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

Abstract

High 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.

Dietary tetradecylthioacetic acid supplementation during the fall prevents an increase in body lipid levels but does not influence precocious male maturation rate in juvenile spring Chinook salmon

“…Although the precocious male maturation rate did not differ between Control and TTA groups, approximately one in five juvenile male Chinook salmon in this study had a high 11-KT level in April indicating that precocious maturation was underway. This is at the low end of the range of precocious maturation rate found in other studies using the same stock of fish (Galbreath, Koch, et al, 2020;Galbreath, Stockton, et al, 2020;Medeiros et al, 2018), which may be due to the smaller tanks used in the present study. The proportion of precociously maturing juvenile male progeny in individual families from the same stock of fish was found to range fully from 0%-100% under hatchery rearing conditions (Galbreath, Koch, et al, 2020).…”

“…This is much higher than rates found in wild fish (Larsen et al, 2013; Pearsons et al, 2009). Gonadal development is underway when these fish are released by the hatcheries, over a period ranging from late February to early May of their second year, and results in the loss of these individuals to the fishery (Harstad et al, 2014; Medeiros et al, 2018). The production of large numbers of minijacks detracts from the goals of conservation and supplementation hatcheries, leading to efforts to reduce the number of minijacks produced.…”

Precocious Maturation of Hatchery‐Raised Spring Chinook Salmon as Age‐2 Minijacks Is Not Detectably Affected by Sire Age

“…The bimodality provided high certainty for characterization of the maturation status of the two types of juvenile males: immature males (lower mode) and maturing minijacks (upper mode; Medeiros et al. 2018).…”

“…2006; Medeiros et al. 2018), which we accounted for by including progeny weight as a covariate in all models. Of primary interest, however, was whether the age of the sire used in cross k influenced the probability that each progeny would be a precociously maturing minijack.…”

“…Male fish maturation status was determined from the distribution of plasma 11‐KT levels at the final sampling using a modification of the method described by Medeiros et al. (2018); instead of the dip test, modality was determined using the excess mass test in the “multimode” R package (Ameijeiras‐Alonso et al. 2018).…”