Bull trout (Salvelinus confluentus) are native to many tributaries of the Snake River in southeast Washington. The Washington Department of Wildlife (WDW) and the American Fisheries Society (AFS) have identified bull trout as a species of special concern which means that they may become threatened or endangered by relatively, minor disturbances to their habitat. Steelhead trout/rainbow trout (Oncorhynchus mykiss) and spring chinook salmon (O.tshawytscha) are also native to several tributaries of the Snake river in southeast Washington. These species of migratory fishes are depressed, partially due to the construction of several dams on the lower Snake river. In response to decreased run size, large hatchery program were initiated to produce juvenile steelhead and salmon to supplement repressed tributary stocks, a practice known as supplementation. There is a concern that supplementing streams with artificially high numbers of steelhead and salmon may have an impact on resident bull trout in these streams. Historically, these three species of fish existed together in large numbers, however, the amount of high-quality habitat necessary for reproduction and rearing has been severely reduced in recent years, as compared to historic amounts. The findings of the first year of a two year study aimed at identifying species interactions in southeast Washington streams are presented in this report. Data was collected to assess population dynamics; habitat utilization and preference, feeding habits, fish movement and migration, age, condition, growth, and the spawning requirements of bull trout in each of four streams. A comparison of the indices was then made between the study streams to determine if bull trout differ in the presence of the putative competitor species. Bull trout populations were highest in the Tucannon River (supplemented stream), followed by Mill Creek (unsupplemented stream). Young of the year bull trout utilized riffle and cascade habitat the most in all four streams. Juvenile bull trout utilized scour pool and run habitat the most in all four streams. YOY bull trout preferred plunge pool and scour pool habitat, as did juvenile bull trout in all four streams. These data show that while in the presence of the putative competitors, bull trout prefer the same habitat as in the absence of the putative competitors. Juvenile bull trout preferred mayflies and stoneflies in Mill Creek, while in the presence of the competitor species they preferred caddisflies, stoneflies, and Oligochaeta. It is felt that this difference is due to the differences in food items available and not species interactions, bull trout consume what is present. Adult bull trout were difficult to capture, and therefore it was difficult to determine the migratory habits in the Tucannon River. It is recommended that future studies use radio telemetry to determine the migratory habitat of these fish. The age, condition, and growth rates of bull trout differed only minimally between streams, indicating that if competitive interactions are occurring between these species it is not reflected by: (1) the length at age of bull trout; (2) the length-weight relationship of bull trout; or (3) the rate of growth of bull trout. The spawning habits of bull trout and spring chinook salmon are similar in the Tucannon River, however it was found that they spawn in different river locations. The salmon spawn below river kilometer 83, while 82% of bull trout spawn above that point. The peak of spawning for salmon occurred 10 days before the peak of bull trout spawning, indicating that very little competition for spawning locations occurs between these species in the Tucannon River. Future species interactions study recommendations include the use of electrofishing to enumerate bull trout populations, snorkeling to identify micro-habitat utilization, seasonal diet analysis, and radio transmitters to identify seasonal migration patterns of bull trout.
