Chinook salmon (Oncorhynchus tshawytscha) were introduced to New Zealand, apparently from a single California population, and have been self-sustaining since about 1905. Salmon from the Waimakariri, Rakaia, Rangitata, and Waitaki rivers differed in various life history traits. The proportion of stream-type adults ranged from 29.1 to 75.6% in Rakaia and Waimakariri tributaries, respectively. Average age at maturity not only differed among the major rivers but also between two tributaries within the Rakaia River catchment. Length at age varied among rivers, the differences being most pronounced in older fish. Age structure and size at age combined to produce 91-, and 73-mm differences in mean length of males and females, respectively, among populations. Waitaki River salmon were not only long at age but also heavier for their length than other populations. The timing of entry into freshwater varied by over 1 mo between the Rakaia and Waitaki rivers, and median date of arrival on spawning grounds varied by 16 d between Rakaia and Waimakariri river tributaries. These life history traits are influenced, to varying degrees, by genetic and environmental factors. However, the suite of differences indicates that considerable adaptation to local conditions has occurred in about 20 generations.
Correlated contemporary evolution of life history traits in New Zealand Chinook salmon, Oncorhynchus tshawytscha
