Juvenile steelhead are known to associate with shiner groups, though they also compete for food. Steelhead form dominance hierarchies within cohorts and aggressively defend feeding territories against all other fish. This study focused on the differential effect of shiner competition on steelhead of different social standing. Survival of subordinate juvenile steelhead was significantly enhanced by the presence of redside shiner under laboratory conditions. A factorial experiment in 80 L tanks examined the relative effects of 0, 3, and 9 shiner at 15�� and 20��C on the growth and survival of 3 juvenile steelhead per tank. No temperature effect was detected and there was no significant difference in steelhead growth though statistical power was low (n=5). The largest steelhead did not die in any treatment and no steelhead died in the presence of 9 shiner. In treatments where no shiner were present mortality in the smallest steelhead was 80% (p-value<0.01). Aggressive interactions between steelhead allowed pathogens to colonize breaks in the skin of stressed fish resulting in death. In 6800 L recirculating stream channels with natural substrate, 10 steelhead were held either alone or with 20 shiner at 15��C. No steelhead died and their growth was not significantly different between treatments, but, in the absence of shiner fin damage was 16 times greater (p-value<0.01) in the smallest three steelhead. If a shiner group was present the smallest steelhead appeared to shoal with shiner to avoid attack by dominant steelhead. / Graduation date: 1999

Three experiments were designed to determine if and how steelhead trout, Oncorhynchus mykiss, may affect chinook salmon, O. tshawytscha, when they are confined together as in a raceway or on a barge. We observed groups of chinook and steelhead together and groups of only chinook in a behavioral experiment to determine if steelhead are aggressive and if their presence changed the behavior of chinook. Two physiological experiments were completed to determine if the loading of steelhead on top of chinook and if the introduction of odor from rainbow trout (steelhead not available) caused a change in plasma cortisol levels in chinook. It was found that chinook showed characteristics of a schooling species, while steelhead exhibited territory holding characteristics. Behavioral changes in chinook were observed when steelhead were present. Chinook grouped with steelhead reduced their movements, darted less, were attacked up to 16 times more often, and were found less frequently in the shade than groups of only chinook. Steelhead were found to establish territories and defend them with chases, charges, and nips. In attempts to establish territories and defend them, steelhead attacked chinook as often as they attacked other steelhead even though chinook showed little aggression toward steelhead. In a physiological experiment, chinook experienced the loading of salmonids into their tank. Chinook had higher levels of plasma cortisol at 2 and 32 hours after the loading of steelhead than chinook that were loaded with chinook or controls (no loading). A second physiological experiment with odor showed that chinook that received rainbow odor and those that received chinook odor had similar levels of plasma cortisol. Cortisol levels (two hours after the odor was introduced) were higher in chinook receiving either of the scented waters than in those that did not receive any odor. In conclusion, all three experiments indicated that the presence of juvenile steelhead trout affect juvenile chinook salmon behavior and physiology. / Graduation date: 1998