Plasma 11‐Ketotestosterone in Individual Age‐1 Spring Chinook Salmon Males Accurately Predicts Age‐2 Maturation Status

Abstract

AbstractHigh rates of precocious male maturation of spring Chinook Salmon Oncorhynchus tshawytscha as 2‐year‐old minijacks have been observed in Columbia River basin hatchery programs. Previously, minijack rates have been assessed by lethal sampling during the spring prior to release of the age‐1 smolts and measurement of plasma levels of 11‐ketotestosterone (11‐KT), the major androgen in salmonids. Analysis of plasma 11‐KT levels has been used for separation of male fish into two distinct groups: low levels for immature males and high levels for precociously maturing minijacks. However, no published studies have confirmed how accurately plasma 11‐KT levels measured in the spring prior to smolt release correspond to the maturation status of individual males the following fall spawning season. We utilized both lethal and nonlethal sampling to determine when to sample hatchery‐reared juveniles to obtain clear separation between immature and precociously maturing males and PIT‐tagged fish to determine whether plasma 11‐KT levels in the spring accurately predicted an individual's maturation status in the fall. The distribution of plasma 11‐KT values from fish sampled in February was not significantly bimodal, whereas that from fish sampled in April was strongly bimodal. Plasma 11‐KT levels measured nonlethally in April accurately predicted the maturation status of 99% of male fish, confirmed via the gonadosomatic index obtained the following September. Plasma 11‐KT levels increased from April to September in both immature and maturing males, although the percent increase in maturing males was 10 times greater than that experienced by the non‐maturing smolts. Our findings support the use of spring plasma 11‐KT level as an indicator of maturation status in Chinook Salmon juveniles, provided that an appropriate sampling time is selected.