Evaluating Minijack Rates in Spring Chinook Salmon
High rates of precocious male maturation of spring Chinook Salmon Oncorhynchus tshawytscha as two-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 the plasma 11-KT levels have 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 non-lethal 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 predict 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 non-lethally in April accurately predicted the maturation status of 99% of male fish confirmed via lethal sampling for gonadosomatic index the following September. Plasma 11-KT levels increased from April to September in both immature and maturing males, though the percent increase in maturing males was 10-times more 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.
Medeiros, L.R., P.F. Galbreath, C.M. Knudsen, C.A. Stockton, I.J. Koch, W.J. Bosch, S.R. Narum, J.J. Nagler and A.L. Pierce. 2018. Evaluating minijack rates in spring Chinook Salmon. Yakima Basin Science and Management Conference, June 13-14, 2018. Ellensburg, WA.