Bias in juvenile salmon survival estimates through hydroturbines due to transmitter presence: Implications for hydroturbine operations and management

Passage through hydropower turbines exposes fish to rapid decompression and the risk of barotrauma. The lowest pressures experienced can range from near vapor pressure to well above atmospheric pressure. Pressure related mortality and injury to fish depend upon the differential between the pressure (depth) to which fish are acclimated prior to turbine passage and the lowest pressure experienced during passage. In addition, the presence of surgically implanted transmitters may increase the likelihood of injury during turbine passage. Following acclimation, juvenile Chinook salmon surgically implanted with one of several different transmitters in combination with a PIT tag were exposed to simulated turbine pressure time histories using computer controlled hyper / hypobaric chambers. The nadir of the simulated turbine pressure profiles ranged between 1 and 15 PSI absolute. Rates of mortal injury for test fish were a function of the ratio between acclimation pressure and nadir pressure, and the size of the transmitter. Understanding the relationship between barotrauma and the presence of an implanted transmitter is critical when evaluating survival study results of juvenile salmonids passing through hydroelectric dams. Illustrations are provided of how the presence of transmitters can bias (higher mortality with bigger transmitters) survival estimates of fish passing through hydroturbines.