Diversity in sympatric chinook salmon runs: timing, relative fat content and maturation.

Since salmon species commonly undertake long spawning migrations and often have multiple runs in the same river system, this study explored the run timing and physiology of sympatric Chinook salmon Oncorhynchus tshawytscha stocks in the Klamath River Basin, CA. Genetic methods revealed that the Trinity-Salmon Spring run entered first, followed by the Klamath Fall run, and the Lower Basin Fall run. The Trinity-Salmon Fall run entered continuously. There was however, considerable overlap in run timing which was thought may be due a morphological threshold being met. Due to longer off-feeding freshwater residency, the earlier-returning fish have higher metabolic demands during their spawning migration and enter freshwater with greater relative fat reserves, estimated by non-water fraction of liver tissue (NWF) and Relative Weight (Wr). They also arrive less mature (as estimated by gonadosomatic index (GSI), which also places a greater demand on their energy reserves for gonadal development. There was a decrease in relative fat reserves and an increase in maturity by freshwater entry date. At spawning however, the Trinity River Hatchery spring- and fall-run fish, after travelling the same distance, had the same relative fat content and size-specific gonad mass and fecundity. The higher energetic cost associated with the early entry of the spring-run fish, and reduced at-sea feeding period, may be the cause for their smaller average body mass. This study suggests that imminent freshwater residency duration is important in determining the relative fat content, maturity leveland size of returning Chinook salmon. [ABSTRACT FROM AUTHOR]