In recent years, soundscapes have been used to evaluate marine and terrestrial environments in order to understand several biotic and abiotic relationships better (Pijanowski et al. 2011). Although it is generally difficult to acoustically assess the marine environment, recent technological advances in equipment and acoustic analysis software have contributed to the knowledge of marine soundscapes and sounds associated with social interactions of different groups of marine organisms (Lammers et al. 2008).

Marine soundscapes are composed of three components: The first component consists of biotic sounds made by the animals themselves, such as fish (Amorim et al. 2006), shrimps (Lammers and Munger 2016), bivalves (Coquereau et al. 2016, Lillis et al. 2016), crabs (Boon et al. 2009), lobsters (Buscaino et al. 2011), sea urchins (Radford et al. 2008) and marine mammals (Frankel 2009). They are associated with several behaviours. The second component is abiotic sounds produced by natural events, such as wind and waves (geophonic). The third component is anthropogenic sounds, such as those of ships.

The soundscape of coastal reefs is important because it is as an orientation for fish larval settlement (Simpson et al. 2004, Radford et al. 2011), crustaceans (Montgomery et al. 2006), molluscs (Lillis et al. 2015, Egglestone et al. 2016) and reef-building corals (Lillis et al. 2016, 2018).

Passive acoustic monitoring (PAM) is a non-invasive and non-destructive observation tool. It has a permanent or long-term remote monitoring capability, providing important information on daily and seasonal patterns (Rountree et al. 2006). Furthermore, PAM can be used as a complementary tool to assess habitat quality and health of ecosystems (Piercy et al. 2014, Harris et al. 2016) and to monitor biodiversity (Kaplan et al. 2015).

Marine protected areas are an effective method for protecting marine biodiversity and habitats. Protected areas of coral reefs, when efficiently and effectively managed, are expected to sustain a high biological diversity and a soundscape composed mainly of biological sounds (Bertucci et al. 2016). Although soundscape studies have been conducted on temperate waters of the South American Atlantic (Sánchez-Gendriz and Padovese 2016, 2017), there are no studies on equatorial Atlantic coastal reef areas. Therefore, the aim of this work was to investigate soundscapes of protected and unprotected Brazilian equatorial coastal reef areas to provide baseline information for future long-term soundscape monitoring programmes that aim to provide information for conservation actions.

Marine soundscapes are composed of physical (e.g. waves), biological (e.g. fish) and anthropogenic components (e.g. ships). Fish sounds are typically clear at frequencies <1 kHz and the sound of benthic invertebrates is clear at >1 kHz (Kennedy et al. 2010). We determined here the soundscape of a marine protected area and of an unprotected area in northern Brazil. Snapping shrimp and six different choruses dominated the soundscape. The choruses ranged from ~200 to 4000 Hz and were likely produced by aggregations of several fish and crustacean species.

Snapping shrimp is a ubiquitous sound source found in all oceans. Depending on the location, they have a varying diurnal snapping activity pattern. Radford et al (2008) found that snapping shrimp dominate the temperate soundscapes of New Zealand at dawn and dusk. In northeastern Brazil, snapping shrimp are more active at dusk and produce a characteristic snap sound of between 1 and 4 kHz, with a dominant frequency of between 2 and 3 kHz, although frequencies can reach 22050 Hz (our sampling limit). This sound was a constant cacophony during the day, but the highest intensity was at night. The “snap” signal has characteristics similar to those found in some marine invertebrates, particularly Alpheus genera, which produce snapping sounds within a wide frequency band: between ~1 and 15 kHz (Schmitz 2002; Coquereau et al. 2016). Snapping shrimp (family Alpheidae) produce the major component of reef noise at frequencies above 2 kHz. The highest intensities occur at the beginning and end of the night in Hawaiian reefs (Lammers and Munger 2016). Another important component of the soundscape is lobster sounds. Species of the genus Palunirus sp. can emit stridulating? sounds at frequencies of between 2 and 5.5 kHz (Mulligan and Fischer 1977); the dominant frequency is between 3.7 and 5.2 kHz (Latha et al. 2005). Kikuchi et al. (2015) suggested that the frequency of stridulating sounds possibly reflects the activity and presence of commercially important lobsters of the same genus.

The “Rap” sound found as an aggregation in Porto de Galinhas during sunset has similar characteristics to those of sounds produced by Ocypodidae (Brachyura). The frequencies are up to 2 kHz (Horch 1975) and dominant frequencies are between 1.1 and 3.2 kHz (Clayton 2001). Males of some Ocypodidae species produce a drumming sound by stridulating their claw during courtship (Mowles et al. 2017). Ocypodidae, Alpheidae and Palinuridae species can be commonly found in our study area, particularly in Porto de Galinhas (Giraldes et al. 2015), as well in other areas along the Pernambuco coast (Coelho et al. 2006, 2007, Barreto et al. 1993). They can contribute to the cacophony of crustaceans found in this study.

The six chorus types detected here have sound characteristics typically representative of fish. They were mainly detected during sunset and after midnight. Chorus III occurred only near the Ilha da Barra reef (MLPZ 1 and 2). It is commonly heard in the study area by divers (authors’ personal observation). This Chorus showed a dominant frequency mean of 414 Hz, which is similar to that of reproductive sounds found in many coral reef damselfish (Pomacentridae) (Mann and Lobel 1997, Maruska et al. 2007, Parmentier and Frederich 2016, Parmentier et al. 2009).

The frequency bandwidth of the choruses varied. It was broad in Choruses I and IV and narrow in Choruses II and VI. Some choruses may therefore have different frequency band distributions and can show spatial and temporal overlapping (Pearson et al. 2016). Due to the absence of vision during the night, the communication sound plays a more important role (Ruppé et al. 2015). The lack of time and chorus frequency partitioning in hours of darkness illustrates the complexity of monitoring different communities of sound-producing fish (Parsons et al. 2016, in Australia).

Similar frequency band of choruses founded in this study have been found in fish detected in marine protected areas in southeastern sub-tropical regions of Brazil (Sánchez-Gendriz and Padovese 2016 ), although they are produced at different times of the day. There was no temporal competition between them, but rather a temporal overlaping observed during the early morning hours (Sánchez-Gendriz and Padovese 2017). This could indicate that fish species emitting this type of signal may have a wide distribution on the Brazilian coast, occurring in both tropical and temperate waters. The timing differences could be related to the migratory patterns of these species.

In this study we present evidence that the acoustic energy of Choruses II, IV, V and VI were higher after the last coastal reef line in Tamandaré. Several habitats showed significantly different energies. There was a decreasing level of energy from the reef towards the coast, showing that closely related habitats separated by 1 km may differ significantly (Bertucci et al. 2015). The imperceptive or low acoustic energy of crustaceans observed in MLPZ 4 and PGA 2 may be related to distance from the reef. Kaplan and Mooney (2016) indicated that the sound of the reef is of low intensity and may not reach distances greater than 1.5 km.

The detection of fish choruses in this area may be associated with the type of substrate at these sites. On coastal reefs on the southern coast of Pernambuco it is possible to find a muddy substrate resulting from river depositions (Kempf 1970). Muddy patches are of commercial interest, as artisanal fishery target these areas for shrimp and fish, catching mainly sciaenids (Silva Júnior et al. 2015). Some of these species can also be found in Brazilian temperate waters (Schmidt and Dias 2012). The genera are widely distributed throughout the western Atlantic.

One of the most interesting and well-known aspects of the Scieanidae family is that they produce sounds and are commonly called “croakers”. Several species of this family produce sounds during the reproductive season, particularly during the night (Lagardère and Mariani 2006, Luczkovich et al. 2008, Mok et al. 2009). Several studies on sciaenid sounds have been conducted in the northwest Atlantic. Field studies using passive acoustics monitored spawning activity of a resident aggregation of Cynoscion nebulosus over a long period (Walters et al. 2009). The critical spawning habitats of Cynoscion regalis, Bairdiella chrysoura and Sciaenops ocellatus have also been mapped (Luczkovich et al. 2008). In the southwest Atlantic, Micropogonias furnieri produces a characteristic seasonal and daily sound of courtship/spawning in the Rio de la Plata estuary from November to March (Tellechea et al. 2011). Larvae of Sciaenidae were highly abundant in areas near our study site (Junior et al. 2011), possibly indicating that the sounds we detected could have been made by species of the family Sciaenidae during the reproductive process.

A comparison of individual calls of Chorus I with those of Larimus breviceps found in the western Atlantic (Fish and Mowbray 1970) showed an acoustic similarity. The harmonic distribution of frequency bands occurred at peak intervals every 100 Hz (Fig. 10). L. breviceps is a widely distributed species with harmonious “hornlike” sounds and frequencies of between 500 and 1000 Hz (Ramcharitar et al. 2006).

Several species of the genus Cynoscion, widely distributed throughout the western Atlantic, are known to emit sounds during the reproductive period. Some species emit a wide range of dominant frequencies, from ~347 to 1046 Hz (Connaughton 1995, Sprague et al. 2000), particularly in the northwest Atlantic. C. gutupaca emits a dominant frequency of 450 Hz in the southwestern Atlantic (Tellechea and Norbis 2012). These differences in frequency could be caused by differences in swim bladder size, which is in turn correlated with fish size (Connaughton et al. 2000). The close relationship between Choruses I, V and VI may also be related to the sound mechanism. Sound production by these fish occurs through a pair of sonic muscles commonly found in males (Chao 1978).

The results show a soundscape composed of crustacean and fish choruses (dominant frequencies <1 kHz). The choruses showed high energy after the last reef line in the protected area but low energy in unprotected areas. Higher levels of acoustic energy in the marine protected area may indicate the importance of these environments to fish populations. This information shows the importance and the usefulness of passive acoustics tools in monitoring and protecting coral reef biodiversity to guarantee sustainable fisheries and improve the management of populations. Greater efforts are still needed in order to improve the identification of the sound sources that compose the soundscape in these areas and in other marine ecosystems, with a potential for fast and non-intrusive biodiversity assessments.