Emergence Timing Phenology and Early Life History Plasticity in Umatilla River Fall Chinook Salmon

Emergence timing and feed regime were manipulated in a laboratory experiment to examine their effect on life history traits in Umatilla River fall (ocean-type) Chinook salmon (Oncorhynchus tshawytscha).  Fry were ponded on the same day but at three different points in the photoperiod cycle, spanning the natural range of emergence timing in Columbia River Chinook salmon (December – May). Two feed regimes (high growth vs. low growth) were maintained throughout the experiment within each photoperiod. Smolt development was measured monthly via gill Na+/K+-ATPase activity and seawater challenge survival.  Early male maturation as age 1 “microjacks” and age 2 “minijacks” was determined through visual inspection of testes and measurement of plasma 11 ketotestosterone levels in males.  Significant increases in ATPase activity and seawater survival were observed in all treatments as both subyearling (1^st autumn) and yearling smolts (2^nd spring). Feed regime, emergence timing and their interaction impacted both smoltification and early male maturation. In particular, early emergence groups displayed a greater degree of smolt development as subyearlings and had a higher incidence of microjacks compared with other treatments.  These results demonstrate how emergence date and growth at first feeding may significantly influence life-history decisions in wild and hatchery fall Chinook salmon.