Free-falling crushing as an underreported marine construction noise source: field characterization and implications for marine mammal exposure assessment

The increasing scrutiny of anthropogenic noise impacts on marine ecosystems demands a more nuanced understanding of all potential noise sources, and a recent study published in *Correction: Climate-driven phenological shifts and biogeographical constraints of the hydrozoan Velella velella in Mediterranean coastal waters highlights the complexities of accurately assessing these impacts. This new research, focusing on free-falling crushing (FFC) – a method gaining traction as a non-explosive alternative to underwater blasting – underscores a critical gap in our current marine noise pollution assessment practices. While FFC is often presented as a less disruptive alternative, this study, conducted in coastal waters of Zhangzhou, China, demonstrates that it generates significant, and surprisingly variable, underwater noise. The findings are particularly important given the growing prevalence of coastal development and the need for environmentally responsible construction techniques, and further complement research such as Title: "Correction: Climate-driven phenological shifts and biogeographical constraints of the hydrozoan Velella velella in Mediterranean coastal waters" which examines the broader effects of environmental changes on marine life.

The core revelation of this research lies in the substantial event-to-event variability observed in the underwater noise produced by the 35-ton crusher. Even under seemingly stable environmental conditions, measurements of impulsiveness, received levels, and time-frequency characteristics fluctuated considerably during the 64 impacts analyzed. The range of kurtosis-to-time ratios (β/Δt) and single-event sound exposure levels (SEL) highlights the challenge of accurately predicting and mitigating the acoustic impact of FFC. This variability likely stems from source-related differences in the fracturing process itself—slight variations in impact angle, reef composition, or the crushing mechanism can all substantially alter the resulting noise profile. The data, meticulously gathered using calibrated hydrophones at varying ranges and depths, provides a valuable baseline for future acoustic modeling and refinement of marine noise exposure assessments. This work serves as a crucial empirical validation that avoids assuming acoustic negligibility simply because a method eschews explosives.

The implications of these findings extend beyond the specific context of reef removal operations. As coastal development intensifies globally, and as regulators increasingly require comprehensive environmental impact assessments, it becomes increasingly important to account for all potential noise sources—including those that are often overlooked. Current marine noise assessment frameworks often rely on simplified models and standardized noise emission levels for various construction activities. This study demonstrates the limitations of such approaches when dealing with complex, variable processes like FFC. Integrated data ecosystems, like those we champion, are crucial for accurately capturing and analyzing these nuanced acoustic profiles, allowing for more precise predictions of marine mammal exposure and the development of effective mitigation strategies. Moreover, the need for longitudinal studies to understand the long-term effects of repeated FFC events on marine life becomes increasingly apparent.

Looking forward, a critical question arises: how can we develop real-time monitoring and adaptive mitigation techniques for FFC operations? Calibrated hydrophones and advanced signal processing algorithms, deployed during construction, could provide valuable data for adjusting operating parameters to minimize noise emissions—perhaps by altering the drop height of the crusher or employing different fracturing strategies. Further research should also focus on characterizing the specific source mechanisms responsible for the observed variability, facilitating the development of more accurate predictive models. Ultimately, the success of environmentally responsible marine construction hinges on a commitment to rigorous scientific assessment and a willingness to adapt our practices based on validated empirical data; this study marks an important step in that direction.