Barging Effects on Sensory Systems of Chinook Salmon Smolts

Abstract

To avoid mortality caused by passage through dam turbines and spillways, juvenile Chinook salmon Oncorhynchus tshawytscha are annually transported downstream by barge through the federal hydropower system on the Snake and Columbia rivers. Survival of transported fish is higher than that of in-river migrants; however, transported fish experience higher rates of postrelease mortality. Increased mortality could result from a decrease in the ability to detect or avoid predators due to stressors associated with the barge environment. This study examined the effects of barging on juvenile Chinook salmon olfaction and auditory function, two sensory systems involved in predator detection. We focused on dissolved metals known to be toxic to the salmon olfactory system and on the level of noise from the barge, which could impair the auditory system. Experimental groups included animals collected (1) before barge loading (control group), (2) at the Bonneville Dam bypass system (migrant fish), (3) immediately after barge transport, and (4) within 7 d postbarging and at or after 7 d postbarging. Measured concentrations of dissolved metals from the water within the barge were below established water quality criteria for the protection of aquatic life. Moreover, ultrastructural examination of the olfactory epithelium surface showed no evidence of injury to olfactory sensory neurons. Noise in the barge holding tanks had levels up to 136 dB referenced to 1 μPa (root mean square) with primary energy below 400 Hz. Auditory sensitivity was measured using the auditory-evoked potentials (AEP) technique. We found a small but statistically significant threshold shift for fish collected within 7 d postbarging, while in the 7-d-and-later postbarging group the AEP thresholds were similar to the control. Our findings indicate that the olfactory systems of transported Chinook salmon are intact and probably functional, while the auditory sensitivities are compromised with probable recovery.