Coral reef soundscapes may not be detectable far from the reef

Exploring offshore particle motion soundscapes

Fishes and aquatic invertebrates utilize acoustic particle motion for hearing, and some additionally detect sound pressure. Yet, few underwater soundscapes studies report particle motion, which is often assumed to scale predictably with pressure in offshore habitats. This relationship does not always exist for low frequencies or near reflective boundaries. This study compared particle motion and sound pressure from hydrophone arrays near the seafloor at six sites on the U.S. Mid and South Atlantic Outer Continental Shelf and assessed predictability of sound pressure and particle motion levels by environmental indicators (wind, vessels, temperature, currents). Unidentified fish sounds (100-750 Hz) had particle motion magnitudes 4.8-12.6 dB greater than those predicted from single hydrophone (pressure) measurements, indicating that these sounds were received in the near field. Excess particle motion attributed to hydrodynamic flow noise (<100 Hz) was also present at all sites. Most sounds (25th-75th percentile) from other sources received in the far field (vessels, mammals), had measured particle motion within ±3 dB of that predicted from single hydrophone measurements. The results emphasize for offshore soundscapes the importance of particle motion measurement for short-time (1 min) and near field signals, and that pressure measurement is sufficient for long-term (1 year) predictive modeling.

Chatting behind the reef: Fish bioacoustic diversity of tropical back-reefs in Fiji, South Pacific

Back-reef habitats are important and fragile transition zones acting as nurseries for many coral reef fishes. In this framework, Passive Acoustic Monitoring (PAM) can be an important tool to evaluate the diversity and dynamics of sonic fish community. Here, we investigated the diversity, spatial and diel dynamics of fish sounds in back-reef habitats at Makogai Island in Fiji, South Pacific. Synchronized underwater recorders were deployed in 4 bays collecting data for about 4 days. The abundance of 12 different sub-categories of fish sounds were quantified. Signals were acoustically characterized and the level of discrimination between the sub-categories was evaluated by Discrimination Function Analysis. Generalized Additive Models showed that the abundance of signals was related to the bay and the hour. Moreover, the Shannon Diversity and Equitability Indices were calculated using acoustic and visual census data to describe fish biodiversity of each bay. The two bays with greater biodiversity based on visual census also showed a greater acoustic diversity at dawn and night. Our results highlight the importance of PAM to reveal the diversity of fish community in back-reef habitats, providing a baseline to understand future changes in these crucial environments.

Soundscape enrichment increases larval settlement rates for the brooding coral Porites astreoides

Coral reefs, hubs of global biodiversity, are among the world's most imperilled habitats. Healthy coral reefs are characterized by distinctive soundscapes; these environments are rich with sounds produced by fishes and marine invertebrates. Emerging evidence suggests these sounds can be used as orientation and settlement cues for larvae of reef animals. On degraded reefs, these cues may be reduced or absent, impeding the success of larval settlement, which is an essential process for the maintenance and replenishment of reef populations. Here, in a field-based study, we evaluated the effects of enriching the soundscape of a degraded coral reef to increase coral settlement rates. Porites astreoides larvae were exposed to reef sounds using a custom solar-powered acoustic playback system. Porites astreoides settled at significantly higher rates at the acoustically enriched sites, averaging 1.7 times (up to maximum of seven times) more settlement compared with control reef sites without acoustic enrichment. Settlement rates decreased with distance from the speaker but remained higher than control levels at least 30 m from the sound source. These results reveal that acoustic enrichment can facilitate coral larval settlement at reasonable distances, offering a promising new method for scientists, managers and restoration practitioners to rebuild coral reefs.

Atlantic cod (Gadus morhua) larvae are attracted by low-frequency noise simulating that of operating offshore wind farms

The number and size of offshore wind (OW) turbines is increasing rapidly. OW turbines produce continuous, low-frequency noise that could impact marine fish dispersing/migrating through the facilities. Any such impact would be relevant for larval stages, which have limited possibility to swim away from OW facilities. If directional movement of fish larvae at sea is impacted by low-frequency continuous sound is unknown. We observe the behavior of Atlantic cod larvae (N = 89) in response to low-frequency sound while they are drifting in a Norwegian fjord inside transparent drifting chambers. We transmit 100 Hz continuous sound in the fjord, in the intensity range of OW turbines' operational noise, and measure the sound pressure and 3-D particle motion. Half of the larvae (N = 45) are exposed to low-frequency (100 Hz) continuous sound, while the other half (N = 44) are observed under the same conditions but without the sound. Exposure does not affect the routine and maximum swimming speeds or the turning behavior of the larvae. Control larvae orient to the northwest. In contrast, exposed larvae orient towards the source of low-frequency sound and particle motion. This provides a basis to assess how OW might impact dispersal in this species.

Soundscapes as heard by invertebrates and fishes: Particle motion measurements on coral reefs

Coral reef soundscapes are increasingly studied for their ecological uses by invertebrates and fishes, for monitoring habitat quality, and to investigate effects of anthropogenic noise pollution. Few examinations of aquatic soundscapes have reported particle motion levels and variability, despite their relevance to invertebrates and fishes. In this study, ambient particle acceleration was quantified from orthogonal hydrophone arrays over several months at four coral reef sites, which varied in benthic habitat and fish communities. Time-averaged particle acceleration magnitudes were similar across axes, within 3 dB. Temporal trends of particle acceleration corresponded with those of sound pressure, and the strength of diel trends in both metrics significantly correlated with percent coral cover. Higher magnitude particle accelerations diverged further from pressure values, potentially representing sounds recorded in the near field. Particle acceleration levels were also reported for boat and example fish sounds. Comparisons with particle acceleration derived audiograms suggest the greatest capacity of invertebrates and fishes to detect soundscape components below 100 Hz, and poorer detectability of soundscapes by invertebrates compared to fishes. Based on these results, research foci are discussed for which reporting of particle motion is essential, versus those for which sound pressure may suffice.

Repairing recruitment processes with sound technology to accelerate habitat restoration

Humanity's ambitions to revive ecosystems at large scales require solutions to move restoration efforts beyond the small scale. There are increasing calls for technological solutions to reduce costs and facilitate large-scale restoration through the use of emerging technologies using an adaptive process of research and development. We show how technological enrichment of marine soundscapes may provide a solution that repairs the recruitment process to accelerate the recovery of lost marine habitats. This solution would solve the problems of current practice that largely relies upon natural recruitment processes, which carries considerable risk where recruitment is variable or eroded. By combining the literature with laboratory experiments, we describe evidence for "highways of sound" that convey navigable information for dispersing life stages in search for adult habitat. We show that these navigational cues tend to be silenced as their habitat is lost, creating negative feedbacks that hinders restoration. We suggest that reprovisioning soundscapes using underwater technology offers the potential to reverse this feedback and entice target organisms to recruit in greater densities. Collective evidence indicates that the application of soundscape theory and technology may unlock the recruitment potential needed to trigger the recruitment of target organisms and the natural soundscapes they create at large scales.

From the Reef to the Ocean: Revealing the Acoustic Range of the Biophony of a Coral Reef (Moorea Island, French Polynesia)

The ability of different marine species to use acoustic cues to locate reefs is known, but the maximal propagation distance of coral reef sounds is still unknown. Using drifting antennas (made of a floater and an autonomous recorder connected to a hydrophone), six transects were realized from the reef crest up to 10 km in the open ocean on Moorea island (French Polynesia). Benthic invertebrates were the major contributors to the ambient noise, producing acoustic mass phenomena (3.5–5.5 kHz) that could propagate at more than 90 km under flat/calm sea conditions and more than 50 km with an average wind regime of 6 knots. However, fish choruses, with frequencies mainly between 200 and 500 Hz would not propagate at distances greater than 2 km. These distances decreased with increasing wind or ship traffic. Using audiograms of different taxa, we estimated that fish post-larvae and invertebrates likely hear the reef at distances up to 0.5 km and some cetaceans would be able to detect reefs up to more than 17 km. These results are an empirically based validation from an example reef and are essential to understanding the effect of soundscape degradation on different zoological groups.

Soundscape of green turtle foraging habitats in Fiji, South Pacific

The soundscape features of the marine environment provide crucial information about ecosystem health for many species, and they are defined by the local biological, geophysical, and anthropogenic components. In this study, we investigated the soundscape at green turtle neritic foraging habitats in Fiji, South Pacific, with the aims of characterizing the contribution of each component and of comparing the levels of acoustic pressure among sites with different abundances of sea turtles. Four sites were selected at two islands, and one hydrophone was deployed at each site. Generalized additive models highlighted that sound pressure levels (SPLs) at low frequencies (125–250 Hz) were especially affected by wind conditions, while at higher frequencies (>250 Hz) SPLs were mostly influenced by fish and crustacean acoustic activity. Higher abundances of green turtles were found at sites with the highest levels of SPLs and the highest number of acoustic emissions by fishes and crustaceans but were not related to maximum seagrass and macroalgae coverage, or the highest number of fish. The selected coastal habitats have negligible anthropogenic noise, thus this study informs physiological and behavioral studies of the acoustic signatures that sea turtles might target and provides a baseline against which potential impact of soundscape changes on sea turtle spatial abundance and distribution can be evaluated.

Listening forward: approaching marine biodiversity assessments using acoustic methods

Ecosystems and the communities they support are changing at alarmingly rapid rates. Tracking species diversity is vital to managing these stressed habitats. Yet, quantifying and monitoring biodiversity is often challenging, especially in ocean habitats. Given that many animals make sounds, these cues travel efficiently under water, and emerging technologies are increasingly cost-effective, passive acoustics (a long-standing ocean observation method) is now a potential means of quantifying and monitoring marine biodiversity. Properly applying acoustics for biodiversity assessments is vital. Our goal here is to provide a timely consideration of emerging methods using passive acoustics to measure marine biodiversity. We provide a summary of the brief history of using passive acoustics to assess marine biodiversity and community structure, a critical assessment of the challenges faced, and outline recommended practices and considerations for acoustic biodiversity measurements. We focused on temperate and tropical seas, where much of the acoustic biodiversity work has been conducted. Overall, we suggest a cautious approach to applying current acoustic indices to assess marine biodiversity. Key needs are preliminary data and sampling sufficiently to capture the patterns and variability of a habitat. Yet with new analytical tools including source separation and supervised machine learning, there is substantial promise in marine acoustic diversity assessment methods.

Particle motion measurements near a rocky shore off Cabo Frio Island

This paper describes the lessons learned from the experiment BIOCOM'19 carried out in January 2019, in a shallow water bay off the island of Cabo Frio (RJ, Brazil). A dual accelerometer vector sensor hydrophone was deployed for two days, near a rocky shore covered with a significant benthic fauna. The results show that the frequency band above approximately 1.5 kHz is mostly associated with invertebrate biological noise and that the acoustic and the particle motion fields have a similar behavior, following the usual dawn-dusk activity pattern, and a coherent directivity content. At low frequencies, below ∼300 Hz, the acoustic pressure and the particle acceleration fields have significantly different spectral content along time. Many of these differences are due to anthropogenic noise sources related with nearby boating activity, while during quiet periods, they may be attributed to the biological activity from the rocky shore.

Spiny lobster sounds can be detectable over kilometres underwater

The detection ranges of broadband sounds produced by marine invertebrates are not known. To address this deficiency, a linear array of hydrophones was built in a shallow water area to experimentally investigate the propagation features of the sounds from various sizes of European spiny lobsters (Palinurus elephas), recorded between 0.5 and 100 m from the animals. The peak-to-peak source levels (SL, measured at one meter from the animals) varied significantly with body size, the largest spiny lobsters producing SL up to 167 dB re 1 µPa². The sound propagation and its attenuation with the distance were quantified using the array. This permitted estimation of the detection ranges of spiny lobster sounds. Under the high ambient noise conditions recorded in this study, the sounds propagated between 5 and 410 m for the smallest and largest spiny lobsters, respectively. Considering lower ambient noise levels and different realistic propagation conditions, spiny lobster sounds can be detectable up to several kilometres away from the animals, with sounds from the largest individuals propagating over 3 km. Our results demonstrate that sounds produced by P. elephas can be utilized in passive acoustic programs to monitor and survey this vulnerable species at kilometre scale in coastal waters.

A Vector Sensor-Based Acoustic Characterization System for Marine Renewable Energy

NoiseSpotter is a passive acoustic monitoring system that characterizes, classifies, and geo-locates anthropogenic and natural sounds in near real time. It was developed with the primary goal of supporting the evaluation of potential acoustic effects of offshore renewable energy projects. The system consists of a compact array of three acoustic vector sensors, which measures acoustic pressure and the three-dimensional particle velocity vector associated with the propagation of an acoustic wave, thereby inherently providing bearing information to an underwater source of sound. By utilizing an array of three vector sensors, the application of beamforming techniques can provide sound source localization, allowing for characterization of the acoustic signature of specific underwater acoustic sources. Here, performance characteristics of the system are presented, using data from controlled acoustic transmissions in a quiet environment and ambient noise measurements in an energetic tidal channel in the presence of non-acoustic flow noise. Data quality is demonstrated by the ability to reduce non-acoustic flow noise contamination, while system utility is shown by the ability to characterize and localize sources of sound in the underwater environment.

Ontogenetic change in predicted acoustic pressure sensitivity in larval red drum (Sciaenops ocellatus)

Detecting acoustic pressure can improve a fish's survival and fitness through increased sensitivity to environmental sounds. Pressure detection results from interactions between the swim bladder and otoliths. In larval fishes, those interactions change rapidly as growth and development alter bladder dimensions and otolith-bladder distance. We used computed tomography imagery of lab-reared larval red drum (Sciaenops ocellatus) in a finite-element model to assess ontogenetic changes in acoustic pressure sensitivity in response to a plane wave at frequencies within the frequency range of hearing by fishes. We compared the acceleration at points on the sagitta, asteriscus and lapillus when the bladder was air filled with results from models using a water-filled bladder. For larvae of 8.5-18 mm in standard length, the air-filled bladder amplified simulated otolith motion by a factor of 54-3485 times that of a water-filled bladder at 100 Hz. Otolith-bladder distance increased with standard length, which decreased modeled amplification. The concomitant rapid increase in bladder volume partially compensated for the effect of increasing otolith-bladder distance. Calculated resonant frequency of the bladders was between 8750 and 4250 Hz, and resonant frequency decreased with increasing bladder volume. There was a relatively flat frequency dependence of these effects in the audible frequency range, but we found a small increase in amplification with increasing excitation frequency. Using idealized geometry, we found that the larval vertebrae and ribs have negligible influence on bladder motion. Our results help clarify the auditory consequences of ontogenetic changes in bladder morphology and otolith-bladder relationships during larval stages.

Habitat degradation negatively affects auditory settlement behavior of coral reef fishes

Climate change is causing widespread damage to the world’s tropical coral reefs, via increases in cyclones and mass bleaching. Healthy populations of reef fishes facilitate recovery from such events, and recruitment of juvenile fish is influenced by acoustic cues that guide larval orientation, habitat selection, and settlement to reefs. Our matched recordings of Australia’s Great Barrier Reef before and after recent severe degradation demonstrate major changes to natural reef sound. In field experiments using these recordings, we show the potential impact of such acoustic changes. Postdegradation reef sounds were less attractive to young fishes than their predegradation equivalents. Reductions in fish settlement, caused by acoustic changes, may threaten the recovery potential of degraded coral reefs.

Coral reefs are increasingly degraded by climate-induced bleaching and storm damage. Reef recovery relies on recruitment of young fishes for the replenishment of functionally important taxa. Acoustic cues guide the orientation, habitat selection, and settlement of many fishes, but these processes may be impaired if degradation alters reef soundscapes. Here, we report spatiotemporally matched evidence of soundscapes altered by degradation from recordings taken before and after recent severe damage on Australia’s Great Barrier Reef. Postdegradation soundscapes were an average of 15 dB re 1 µPa quieter and had significantly reduced acoustic complexity, richness, and rates of invertebrate snaps compared with their predegradation equivalents. We then used these matched recordings in complementary light-trap and patch-reef experiments to assess responses of wild fish larvae under natural conditions. We show that postdegradation soundscapes were 8% less attractive to presettlement larvae and resulted in 40% less settlement of juvenile fishes than predegradation soundscapes; postdegradation soundscapes were no more attractive than open-ocean sound. However, our experimental design does not allow an estimate of how much attraction and settlement to isolated postdegradation soundscapes might change compared with isolated predegradation soundscapes. Reductions in attraction and settlement were qualitatively similar across and within all trophic guilds and taxonomic groups analyzed. These patterns may lead to declines in fish populations, exacerbating degradation. Acoustic changes might therefore trigger a feedback loop that could impair reef resilience. To understand fully the recovery potential of coral reefs, we must learn to listen.

Singing whales generate high levels of particle motion: implications for acoustic communication and hearing?

Acoustic signals are fundamental to animal communication, and cetaceans are often considered bioacoustic specialists. Nearly all studies of their acoustic communication focus on sound pressure measurements, overlooking the particle motion components of their communication signals. Here we characterized the levels of acoustic particle velocity (and pressure) of song produced by humpback whales. We demonstrate that whales generate acoustic fields that include significant particle velocity components that are detectable over relatively long distances sufficient to play a role in acoustic communication. We show that these signals attenuate predictably in a manner similar to pressure and that direct particle velocity measurements can provide bearings to singing whales. Whales could potentially use such information to determine the distance of signalling animals. Additionally, the vibratory nature of particle velocity may stimulate bone conduction, a hearing modality found in other low-frequency specialized mammals, offering a parsimonious mechanism of acoustic energy transduction into the massive ossicles of whale ears. With substantial concerns regarding the effects of increasing anthropogenic ocean noise and major uncertainties surrounding mysticete hearing, these results highlight both an unexplored pathway that may be available for whale acoustic communication and the need to better understand the biological role of acoustic particle motion.