Early Male Maturation of Summer Chinook Salmon in the Upper Columbia Basin: an Examination of Minijack Rates Among Five Hatchery Release Populations

We have assessed the rate of early male maturation (age 2 minijacks) of hatchery produced summer Chinook salmon in the upper Columbia River Basin since 2008 and have up to three years of data for these populations.  The populations examined include Dryden (BY06-08), Carlton (BY06-08), Similkameen (BY06-08) and Bonaparte (BY08) rearing ponds and Chelan Net Pens (BY06-07). Minijack rates have ranged from 4 to 46 percent among males across sites and years.  Differences in rearing of these populations include seasonal water temperatures, feeding rates and time spent in raceways, acclimation ponds or net pens. Previous studies on spring Chinook have shown that high growth rates at critical periods can provide the opportunity for initiation of early maturation.  Water temperature and feeding rates in particular may contribute to differences in growth rates and contribute to variation in minijack rates.  Also genetic differences may be present across the populations as the Dryden population is sourced from Wenatchee stock of summer Chinook salmon, the fish released from the Chelan Net Pens comes from Wells stock that returned to the hatchery facility there, and the other three populations (Carlton, Similkameen and Bonaparte) come from natural origin summer Chinook salmon that are collected as they pass Wells Dam.  I will present here an examination of differences in rearing and minijack rates across these hatchery release populations.