Considering the Soundscape from the Larval Fish Perspective: Predicting the Depth and Frequency Dependence of the Acoustic Cues Received during the Settlement Process

The biogenic soundscape is created by the soniferous activities of individual fish and invertebrates, with each sound having the potential to act as an acoustic cue or signal for an organism in the path of that sound’s propagation. Sounds propagating away from coastal habitats into pelagic waters can be intercepted by larval fishes, who use acoustic cues during settlement. We used a field-calibrated acoustic propagation model from coral reefs in Caribbean Panama to predict the transmission loss of frequencies detectable by fishes. Modelling results highlight that transmission loss is not monotonic as predicted by geometric spreading models typically used. Instead, larval fishes are likely navigating through alternating regions of high and low amplitude sound even as they approach the reef. The soundscape encountered by a fish is dependent on both frequency and depth; lower frequencies provide more directional cues, but there is depth dependence in amplitude variation with range. Another key consideration is whether or not the sound level received by a fish is greater than the frequency-dependent hearing threshold. We combined the modeled transmission losses with source level measurements for a low- and high-frequency source to predict the range to which the propagation patterns might be relevant. We used field measurements of toadfish (Amphicthyses cryptocentrus) calls, the dominant low-frequency sound observed at the study reefs. Snapping shrimps dominantly produce coastal high-frequency sounds, and we measured the snap source level of three species. We tested two species of Alpheidae in a tank setting, in addition to field and tank measurements of a Palaemonid shrimp collected in the U.S. Virgin Islands. We predict that the low-frequency tuning of larval fishes allows a greater distance of detection for fish calls compared to shrimp snaps, supporting the importance of the more directional, low-frequency components of the soundscape in the early life history of fish larvae.