Ground-truthing daily and lunar patterns of coral reef fish call rates on a US Virgin Island reef

Abstract

Coral reefs comprise some of the most biodiverse habitats on the planet. These ecosystems face a range of stressors, making quantifying community assemblages and potential changes vital to effective management. To understand short- and long-term changes in biodiversity and detect early warning signals of decline, new methods for quantifying biodiversity at scale are necessary. Acoustic monitoring techniques have proven useful in observing species activities and biodiversity on coral reefs through aggregate approaches (i.e. energy as a proxy). However, few studies have ground-truthed these acoustic analyses with human-based observations. In this study, we sought to expand these passive acoustic methods by investigating biological sounds and fish call rates on a healthy reef, providing a unique set of human-confirmed, labeled acoustic observations. We analyzed acoustic data from Tektite Reef, St. John, US Virgin Islands, over a 2 mo period. A subset of acoustic files was manually inspected to identify recurring biotic sounds and quantify reef activity throughout the day. We found a high variety of acoustic signals in this soundscape. General patterns of call rates across time conformed to expectations, with dusk and dawn showing important and significantly elevated peaks in soniferous fish activity. The data reflected high variability in call rates across days and lunar phases. Call rates did not correspond to sound pressure levels, suggesting that certain call types may drive crepuscular trends in sound levels while lower-level critical calls, likely key for estimating biodiversity and behavior, may be missed by gross sound level analyses.

Unidentified fish sounds as indicators of coral reef health and comparison to other acoustic methods

“…Yet in other studies, low frequency SPL alone had little relationship with reef composition (Lamont et al, 2022b;Williams et al, 2022) or had partial or mixed success (Bertucci et al, 2016;Dimoff et al, 2021). Additionally, while fish call rates have revealed similar trends to low frequency SPL at a single site, Ferguson et al (2022) showed that the direct relationship between these two variables was not significant. Fewer studies have examined the daily fluctuations in SPL, despite documented increases in fish sound production around dawn and dusk (Winn et al, 1964;Parmentier et al, 2010).…”

“…Summaries of the reef community assessments are briefly described below but can be found in more detail in earlier work (Kaplan et al, 2015;Dinh et al, 2018;Formel et al [in preparation]). Tektite Reef (18.30962 N, 64.72218 W) is considered an acoustically rich reef, correlating with high fish diversity and coral cover (Kaplan et al, 2015;Ferguson et al, 2022; Table 1). Yawzi Point (18.31458 N, 64.72609 W) was selected as an intermediate quality reef and is characterized by slightly lower fish abundances and diversity (Friedlander and Beets, 2008) and generally lower hard coral cover (Edmunds, 2013; Table 1).…”

“…First, sound files were low-pass filtered and decimated to 4 kHz to focus on the lower frequency range typical of fish calls. Nearly all reef fish sounds are below 2000 Hz, particularly between 50 and 1,200 Hz (Tricas and Boyle, 2014;Ferguson et al, 2022;Rice et al, 2022), which was the focus of this study. Spectrograms were then created (128 pt, 32 ms Hamming window, 75% overlap, 1,024 pt FFT size) and visualized in 10-s increments to distinguish characteristically short-duration fish calls (Tricas and Boyle, 2014).…”

“…Sound is a dynamic component of coral reef communities. The two persistent sources of biological sounds on coral reefs are marine invertebrates, particularly snapping shrimp, which occupy the midto high-frequencies (~2-20 kHz) (Au and Banks, 1998), and lower frequency fish sounds, typically below 2 kHz (Tricas and Boyle, 2014;Ferguson et al, 2022). Soundscape signatures can vary on daily (Staaterman et al, 2014;Kaplan et al, 2015), celestial, (Staaterman et al, 2014;Parsons et al, 2016;McWilliam et al, 2017), and seasonal scales (Kaplan et al, 2018), and are biologically important for coral reef animals.…”

Moonlight-driven biological choruses in Hawaiian coral reefs

“…Acoustic monitoring is a reliable and noninvasive method of observing coral reef ecosystems, with increased noise from fish and invertebrates associated with healthier reefs [2,[4][5][6]. A single acoustic sensor in a reef may record hundreds of fish sounds and thousands of invertebrate sounds per minute, providing an aggregated measurement of biological activity which may allow for the early detection of changes in population abundance and behavior [7,8]. Coral reef soundscapes also act as a beacon for recruiting pelagic fish and crustacean larva [9][10][11], and recent restoration efforts have been aided by artificial playback of healthy reef sounds to promote recruitment [12].…”