EFFECTS OF WILD JUVENILE SPRING CHINOOK SALMON ON GROWTH AND ABUNDANCE OF WILD RAINBOW-TROUT

We investigated some of the ecological impacts to rainbow trout Oncorh ynchus 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 aff ect growth of wild rainbow trout in high-elevation tributaries. Experi ments 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 grow th rates (SGRs) of wild rainbow trout paired with wild juvenile spring chinook salmon were not significantly lower than SGRs of their unpair ed counterparts (1993: P = 0.360; 1994: P = 0.190). Stream reach exper iments in another Yakima River tributary in 1995 also indicated that i ntroductions of wild juvenile spring chinook salmon into 100-m-long en closures, at a numerical density equal to the preexisting wild rainbow trout, did not adversely affect rainbow trout growth or abundance. Th e mean fork length (FL) and instantaneous growth rate (IGR) of age-0 w ild rainbow trout in stream reach enclosures were unaffected by introd uced 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 a ge-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 we eks (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 r ainbow trout size more than the addition of juvenile spring chinook sa lmon. 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 ra inbow trout have a refuge from interactions with juvenile spring chino ok salmon in high-elevation portions of tributaries (e.g., over 700 m) .