Temporal and Spatial Comparisons of Underwater Sound Signatures of Different Reef Habitats in Moorea Island, French Polynesia

Abstract

As environmental sounds are used by larval fish and crustaceans to locate and orientate towards habitat during settlement, variations in the acoustic signature produced by habitats could provide valuable information about habitat quality, helping larvae to differentiate between potential settlement sites. However, very little is known about how acoustic signatures differ between proximate habitats. This study described within- and between-site differences in the sound spectra of five contiguous habitats at Moorea Island, French Polynesia: the inner reef crest, the barrier reef, the fringing reef, a pass and a coastal mangrove forest. Habitats with coral (inner, barrier and fringing reefs) were characterized by a similar sound spectrum with average intensities ranging from 70 to 78 dB re 1 μPa.Hz(-1). The mangrove forest had a lower sound intensity of 70 dB re 1 μPa.Hz(-1) while the pass was characterized by a higher sound level with an average intensity of 91 dB re 1 μPa.Hz(-1). Habitats showed significantly different intensities for most frequencies, and a decreasing intensity gradient was observed from the reef to the shore. While habitats close to the shore showed no significant diel variation in sound intensities, sound levels increased at the pass during the night and barrier reef during the day. These two habitats also appeared to be louder in the North than in the West. These findings suggest that daily variations in sound intensity and across-reef sound gradients could be a valuable source of information for settling larvae. They also provide further evidence that closely related habitats, separated by less than 1 km, can differ significantly in their spectral composition and that these signatures might be typical and conserved along the coast of Moorea.

Fig 1. Maps of the North (a) and West (b) coasts of Moorea Island.

Locations of the sound recordings for each of the five habitats (red dots = reef crest; blue dots = barrier reef; purple dots = pass; orange dots = fringing reef and green dots = mangrove forest). Maps drawn by the authors from an aerial photograph of Moorea taken by the CRIOBE in 2008 from a private plane. For representative purposes only.

Fig 2. Comparison of sound intensities between day periods.

Results of sampled frequencies (Fast Fourier Transform FFT, 512 points Hamming window, 86.13 Hz resolution, in the 20–5000 Hz range) at the West Coast in 2010 (top), the North Coast in 2010 (middle) and the North coast in 2011 (bottom). Black = P < 0.05. Grey = non-significant difference (Tukey’s multiple comparison post-hoc test). White = no data available.

Fig 3. Average power spectra for the five habitats recorded at the North Coast in 2011.

Recordings were performed during the day (a), at dusk (b) and at night (c). CR = inner reef crest; BR = barrier reef; PA = pass; FR = fringing reef and MG = mangrove forest. Values are mean ± SE for the 20 to 5000 Hz frequency range. Error bars are not shown for each frequency point for CR, BR, FR and MG but are staggered along the spectra lines in order to avoid overlapping lines and ensure greater clarity in their visualisation. All error bars are displayed for PA. Bottom: significant differences between habitats (multiple comparison post-hoc test); black = P < 0.05. Grey = non-significant difference.

Fig 4. Comparison of sound intensities between the North and West Coasts in 2010.

Results of sampled frequencies (Fast Fourier Transform FFT, 512 points Hamming window, 86.13 Hz resolution, in the 20–5000 Hz range). Black = P < 0.05. Grey = non-significant difference (Tukey’s multiple comparison post-hoc test). White = no data available.