American shad sensitivity to split-beam hydroacoustic monitoring

Split-beam hydroacoustic systems are often employed as a passive technique to estimate runs of migrating fishes. Further study, however, is required to determine what signal frequencies (and intensities) are truly inaudible to various fish species. American shad, among other clupeid fishes, are capable of detecting ultrasonic frequencies and hydroacoustic systems ideal for riverine applications often operate at 200kHz; uncomfortably close to the upper threshold of this fish’s auditory range.  As part of the long-term monitoring effort associated with the Penobscot River Restoration Project, we examined the behavioral responses of upstream migrating American shad to split-beam hydroacoustic signals. Utilizing a DIDSON, we counted the number of fish migrating upriver, through transducer signals of 120, 200 and 420 kHz at multiple source levels between 208 and 220 dB. A two-way ANOVA was then used to test for differences in American shad passage rates in the presence of these various signal frequencies and intensities.  We also provide a qualitative summary of reactions observed. The results of this work are shaping our approach to long-term assessment of the suite of eleven native diadromous fishes that stand to benefit from habitat restoration via dam removal.