Variability in Agonistic Behaviour and Salinity Tolerance between and within Two Populations of Juvenile Chinook Salmon, Oncorhynchus tshawytscha, with Contrasting Life Histories

Agonistic behaviour and salinity tolerance were investigated in eight families of chinook salmon, Oncorhynchus tshawytscha, from two populations in the Nanaimo River, British Columbia. From emergence to 3 mo, there were small differences in agonistic behaviour among families within populations, but "First Lake" chinook had strikingly higher levels of agonistic behaviour than salmon from the "lower Nanaimo" population. Lower Nanaimo chinook were larger, lost less weight, and had higher survival after 24 h in 20 ppt and 30 ppt seawater than First Lake salmon. Within populations, families of larger than average size tended to perform better in seawater than families of smaller size. At a common body size, salinity tolerance was similar in salmon from the two populations, but significant differences among families within populations suggest that families may differ in factors other than size that influence salinity tolerance. It was concluded that the greater agonistic behaviour of First Lake chinook has a genetic basic and is related to their longer period of territorial stream residence before seaward migration. By contrast, the two populations exhibited only small differences in salinity tolerance, but significant interfamily variation illustrates the importance of considering family effects in studies of interpopulation differentiation.