Eliminating Bias in the Study of Barotrauma Associated with Passage Through Hydroturbines Using a Novel Transmitter Design

During hydroturbine passage, rapid decompression is likely to occur causing the gases in the swim bladder and tissues of fish expand.  Rapid gas expansion can result in barotraumas and associated mortality (mortal injury).  In previous laboratory studies, the presence of a negatively buoyant intra-ceolomically implanted acoustic transmitter has been shown to increase susceptibility of mortal injury for juvenile Chinook salmon when exposed to pressure changes present during hydroelectric turbine passage.  This increased susceptibility is likely due, in part, to the combination of increased swim bladder gas volume due to buoyancy compensation for the excess mass of the transmitter, and the reduction of space within the body cavity due to the presence of the transmitter.  A neutrally buoyant, externally attached telemetry tag theoretically would not require the fish to increase the volume of gas within the swim bladder during acclimation, and would not reduce the space within the body cavity.  The objective of this study was to determine the susceptibility of juvenile Chinook salmon tagged with a neutrally buoyant, external tag to mortal injury associated with rapid decompression.  Both untagged and externally tagged fish were exposed to changes in pressure similar to those observed in hydroturbines in the Columbia and Snake Rivers.  Exposed fish were observed for immediate mortality following exposure and were examined for mortal injuries (e.g., ruptured swim bladder, exophthalmia, internal hemorrhaging) at necropsy.  There was no significant difference in the rates of mortal injury for externally tagged and untagged fish exposed to rapid decompression.  The results of the current study support the need for future field studies to evaluate the efficacy of this new tag design to provide unbiased estimates of survival through hydroturbines.