Examining the Conflict between Smolting and Precocious Male Maturation in Spring (Stream‐Type) Chinook Salmon

Abstract

AbstractPrecocious male maturation is a natural life history strategy for spring Chinook salmon Oncorhynchus tshawytscha. During spawning, precocious males employ a “sneaker” strategy to fertilize eggs in competition with full‐size anadromous adults. Hatchery rearing practices may increase the incidence of this phenotype beyond its natural levels. Previous research reported high rates (>40%) of precocious male maturation at age 2 (minijacks) in the Yakima River spring Chinook salmon supplementation program in Washington State. Minijack rates in wild populations are believed to be less than 5%. We compiled seasonal profiles for size, condition factor (K), gill Na+,K+‐ATPase activity, whole‐body lipid levels, plasma 11‐ketotestosterone (11‐KT), insulin‐like growth factor‐I (IGF‐I), and thyroxine (T4) in minijacks and immature smolts in the hatchery and during out‐migration. In the hatchery, minijacks were larger and had higher K, whole‐body lipid, plasma 11‐KT, and IGF‐I levels than smolts. Plasma T4 and gill Na+,K+‐ATPase activity increased in minijacks in spring, but the levels were slightly lower than those of smolts. Most minijacks are thought to remain resident in headwater streams throughout the summer in preparation for autumn spawning. A subset of these minijacks migrate hundreds of kilometers toward the ocean in the spring, only to reverse course later in the summer in an effort to return to their natal spawning grounds. These migrating minijacks had elevated plasma 11‐KT, IGF‐I, and T4 levels and gill Na+,K+‐ATPase activity. It is generally thought that smoltification and reproductive maturation are mutually exclusive life history events in salmonid fishes. This investigation examined the physiology of a unique phenotype in which smoltification and downstream migration appear to occur in fish that have already initiated the maturation process. These results suggest that hatchery programs with high minijack rates may produce significant numbers of fish that are maladapted for either smoltification or competing on the spawning grounds, and it is likely that they die in the freshwater environment before contributing to subsequent generations.