Cooperative Fish and Wildlife Research Units Program: Montana Fishery
Education, Research and Technical Assistance for Managing Our Natural Resources

Montana Fishery Project

Bull Trout Emigration Study

May 2018 - December 2021


Participating Agencies

  • Avista Corporation

Currently, we have a limited and potentially biased understanding of when juvenile Bull Trout emigrate from the Montana adfluvial streams. This understanding is limited to inferences based on the literature (i.e., what has been documented in other systems), preliminary information from direct tributaries to Lake Pend Oreille, and the timing of fish captured in traps in the Montana adfluvial streams. The literature suggests that juvenile Bull Trout emigration timing can be variable, potentially system specific, and potentially influenced by numerous factors. Of particular interest, numerous adfluvial Bull Trout populations exhibit a large spring emigration associated with the freshet and peak flows. The closest and most well-documented evidence for this comes from Trestle Creek which is a direct tributary to Lake Pend Oreille. Downs et al. (2006) documented substantial spring emigration events in Trestle Creek that have never been observed in the Montana adfluvial streams; albeit, the spring events were predominated by age-0 emigrants which purportedly have survival rates approaching zero. Further information is currently being collected in both Trestle Creek and another direct tributary to Lake Pend Oreille, Granite Creek, using passive integrated transponder (PIT) tagged fish and PIT arrays. Preliminary evidence from these two streams suggests there are not meaningful spring emigration events and that nearly all juvenile fish emigrate during the fall and winter (Ken Bouwens, Idaho Fish and Game, personal communication). Understanding whether or not there is a spring emigration has numerous management implications. First, if it is learned that there is a meaningful spring emigration, efforts could be directed toward learning and increasing capture efficiency during this time of year. Conversely, if there is not a meaningful spring emigration from the Montana adfluvial streams, trap and transport operations could be discontinued during all times of the year except the fall. Under this scenario, the time and money saved would be available to do more prudent activities in the interest of increasing connectivity and otherwise enhancing the adfluvial Bull Trout populations. Developing this understanding will also give managers the information they need to develop design criteria for future traps (e.g., do traps need to be capable of effectively fishing through the spring freshet, or only through flashy fall rain events). This has implications in terms of both cost, and the footprint of future traps. Thus, the objectives of this study are to evaluate capture efficiency of tributary traps in Graves Creek and East Fork Bull River.