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Soundings Archive

Shipping Contributes Most Noise To The Oceans, Effects Unknown
The consensus is that the oceans are growing noisier, and that the noise comes from human sources. Concern over the impact of this noise on marine mammals has so far focused on navy sonar and oil-and-gas seismic operations.  More attention should be paid to another noise source: shipping noise.  The National Academy of Sciences recently published a report that discusses this issue in detail:

First, there is no doubt that ships generate noise, principally by propeller cavitation and machinery. Second, it is well known that aging ships tend to generate more noise as mechanical and electrical systems deteriorate over time. Third, newer ships have a number of noise-mitigating characteristics, including quieter diesel-electric propulsion systems and deeper propellers that are less prone to cavitation. Fourth, and most important, the number of ships and gross tonnage of the world fleet have increased substantially since 1950 (Figure 2-9) (McCarthy and Miller, 2002). During this period, the number of ships almost tripled (from 30,000 to 87,000 ships), while the gross tonnage increased by a factor of about 6.5 (from 85 to 550 million gross tons). Interestingly, the logarithmic (dB) equivalent of a factor of 6.5 is about 16 dB, exactly corresponding to the observed increase in low-frequency noise levels [in the oceans]. 1

Other analysts agree with the NAS  that the increase in ocean noise comes predominantly from increased shipping:  “At low frequencies (5 to 500 Hz), commercial shipping is the major contributor to noise in the world’s oceans.”2

The NAS report further concludes that the impact of increased shipping noise on marine mammals is currently unknown:

Regarding anthropogenic noise sources, the previous sections of this chapter show that educated speculation (Ross, 1976; Mazzuca, 2001) and measurements at one location (Andrew et al., 2002) suggest that shipping noise at low frequencies (20-80 Hz) has increased by about 10-15 dB over a 25-50-year period. Although the decrease in detection range associated with this increase in noise can be calculated from a sonar systems perspective, the degree to which this change has an adverse impact on the marine environment is unknown. The change in level itself is not a cause of great concern given that naturally occurring processes can change noise levels by 20-30 dB over short periods (e.g., Plate 1). However, other properties of this increase in shipping noise may be biologically important, such as the increase in the prevalence of noise (decrease in time intervals between shipping-noise-dominated periods or increase in the number of locations where shipping noise is a significant contributor), the character of the shipping-generated signals themselves, and so on. Increases in the number and size of commercial and recreational craft have resulted in noise-level increases substantially greater than 10 dB in some areas (e.g., 30 dB or so in the frequency band from 10 to 100 Hz in Singapore Harbor) (Potter and Delory, 1998), but the potential impact on these environments is unknown. 3

This uncertainty regarding the impact of shipping noise comes in part from the fact that  “[t]he effects of distributed sources, such as shipping and wind, on marine mammals are not yet well modeled.” 4

If marine mammals are to be protected from adverse effects of anthropogenic sound, then the impact of increased shipping noise must be studied as much as the impact of sonar and seismic exploration.  This study will require better sound models and better data.5 

            1           Ocean Noise and Marine Mammals, pages 78-80  (NAS 2003) available online. Click here.

            2           Hildebrand, J., Sources of Anthropogenic Noise in the Marine Environment, pages 1 and 4,  available online. Click here.

            3           Id.

            4           Id. at page 126.

            5           See NAS report, supra footnote 1 at pages 78-80 and 126; Hildebrand supra footnote 2 at pages 1-4.

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