Underwater noise mitigation in the Santa Barbara Channel through incentive-based vessel speed reduction

Vanessa M. ZoBell,Lindsey E. Peavey Reeves,Kaitlin E. Frasier,Sean M. Wiggins,John A. Hildebrand,Jessica A. Morten,Sean P. Hastings

doi:10.1038/s41598-021-96506-1

Abstract

Commercial shipping is the dominant source of low-frequency noise in the ocean. It has been shown that the noise radiated by an individual vessel depends upon the vessel’s speed. This study quantified the reduction in source levels (SLs) and sound exposure levels (SELs) for ships participating in two variations of a vessel speed reduction (VSR) program. SLs and SELs of individual ships participating in the program between 2014 and 2017 were statistically lower than non-participating ships (p < 0.001). In the 2018 fleet-based program, there were statistical differences between the SLs and SELs of fleets that participated with varying degrees of cooperation. Significant reductions in SL and SEL relied on cooperation of 25% or more in slowing vessel speed. This analysis highlights how slowing vessel speed to 10 knots or less is an effective method in reducing underwater noise emitted from commercial ships.

Highlights

Commercial shipping is the dominant source of low-frequency noise in the ocean
From 2014 through 2018, paired Automatic Identification System (AIS) and acoustic recordings were extracted for 9297 vessel transits, including all types of vessels travelling through the Santa Barbara Channel (SBC)
Our source levels (SLs) estimates are higher than SLs from other vessel noise studies at Site B because our SL estimates included a transmission loss (TL) model which corrected for surface reflections that occur at sites with low inclination angles (Lloyd’s mirror) between ship near-surface sources and the seafloor-mounted acoustic r ecorder[7,8,22]

Summary

Introduction

Commercial shipping is the dominant source of low-frequency noise in the ocean. It has been shown that the noise radiated by an individual vessel depends upon the vessel’s speed. In regions exposed to ship noise, ambient sound levels have risen over the past several decades due to increases in the number, gross tonnage, and horsepower of commercial v essels[3,4] In addition to these parameters, vessel underwater radiated noise levels and vessel speed are positively correlated, suggesting noise pollution may be mitigated by reducing vessel speed[5,6,7,8]. The SBC is an ideal region for the study of underwater noise pollution due to its position as a basin shielded from deep ocean noise by the presence of the Channel Islands and its proximity to the San Pedro Bay Port Complex (i.e., the ports of Los Angeles and Long Beach), which results in an abundance of low-frequency ambient noise that is directly correlated with commercial vessel traffic (Fig. 1)[14]. Because of the ecologically important habitats in the SBC, noise pollution from commercial vessel traffic is a continuing management concern

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Conclusion