Behavioral Interaction between Chinook Salmon and Brown Trout Juveniles in a Simulated Stream

The behavioral interaction between underyearling Chinook salmon Oncorhynchus tshawytscha and brown trout Salmo trutta, both exotics in an unnatural sympatry in New Zealand, was tested during spring and summer in a simulated stream containing natural food. Both species were highly territorial and actively defended preferred drift-feeding sites and resting areas in the pool. Species dominance differed with season and was influenced by prior residence. In spring, Chinook salmon were larger (because of their earlier emergence) and socially dominant, but they did not dominate when brown trout had prior residence. In summer, interspecific differences in size no longer existed, and brown trout were always socially dominant, although dominance required more time to establish when Chinook salmon were prior residents. Most aggressive attacks of both species were initiated and won by the dominant fish in both spring and summer. Yearling brown trout in the pool in summer did not influence the interaction between underyearlings. Tactics in maintaining territory differed between species, with brown trout using a more energy-conserving strategy (frequent resting in preferred cover areas) than did Chinook salmon (primarily free-swimming). Brown trout could adversely affect juvenile Chinook salmon populations; therefore, this species should not be added to streams with Chinook salmon.