Evaluating Minijack Rates in Spring Chinook: Comparing Minijack Rates Based on Spring Plasma 11-ketotestosterone Levels with Rates Based on Fall Gonadosomatic Index
High rates of precocious male maturation of spring Chinook salmon Oncorhynchus tshawytscha as two-year old minijacks have been observed in Columbia River Basin conservation and supplementation hatchery programs. Minijacks are observed in the wild, with rates believed to be between 2-5%, whereas minijack rates measured in hatchery programs range from approximately 8 to 71%. This difference contrasts with the goal of a supplementation program to rebuild depressed fish populations with fish whose physical and behavioral characteristics mimic those of the wild population. Previously, minijack rates have been assessed by lethal blood sampling of age 1+ juveniles, just prior to smolt release in the spring, and assay of plasma levels of 11-ketotestosterone (11-KT), the major androgen in salmonids. However, to our knowledge, no studies have provided confirmation that individual males with elevated springtime plasma 11-KT levels in the spring do in fact complete maturation and are prepared to spawn the next fall. As part of a study to assess the effect of parent age on minijack rate, we PIT tagged and non-lethally blood sampled a portion of spring Chinook smolts in April, and reared these fish to the subsequent spawning season in September. We also evaluated growth and morphometric characteristics to determine whether the growth trajectories of minijacks and immature males differ.
Medeiros, L.R., A. Pierce, C. Knudsen, C. Stockton, B. Bosch, P. Galbreath. 2016. Evaluating Minijack Rates in Spring Chinook: Comparing Minijack Rates Based on Spring Plasma 11-ketotestosterone Levels with Rates Based on Fall Gonadosomatic Index. Northwest Fish Culture Conference, Centrailia, WA.