Acoustic and Biological Trends on Coral Reefs Off Maui, Hawaii
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Coral Reefs DOI 10.1007/s00338-017-1638-x REPORT Acoustic and biological trends on coral reefs off Maui, Hawaii 1 2,3 2 1 Maxwell B. Kaplan • Marc O. Lammers • Eden Zang • T. Aran Mooney Received: 6 May 2017 / Accepted: 31 October 2017 Ó Springer-Verlag GmbH Germany 2017 Abstract Coral reefs are characterized by high biodiver- in the high-frequency band sound level closely tracked sity, and evidence suggests that reef soundscapes reflect water temperature. On shorter timescales, the magnitude of local species assemblages. To investigate how sounds the diel trend in sound production was greater than that of produced on a given reef relate to abiotic and biotic the lunar trend, but both varied in strength among reefs, parameters and how that relationship may change over which may reflect differences in the species assemblages time, an observational study was conducted between present. Results indicated that the magnitude of the diel September 2014 and January 2016 at seven Hawaiian reefs trend was related to fish densities at low frequencies and that varied in coral cover, rugosity, and fish assemblages. coral cover at high frequencies; however, the strength of The reefs were equipped with temperature loggers and these relationships varied by season. Thus, long-term acoustic recording devices that recorded on a 10% duty acoustic recordings capture the substantial acoustic vari- cycle. Benthic and fish visual survey data were collected ability present in coral-reef ecosystems and provide insight four times over the course of the study. On average, reefs into the presence and relative abundance of sound-pro- ranged from 0 to 80% live coral cover, although changes ducing organisms. between surveys were noted, in particular during the major El Nin˜o-related bleaching event of October 2015. Acoustic Keywords Coral reefs Á Soundscapes Á Biodiversity Á analyses focused on two frequency bands (50–1200 and Soniferous 1.8–20.5 kHz) that corresponded to the dominant spectral features of the major sound-producing taxa on these reefs, fish, and snapping shrimp, respectively. In the low-fre- Introduction quency band, the presence of humpback whales (Decem- ber–May) was a major contributor to sound level, whereas Coral reefs vary in their species assemblages in space and time (Parravicini et al. 2013; Williams et al. 2015), and identifying the drivers of this variability has long been a Communicated by Biology Editor Mark R. Patterson focus of the ecological literature. Much effort has gone into characterizing links between biophysical attributes of coral Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00338-017-1638-x) contains supple- reefs and fish species assemblages. Parameters such as mentary material, which is available to authorized users. depth, substrate complexity (rugosity), live coral cover, and coral species richness appear to be important correlates & Maxwell B. Kaplan with fish species richness and abundance (McCormick [email protected] 1994; Friedlander et al. 2003; Messmer et al. 2011; 1 Biology Department, Woods Hole Oceanographic Institution, Komyakova et al. 2013). Woods Hole, MA 02543, USA Just as the biological composition of coral reefs changes 2 Oceanwide Science Institute, Honolulu, HI 96839, USA over time, so too do the associated ambient soundscapes 3 Hawaii Institute of Marine Biology, Kaneohe, HI 96744, (Staaterman et al. 2014; Kaplan et al. 2015; Nedelec et al. USA 2015). For example, in an approximately year-long study 123 Coral Reefs of two Caribbean reefs, sound levels were found to vary on level (Staaterman et al. 2017). For example, a recent effort diel and lunar scales (Staaterman et al. 2014). However, the attempted to apply the acoustic complexity index (ACI) to relationship to species present was not well characterized, recordings of coral reefs (Bertucci et al. 2016), but its which limited understanding of the influence of species utility was not obvious. Higher ACI values were found in assemblages on local soundscapes. Contemporaneous work recordings of marine protected areas (MPAs) compared to sought to link visual survey data to soundscape measure- non-MPAs. However, there were no significant differences ments and found a relationship between the strength of diel between protected and unprotected areas in any visual trends in sound production to fish density and coral cover survey parameter, suggesting that differences in ACI val- on Caribbean reefs (Kaplan et al. 2015). However, that ues between protected and non-protected reefs were not study was relatively short (four months) and was conducted reflective of the species assemblages observed in the study using only three reefs (Kaplan et al. 2015). While others (Bertucci et al. 2016). Furthermore, previous work has have compared acoustic parameters to biophysical vari- shown that these indices may be predominately influenced ables such as coral cover, fish density, and sea state across by snapping shrimp activity, which is a major component several reefs (Nedelec et al. 2014; Bertucci et al. 2016), of coral-reef and temperate soundscapes (Kaplan et al. this has often come with the trade-off of using relatively 2015). At present, more traditional bioacoustic metrics brief recordings that likely overlook the appreciable vari- such as sound pressure level (SPL) and the variability in ation in sound cues over longer timescales (Radford et al. sound level in specific frequency bands over time are likely 2008; Staaterman et al. 2014; Kaplan et al. 2015). to be more robust and easier to comprehend than indices Collectively, these studies present initial characteriza- such as the ACI. tions of some of the putative drivers of this acoustic vari- In addition to biological sounds, anthropogenic noise ability, such as water temperature and the biota present, can modify reef soundscapes in significant ways (e.g., suggesting a link between reef species assemblages and the Kaplan and Mooney 2015). The extent of human activity local soundscape. Individually, however, these studies can and does vary among reefs because of differing degrees often do not adequately characterize the temporal or spatial of remoteness, protection (e.g., areas closed to vessels), and variability that is likely present. For example, the relevant heterogeneous utilization rates. Recent work suggests that factors influencing biological sound production may vary noise from small vessels may increase the predation risk in importance over multiple timescales and among com- for some reef fish (Simpson et al. 2016). Accordingly, these munities of sound-producing organisms (Radford et al. human-mediated elements could also influence biological 2008, 2014). Thus, data collected from several sites over sound production and species assemblages on coral reefs. relatively long timescales are most likely to yield insight To parameterize the factors that might influence sound into the relationships between biodiversity and ambient production on reefs across space, time, and ecological soundscapes. gradients such as live coral cover and fish density, long- Despite the limitations of the aforementioned studies, term assessment of a range of geographically and ecolog- this growing body of work suggests that monitoring the ically disparate reefs is needed. This study measured sounds produced by the diverse array of soniferous coral soundscapes and examined visually observable species reef organisms may be a cost-effective and efficient means assemblages at seven Hawaiian reefs that varied in benthic of assessing reef community assemblages and their changes cover and fish species assemblages over an approximately over time (Lammers et al. 2008; Radford et al. 2014). 16-month period. Here, we present results from visual and Acoustic observations could supplement or reduce the need acoustic surveys of these reefs and describe a new method for frequent, traditional, diver-based visual surveys. How- to quantitatively assess the magnitude of sound production ever, to develop the capability to infer species assemblages on coral reefs. and ecological dynamics from acoustic data (i.e., inverse prediction), it is first necessary to examine the relationship between biological sounds on reefs and fundamental eco- Methods logical parameters, such as fish species richness and abundance and benthic cover. Site selection In recent years, there has been interest in applying acoustic biodiversity metrics developed for terrestrial Reefs were selected for study on the west side of Maui, ecosystems to marine soundscapes (e.g., Parks et al. 2014; Hawaii, in September 2014. The sites were chosen to be Bertucci et al. 2016; Staaterman et al. 2017); however, similar in depth but different in terms of benthic cover, fish there has been little compelling evidence to suggest that species richness and abundance, structural complexity, such metrics provide valuable information not available geographic location, and degree of protection. These from more traditional measurements like sound pressure parameters were assessed in an ad hoc manner during the 123 Coral Reefs site selection period and confirmed ex post using visual macroalgae, crustose coralline algae, turf algae, and surveys described below. Because of an instrument mal- ‘‘other’’ (e.g., bare rock, sand, dead coral, other function, one reef was ultimately excluded from the study, invertebrates). leaving six reefs and one sandy control site (MM17) for To quantify structural complexity, the straight line dis- data analysis (Fig. 1a; Table 1). Of these, one (Ahihi) was tance of the lead line was measured with a fiberglass tape, completely closed to vessel traffic, two were Marine Life and rugosity was then calculated as the ratio of the length Conservation Districts closed to some forms of fishing of the lead line to the length of the straight line distance. (Honolua and Molokini), and one was a Fishery Manage- Belt transect surveys for fish were carried out concur- ment Area closed to the fishing of herbivores (Kahekili). rently. These consisted of four transects (30 m long by 2.5 m on either side of the transect). Start points adjacent to Visual surveys the acoustic recorder were selected randomly. Each fish transect took approximately 10 min to complete.