Effects of frequency-dependent spatial variation in soundscape settlement cues for reef fish larvae

Abstract

The mechanisms that link reef soundscapes to larval fish settlement behaviors are poorly understood, yet the management of threatened reef communities requires we maintain the recruitment processes that recover and sustain populations. Using a field-calibrated sound propagation model, we predicted the transmission loss in the relevant frequency band as a function of range, depth, and azimuth to estimate the spatial heterogeneity in the acoustic cuescape. The model highlighted the frequency- and depth-dependence of the sound fields fishes may encounter, and we predict these complex spatial patterns influence how sounds function as settlement cues. Both modeling and field measurements supported a non-monotonic decline in amplitude with distance from the reef. We modeled acoustic fields created by sounds at frequencies from 2 common soniferous reef-based animals (snapping shrimps and toadfish) and estimated detection spaces of these sounds for larvae of 2 reef fish species. Results demonstrated that larval depth will influence cue availability and amplitude, and these spatial patterns of detection depend on cue frequency and the larval receiver’s auditory sensitivity. Estimated spatial scales of detection coupled with field measurements suggest cue amplitudes might allow some larvae to detect reef-based sounds at a range exposing them to the predicted spatial variation in the acoustic cuescape. In an individual-based model, cues available to even the shortest modeled distances improved settlement success. Our results emphasize the need to consider the frequency- and depth-dependence of the acoustic cues larval fishes encounter to increase understanding of the role of soundscapes in larval settlement.