MEPS 472:117-127 (2013)  -  DOI: https://doi.org/10.3354/meps10045

ABSTRACT: It has been widely accepted that fish reside within seagrass meadows because of the visual protection and structure provided by the standing biomass. However, for many larger species of estuarine fish, marine mammals that use echolocation to forage represent a significant threat. We hypothesized that seagrasses may serve as an acoustic refuge to fish from dolphin predators by sufficiently attenuating the high-frequency sounds used in echolocation. To test this hypothesis, we measured the attenuation of a 100 kHz acoustic signal with increasing distance into a seagrass meadow. The transmission loss of low-frequency sound energy relevant to fish calls (300 to 500 Hz) was also investigated to address the hypothesis that marine mammals may use passive acoustic detection to locate fish in vegetated substrates. Our results show that seagrasses attenuate high-frequency sounds during summer months and reduce prey detection thresholds by 58 to 88% relative to bare substrates. Also, based on the low-frequency hearing threshold of bottlenose dolphins, we calculated that dolphins are capable of detecting low-frequency fish calls within vegetation at a distance of roughly 2.3 to 4.2 m, which is within the detection threshold of 2.8 to 9.8 m for dolphins using echolocation. This study demonstrates that acoustically complex features, such as seagrass meadows, can significantly alter bioacoustic signal transmission, possibly providing an important seasonal refuge to fish from marine mammal predators.

KEY WORDS: Seagrass meadows · Acoustics · Echolocation