Unmasking Motorboat Noise on Coral Reefs (A Maldivian Case Study)

Abstract

Coral reefs sustain exceptional biodiversity and support coastal economies yet are increasingly threatened by cumulative human pressures, including noise pollution. Sound is a crucial sensory cue for marine life within coral reef ecosystems. Nonetheless, noise pollution from small motorboats introduces low-frequency energy that often overlaps with frequencies used for fish hearing and communication. As such, motorboat passages and prolonged idling near jetties can mask biological signals and compress communication space.

Noise can be reduced through speed and spatial restrictions alongside quieter operating practices yet site-specific evidence is needed to quantify exposure effects and evaluate mitigation. The Maldives provides a valuable case study: tourism depends on small-vessel transfers, concentrating traffic near jetties, while nearby reefs with lighter traffic present contrasting biophony-dominated soundscapes. This juxtaposition enables assessment of motorboat disturbance and mitigation.

We ask three questions: (i) how motorboat passages alter acoustic patterns after exposure, (ii) whether spatial separation improves acoustic conditions, and (iii) whether responses vary with diel phase. We combine Passive Acoustic Monitoring (PAM), ecoacoustic indices (fish-band and full-band ACI, NDSI, Snap Rate) and machine-learning detectors to quantify acute noise responses at a natural reef and evaluate a ~100 m buffer at two resort jetties.

Boat passages suppressed acoustic activity with minute-scale minimum recovery estimates (Fish-ACI ~1.5 min; Full-ACI ~2.1 min), with clearer effects at night. At the jetties, all indices were consistently higher ~100 m from the source, consistent with reduced low-frequency masking. These findings provide field evidence that small spatial separations can rapidly improve acoustic conditions near reef-adjacent operations and demonstrate a scalable workflow for acoustic monitoring and management. Replication across reefs, seasons and lunar cycles, and linking ecoacoustic trends to biological surveys will test generality and connect acoustic change to ecological outcomes.

Date of Award	17 Mar 2026
Original language	English
Awarding Institution	University of Bristol
Supervisor	Hugo B Harrison (Supervisor) & Steve Simpson (Supervisor)