Relationships between boron concentrations and trout in the Firehole River, Wyoming - Historical information and preliminary results of a field study

The Firehole River (FHR) in Yellowstone National Park (YNP) is a world-reno wned recreational fishery that predominantly includes rainbow trout (RBT, O ncorhynchus mykiss) and brown trout (BNT, Salmo trutta). The trout populati ons apparently are closed to immigration and have been self-sustaining sinc e 1955. Inputs from hot springs and geysers increase the temperature and mi neral content of the water, including elevating the boron (B) concentration s to a maximum of similar to 1 mg B/L. Both RBT and BNT reside in warm-wate r reaches, except when the water is extremely warm (greater than or equal t o similar to 25 degrees C) during midsummer. They spawn in late fall and ea rly winter, with documented spawning of BNT in the cold-water reach upstrea m from the Upper Geyser Basin and of RBT in the Lower Geyser Basin reach, w here water temperatures presumably are the warmest; however, successful rec ruitment of RBT in waters containing similar to 1 mg B/L has not been demon strated conclusively. Thus, we began investigating the relationships among temperature, B concentrations, other water-quality parameters, and the dist ribution and reproduction of trout in the FHR in spring 1997. However, atyp ical high water flows and concomitant lower than historical temperatures an d B concentrations during summer 1997 preclude conclusions about avoidance of high B concentrations.