Acoustic and biological trends on coral reefs off Maui, Hawaii

Abstract

13Coral reefs are characterized by high biodiversity and evidence suggests that reef soundscapes 14 reflect local species assemblages. To investigate how sounds produced on a given reef relate to 15 abiotic and biotic parameters and how that relationship may change over time, an observational 16 study was conducted between September 2014 and January 2016 at seven Hawaiian reefs that 17 varied in coral cover, rugosity, and fish assemblages. The reefs were equipped with temperature 18 loggers and acoustic recording devices that recorded on a 10% duty cycle. Benthic and fish 19 visual survey data were collected four times over the course of the study. On average, reefs 20 ranged from 0 to 80% live coral cover, although changes between surveys were noted, in 21 particular during the major El Niño-related bleaching event of October 2015. Acoustic analyses 22 focused on two frequency bands (50-1200 Hz and 1.8-20.5 kHz) that corresponded to the 23 dominant spectral features of the major sound-producing taxa on these reefs, fish and snapping 24 shrimp, respectively. In the low-frequency band, the presence of humpback whales (December-25 May) was a major contributor to sound level, whereas in the high-frequency band sound level 26 closely tracked water temperature. On shorter timescales, the magnitude of the diel trend in 27 sound production was greater than that of the lunar trend, but both varied in strength among 28 reefs, which may reflect differences in the species assemblages present. Results indicated that the 29 magnitude of the diel trend was related to fish densities at low frequencies and coral cover at 30 high frequencies; however, the strength of these relationships varied by season. Thus, long-term 31 acoustic recordings capture the substantial acoustic variability present in coral-reef ecosystems 32 and provide insight into the presence and relative abundance of sound-producing organisms. 33 34

Listening forward: approaching marine biodiversity assessments using acoustic methods

“…Notably, snapping shrimp patterns (1.5–24 kHz) were not associated with either metric. A 16-month follow-up study at additional Pacific Ocean sites showed similar patterns [ 7 ]. Desiderà et al [ 107 ] found a relationship between sound types (not indices) and taxonomic diversity.…”

“…Calling patterns vary from daily to seasonally, and have substantial spatial variability [ 101 , 102 ]. In some low-latitude sites, fish and invertebrate sounds that are easily detectable in the summer are overwhelmed and masked by the persistent song of humpback whales in the winter [ 7 ].…”

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Figure 1. Molokini reef, Maui, Hawaii, USA, in October 2016 during a prolonged warming event with extended coral bleaching evident, potentially stressing the local community (Photo: M. Kaplan) [ 7 ].

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Coral Reef Monitoring, Reef Assessment Technologies, and Ecosystem-Based Management

“…Acoustic and visual indicators of reef health can be linked across spatial and temporal scales. Sound-pressure level (particularly at frequencies below 2 kHz) can positively correlate with coral cover, invertebrate abundance, and fish diversity (Kennedy et al, 2010;Kaplan et al, 2015Kaplan et al, , 2018Nedelec et al, 2015;Freeman and Freeman, 2016). Several other acoustic indices such as rates of invertebrate snapping sounds (Butler et al, 2017) and the acoustic complexity index (Pieretti et al, 2011) may also correlate with visual measures of diversity (Nedelec et al, 2015;Bertucci et al, 2016), although the generality of these trends is not fully established (Staaterman et al, 2017).…”

“…Future challenges for acoustic monitoring include collating global soundscape databases, identifying, and standardizing the most useful analysis techniques, and improving accessibility for practitioners and managers. Sharing and compiling libraries of recordings will increase the resolution of acoustic monitoring programmes, facilitating more meaningful analyses that account for spatial and temporal variation in reef soundscapes (Kaplan et al, 2018). Engaging local stakeholders, ecosystem managers, and citizen science with coordinated and standardized protocols could also greatly increase the scope of surveying efforts.…”