Estimating the number of wild adult fish that return to spawn is a crucial piece of information for gauging the health of a salmonid population, and the effectiveness of any restoration actions. At Lower Granite Dam, we have developed a state-space model to estimate the total number of wild adults passing over the dam. This incorporates data from window counts, a trapping operation and observed PIT tags moving through the fish ladder to estimate the true number of wild fish passing each day.
Once past Lower Granite, we utilize the vast detection array infrastructure (PIT tag arrays, weirs, hatchery ladders, etc.) in the Snake basin to estimate the probability that any particular fish crossing Lower Granite ends their upstream migration in one of the many tributary populations of interest. We developed a multinomial branching model to match the detection infrastructure, and use data from PIT tagged fish, most of whom were tagged at Lower Granite dam. These probabilities can then be expanded across the entire estimated wild population crossing Lower Granite to reconstruct the run of adult Chinook or steelhead.