Effects of Wild Juvenile Spring Chinook Salmon on Growth and Abundance of Wild Rainbow Trout

Abstract

We investigated some of the ecological impacts to rainbow trout Oncorhynchus mykiss that could occur by supplementing spring chinook salmon O. tshawytscha in the upper Yakima River basin, Washington. Controlled field experiments conducted in three different streams indicated that presence of wild juvenile spring chinook salmon did not adversely affect growth of wild rainbow trout in high-elevation tributaries. Experiments at two spatial scales, habitat subunit and stream reach scales, were used to detect impacts. In small-enclosure experiments conducted in two tributaries to the Yakima River in 1993 and 1994, specific growth rates (SGRs) of wild rainbow trout paired with wild juvenile spring chinook salmon were not significantly lower than SGRs of their unpaired counterparts (1993: P = 0.360; 1994: P = 0.190). Stream reach experiments in another Yakima River tributary in 1995 also indicated that introductions of wild juvenile spring chinook salmon into 100-m-long enclosures, at a numerical density equal to the preexisting wild rainbow trout, did not adversely affect rainbow trout growth or abundance. The mean fork length (FL) and instantaneous growth rate (IGR) of age-0 wild rainbow trout in stream reach enclosures were unaffected by introduced spring chinook salmon after 7 (FL: P = 0.318) and 14 weeks (FL: P = 0.387, IGR: P = 0.265) in sympatry. Mean fork lengths and IGRs of age-1 rainbow trout were also unaffected by the addition of the spring chinook salmon after 7 weeks (FL: P = 0.553, IGR: P = 0.124) and 14 weeks (FL: P = 0.850, IGR: P = 0.084) of cohabitation. Furthermore, the stream reach experiment showed that spring chinook salmon introduction did not affect rainbow trout abundance (P = 0.298) or biomass (P = 0.538). Site elevation in the stream reach tests appeared to influence rainbow trout size more than the addition of juvenile spring chinook salmon. Site elevation was negatively correlated with length of wild age-0 (P < 0.001) and age-1 (P < 0.001) rainbow trout in October 1995. It appears that rainbow trout and spring chinook salmon partitioned the resources so that impacts were not detected. Our work suggests that rainbow trout have a refuge from interactions with juvenile spring chinook salmon in high-elevation portions of tributaries (e.g., over 700 m).