Temporal and Spatial Patterns in Coral Reef Soundscapes and their Relevance for Larval Fish Orientation

Most coral reef fish adults have limited home ranges, but their pelagic larvae have the potential to disperse over great distances. At the end of the pelagic phase, these larvae must seek appropriate settlement habitat. Which environmental signals do they use to find the reef? It has been suggested that fish larvae utilize a combination of visual, olfactory, and acoustic cues at different ontogenetic stages and different distances from the reef. At least ten experiments in the last decade have tested the response of reef fish larvae to sounds of a coral reef, resulting in more than 650 citations. This dissertation focuses on the potential role of acoustic cues in the orientation behavior of larval reef fish from the open ocean. First, a biophysical model was used to examine the consequences of orientation behavior if larvae could detect acoustic signals from 1-10 km from the reef. When larvae oriented early during ontogeny and from larger distances, they greatly increased their settlement success and settled closer to home. These findings suggest that early orientation is critical to the survival of fish larvae, which must be active agents of their own dispersal. Second, a time-series of coral reef soundscapes was conducted for two nearby coral reefs in the Northern Florida Keys. The reef soundscapes were highly variable over daily, lunar, and seasonal time-scales, and the highest amplitudes coincided with new moons of the wet season - the time when the larvae of most coral reef fish species settle. Interestingly, the wind-based contribution to the soundscape also had a lunar period. Third, an acoustic playback experiment was conducted at Dean’s Blue Hole in the Bahamas, a relatively “quiet” environment. Larvae from Apogonidae (cardinalfish) and Acanthuridae (surgeonfish) families were exposed to reef sounds recorded in the Bahamas and in Florida and played back at ambient levels. The acanthurid species demonstrated no response to the playbacks, but the apogonids exhibited a disruption of their orientation behavior. This finding suggests that apogonids were able to detect the playbacks, but had no directional response, as was anticipated based on previous studies where sounds were broadcast at higher amplitudes. Finally, an acoustic propagation experiment was conducted in the Upper Florida Keys. Both acoustic pressure and particle acceleration diminished gradually with distance from the reef, but the amplitude of the signal, particularly for particle acceleration, was lower than the detection thresholds of most fish larvae. Furthermore, the particle acceleration field (measured 1-1000 m from the reef) was not highly directional, which may restrict the use of acoustic signals to animals that can detect acoustic pressure. These findings suggest that most fish larvae in the pelagic zone near Florida reefs would have a difficult time locating the reef using acoustic cues alone. However, this may not be the case for species with particularly sensitive hearing (e.g., those that can detect acoustic pressure), and for reefs with higher-amplitude soundscapes. The results of this study challenge research from the past decades that demonstrated a clear attraction of larval fishes to sounds played-back at high amplitudes. Further work is needed, specifically hearing thresholds in other fish larvae, and particle acceleration measurements over longer time periods and near additional coral reefs, to determine whether the trends found in the Florida Keys are consistent with other parts of the world