during the expanded status review 

 focused on winter-run steelhead. However, other genetic studies that considered both winter and 

 summer steelhead from other areas have failed to find consistent genetic differences between 

 run-types within individual regions. Therefore, NMFS concludes that all runs of steelhead 

 within the Klamath Mountains Province should be considered part of the same ESU. 



Patterns of ocean migration of salmon and steelhead may reflect reproductive isolation of 

 spawning populations. Chinook salmon populations from south of Cape Blanco are generally 

 considered south-migrating (e.g.. to ocean areas off southern Oregon and California), whereas 

 stocks from north of Cape Blanco are considered north-migrating. Other studies suggest that 

 coho salmon and steelhead from south of Cape Blanco may not be highly migratory, remaining 

 instead in the highly productive oceanic waters off southern Oregon and northern California. 



NMFS is not aware of any direct evidence about the relationship between the anadromous and 

 non-anadromous life history forms of O. mykiss within the Klamath Mountains Province. 

 Although it has been reported that these two life history forms within a geographic area may be 

 more genetically similar to each other than either is to the same form from outside the area, other 

 studies have found evidence for reproductive isolation between anadromous and non- 

 anadromous O. mykiss . NMFS' policy contained in "Pacific Salmon and the Definition of 

 Species under the ESA" states that anadromous and non-anadromous forms should be considered 

 separately if they are reproductively isolated. Reproductive isolation, as previously noted, is a 

 question of degree. NMFS has determined that, until specific information regarding these two 

 life history forms within the Klamath Mountains Province becomes available, non-anadromous 

 fish will not be considered part of the ESU. This determination may be reconsidered if 

 information demonstrating that the two forms share a common gene pool becomes available. 



SPECIES BIOLOGY 



Steelhead exhibit a wide variety of life history strategies. In general, steelhead migrate to the sea 

 after spending 2 years in fresh water and then spend 2 years in the ocean prior to returning to 

 fresh water to spawn. Variations of this pattern are common. Some spawners survive and return 

 to the ocean for 1 or more years between spawning migrations. Some steelhead return to fresh 

 water after only a few months at sea and are termed "half-pounders." having attained the 

 approximate size that inspired this term. Half-pounders generally spend the winter in fresh water 

 and then return to sea for several months before returning to fresh water to spawn. 



Steelhead exhibit several spawning migration strategies. "Summer-run steelhead" enter fresh 

 water between May and October, and begin their spawning migration in a sexually immature 

 state. After several months in fresh water, summer steelhead mature and spawn. "Winter-run 

 steelhead" enter fresh water between November and April with well-developed gonads. In 



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