Contemporary divergence in migratory timing of naturalized populations of chinook salmon, Oncorhynchus tshawytscha, in New Zealand

Hypotheses: Recently established populations, sharing common ancestry but facing different reproductive habitat seasonality, will genetically diverge in the timing of their reproductive migrations. Organism: Exotic populations of chinook salmon (Oncorhynchus tshawytscha) established from common ancestors introduced to South Island of New Zealand between 1901 and 1907. Site of experiments: Early rearing at Silverstream Hatchery, Waimakariri River, New Zealand, followed by release to sea, growth in coastal ocean, and return to freshwater. Methods: We reared early- and late-spawned juveniles from two populations that differ in spawning time in the wild, and released those fish to mature at sea. We obtained dates of migratory return to freshwater, fish age, and experimental group identity from angler captures at river mouths. Analyses involved assessment of population, spawning date, and age at return effects for the current experiment, plus an expanded analysis of data from previous release experiments. Results: After 26 generations, the population that reproduces later in the wild now returns to freshwater an average of 18 days later than the early spawning population. This divergence is likely genetic, given the common-garden nature of our experimental design, and was apparent even after accounting for significant effects of age at return and marginal effects of initial spawning date.