Transmission loss of Fish Spawning Vocalizations and the Detection Range of Passive Acoustic Sampling in Very Shallow Estuarine Environments

Acoustic signaling in fish is commonly associated with spawning and sound production has been used to identify spawning sites for multiple species. However, the transmission loss of those signals and subsequent ranges at which those sounds can be detected are often not accounted for, confounding the spatial resolution of those studies. We examined transmission loss of spotted seatrout ( Cynoscion nebulosus ) vocalizations in four habitat types (mud, seagrass, channel edges, and structures) within the very shallow (< 10 m) environment of an estuary. Recorded C. nebulosus vocalizations were projected through an underwater speaker while hydrophones measured the sound pressure levels along transects at doubling distances 2–512 m from the source. A linear-log regression was used to compare transmission loss across sites as a function of distance, and a mixed effects model was used to estimate the impact of distance, water depth, temperature, and salinity on the rate of transmission loss. Sounds were rapidly attenuated as the slopes of the regression lines ranged from − 14.9 to − 22.9 and were significantly different among sites. After distance from the source, water depth had the greatest effect on transmission loss in the mixed effects model, although sound is likely propagating through the sediment at all sites. Sound was most quickly attenuated in the channel edge habitat and least attenuated in the mud habitat. Considering the rate of transmission loss, the estimated source level of C. nebulosus sounds, and the background noise level during spawning, the detection range of passive acoustic sampling varied between 44 and 281 m from the source. These results indicate that the spatial precision of passive acoustic sampling of fish within the estuary is < 281 m, but that habitat type and water depth should also be considered when designing sampling protocols and estimating the location of vocalizing fish.