Geographic Variation in Size and Age of North American Chinook Salmon

Patterns of mean length and length at age were characterized and compared for 108 populations of chinook salmon Oncorhynchus tshawytscha located between 42 and 65° north latitude in western North America. Mean overall fish length in populations ranged over 45.5 cm for adult males and 28.8 cm for females. Mean length at a given marine age and freshwater life history type varied by up to 26 cm. Mean length at marine age did not differ between stream‐type and ocean‐type life histories, which indicates that differences in size and date of ocean entry had little effect on final size. Hatchery‐reared fish were significantly smaller at a given age than naturally produced fish. Chinook salmon from the Kenai, Kitsumkalum, and Wannock rivers had the largest total mean lengths of the 108 populations, and were also the largest at marine ages 4 and 5. These populations appear to represent extremes in both body size and reproductive life history. There are many selective advantages of large body size, but no consistent relationship was detected between latitude or migration distance and length. Therefore, local adaptations of body size in chinook salmon are probably the result of other factors in the freshwater portion of their life history. This large variation in length among populations further emphasizes the need for stock‐specific fisheries management and nonselective fisheries to protect unique stocks of chinook salmon and diversity within these stocks.