The Development of a Reference Collection of Bull Trout Otoliths, Scales and Fin Rays As a Training Tool for Fisheries Biologists

Bull trout are an ESA-listed threatened species which may be vulnerable to climate change, especially non-migratory, headwater resident populations adapted to extremely cold conditions. Life history information (e.g. size-at-age) is of particular interest to biologists and resource managers. Otoliths are the most reliable aging structure, but require lethal sampling. Accordingly, there has been great interest in developing non-lethal aging techniques using scales and fin rays to evaluate bull trout populations. The precision of fin ray aging approaches that of otoliths, while scales are easier to collect but have been shown to be more difficult to read and the least reliable aging structure in bull trout. We obtained archived samples of incidental bull trout mortalities collected by the National Park Service during several years of management and research activities in Sun Creek, Crater Lake National Park.  Using otoliths, fin rays and scales from these fish, we set out to develop protocols, recommendations, and training tools for non-lethal aging to evaluate bull trout populations. The process and products associated with this project will be presented.