Limiting motorboat noise on coral reefs boosts fish reproductive success

Anthropogenic noise is a major global issue affecting human health and wildlife, including birds, amphibians, fishes and invertebrates. Transportation is the primary source of noise above and below water, and traffic noise has widespread detrimental impacts across taxa and ecosystems. Noise levels can be rapidly reduced through changes in human behavior, as observed during COVID-19 activity reductions, suggesting potential benefits for wildlife if managed. Coral reefs are highly biodiverse yet threatened ecosystems with major biodiversity and socioeconomic value, and reef health relies on functionally diverse fish communities. Motorized traffic is a growing source of reef noise that threatens fish throughout their life cycle, impacting development, competition and species interactions, with evidence of negative consequences for survival and reproductive success. This study tests whether limiting motorboat noise on coral reefs increases fish reproductive success and juvenile survival, using a field manipulation on the Great Barrier Reef and a complementary laboratory experiment to isolate the role of sound.

Prior work shows that noise pollution can cause stress, distraction and injury, disrupting biological organization and affecting behavior, physiology, reproduction and survival across taxa. Transportation noise is a dominant source above and below water. In coral reef fishes, traffic noise has been shown to affect early life history, orientation behavior, parental behavior, and offspring survival. Natural and anthropogenic soundscapes influence key processes such as settlement and recruitment. While some fishes can show increased tolerance with repeated exposure, this is not universal and not observed in all species. Managing local anthropogenic stressors, such as motorboat noise, is a component of resilience-based management for coral reefs.

Field study: Conducted at Lizard Island Research Station, Great Barrier Reef, Australia (14° 40′ S; 145° 28′ E) over a full spiny chromis (Acanthochromis polyacanthus) breeding season (29 Oct 2017–20 Jan 2018). Reefs were assigned to two treatments: limited-boating reefs (motorized vessels requested to avoid reefs or approach slowly and anchor >200 m away; low-speed/no-wake access when necessary) and matched busy-boating reefs (motorboats used for approximately 1.25 h per day; typical exposure included boat passes at 10–30 m from reef edge). Acoustic measurements of sound pressure and particle motion were made in both field and tank environments. Wild breeding pairs and nests were monitored throughout the season to assess timing of breeding, brood size near hatching, predator presence, juvenile survival and growth. Eggs were laid in caves and thus not observable in situ; juvenile counts commenced shortly after hatching when juveniles emerged above the substrate. Juvenile survival was assessed by repeated counts every 4–8 days until no longer seen alive; juvenile size was measured at a subset of nests by capturing up to 10 juveniles per nest weekly and measuring standard length under magnification. Predator presence was quantified using GoPro video at a subset of nests. Statistical analyses included Chi-square tests, Welch’s t-tests, linear and generalized linear mixed-effects models with treatment, age, and initial brood size as fixed effects, and nest and site as random effects; survival analyses included Kaplan–Meier curves and Cox models. Laboratory study: Conducted at MARFU, James Cook University, Townsville (Mar–Jul 2018). Breeding pairs and broods of spiny chromis were exposed to controlled acoustic playbacks: busy-boating (intermittent motorboat noise) versus no-boating (ambient reef sound). Embryonic and juvenile development were monitored, including egg/embryo metrics (egg area, yolk sac area, dry weight, dorsal spine length) at days 1 and 10, hatching success, and juvenile growth (standard length at 21 and 42 days post-hatching). Parental care behaviors (egg-fanning rates and parental activity) were video-recorded during baseline quiet periods and during noise playback. Acoustic fields in tanks were characterized for pressure and particle acceleration. Analyses used LMMs and Welch’s t-tests to evaluate treatment effects and interactions with age.

• Reproductive output: Similar numbers of breeding pairs produced offspring on limited-boating (N = 46) and busy-boating (N = 40) reefs, but pairs on limited-boating reefs were almost twice as likely to have surviving offspring by season’s end (e.g., Chi-square test: χ² = 4.67, p = 0.031). Survival curve analyses indicated strong treatment effects (Cox model χ² = 34.87, p < 0.001).
• Hatching and brood size: No evidence that timing of breeding or clutch size differed between treatments; hatching success was equivalent in captivity. In the wild, initial brood sizes showed a trend toward fewer hatchlings on limited-boating reefs compared to busy-boating (mean ± SE: 113 ± 12 vs 139 ± 9; t(53) = 1.81, p = 0.076). Predator presence did not differ significantly between treatments.
• Post-hatching survival: Enhanced juvenile survival drove higher end-of-season success on limited-boating reefs. Survival depended on an interaction between treatment and hatch count, with better survival for smaller broods under limited-boating; overall survival was higher in limited-boating than busy-boating conditions (e.g., X² values reported < 0.001). In captivity, survival was highest for smaller broods.
• Juvenile growth: Offspring in limited/no-boating conditions grew faster. Wild: treatment × age interaction for standard length (LMM χ² = 8.97, p = 0.033; effect size ≈ 0.004 ± 0.001 mm/day). Captivity: treatment × age interaction (LMM χ² = 7.95, p = 0.005; effect ≈ 0.09 ± 0.04 mm/day). Body weight increased from day 21 to 42 in captivity without significant treatment effects.
• Embryonic development: By day 10 post-fertilization, embryos from no-boating treatment were longer (e.g., dorsal spine/standard length differences: LMM X² = 8.10, p = 0.004; also reported χ² = 21.80, p = 0.004) and had larger yolk sac area (LMM X² = 11.19, p < 0.001). Development time and initial egg metrics at laying (area, yolk sac area, dry weight) were similar between treatments.
• Parental care: Protection from motorboat noise increased parental egg-fanning; onset of motorboat playback reduced fanning (mean difference ≈ 9 min⁻¹; Welch’s t = 4.24, p < 0.001). Parental activity showed context-dependent differences, with trends for greater activity in quiet and increases in activity in busy-boating tanks at noise onset to similar levels as no-boating. Overall, limiting motorboat noise increased the proportion of nests with surviving offspring, improved post-hatching survival and juvenile growth, and enhanced embryonic development and parental care.

Complementary field and laboratory experiments demonstrate that limiting motorboat activity on coral reefs increases fish reproductive success and juvenile survival. In the wild, more nests produced viable offspring under limited-boating, with improved within-nest survival and faster juvenile growth. In the laboratory, isolating sound as the disturbance showed that protection from motorboat noise increased parental egg-fanning, improved embryonic development (longer embryos with larger yolk sacs), and enhanced juvenile growth, indicating direct effects of noise on offspring and indirect effects via altered parental care. These findings align with broader evidence that chronic anthropogenic noise compromises behavior, physiology, reproduction and survival across taxa. The benefits of noise mitigation are unlikely to be species-specific and could contribute to resilience-based management by reducing local anthropogenic stressors. Although some fishes can show tolerance to repeated noise exposure, responses are species- and context-dependent and not universal. By increasing adult reproductive output and offspring growth, noise mitigation has the potential to deliver population-level benefits and enhance coral reef ecosystem resilience.

Protecting coral reefs from motorboat noise nearly doubled the likelihood that fish nests produced surviving juveniles, improved post-hatching survival, accelerated juvenile growth, and enhanced embryonic development and parental care. Together, field and laboratory evidence identify motorboat noise as a manageable local stressor and demonstrate that noise mitigation can bolster reproductive success with potential population-level benefits. Management actions that limit motorized traffic near reefs represent a practical tool to support reef fish populations and ecosystem resilience. Future work should quantify long-term, population-level outcomes across multiple species and reef systems, and refine best practices for implementing noise mitigation in conservation and fisheries management.

• In the wild, eggs were laid in caves and could not be observed directly, preventing assessment of clutch characteristics and embryonic development in situ.
• Predator presence metrics were heavily confounded in the analyzed videos, limiting inference about predation pressure differences between treatments.
• Reliability of juvenile counts by snorkelers decreased when broods exceeded ~20 individuals, introducing uncertainty in some survival estimates.
• Laboratory findings were based on acoustic playbacks that may not capture all aspects of real-world soundscapes.