**5. Acoustic habitat degradation**

The sound scape in the marine environment is composed of two main sources. The natural source comprising of the physical activities like wind, wave, ice, rain and others and the biological sources as discussed above. The alternate source is the anthropogenic or manmade sources that primarily comprise of distant shipping, seismic surveys for oil and gas sector and the sonar transmissions for military and commercial applications. Additionally, there are industrial activities like deep sea

mining, pile driving, dredging and many more that also contribute significantly to anthropogenic sources in the ocean [24].

The acoustic signals are the only signals that propagate effectively and efficiently underwater, so any disruption of the soundscape causes serious acoustic habitat degradation for the marine species. The marine species adapt very well to the natural sources of the soundscape in their habitat; however, the anthropogenic noise associated with the so-called human development index directly abets the acoustic habitat degradation. The rapid rise in the maritime activities has resulted in massive increase in the ambient noise having serious impact on the marine species' ability to adapt to the changes. The impact varies from minor discomfort to serious injuries and even fatalities and long-term species degradation [8].

The stressors impact the marine animal based on the acoustic characteristics of the noise ranging from the intensity, spectral content, duration, duty cycle and more. The sound propagation characteristics of the underwater medium also have a profound impact on the acoustic characteristics of the signal projected on the animal. The most important of all is the acoustic characteristics of the receptor (the marine animal likely to be impacted). Thus, a comprehensive source-path receiver model needs to be studied for a realistic assessment of the precise acoustic habitat degradation of any stressor on the marine environment. The IOR with its tropical littoral characteristics will have significant influence on the sound propagation. Certain marine species may be directly impacted, while others may get influenced through the ecosystem changes. Most of the conservation studies are species specific and have limited impact due to the dynamic interaction between the multiple components of the ecosystem and the stressors [12].

Among the stressors, the distant shipping, seismic activities due to the oil and gas industry and the sonar transmissions are considered among the primary sources of underwater noise that impact large-scale acoustic habitat degradation. Among these, the distant shipping is the single ubiquitous source of noise source that has widespread implications on the acoustic habitat degradation. The others are transient and localized in nature so can be managed to some extent. The distant shipping has the following characteristics that make it extremely complex, when we look at the management of the stressor:


so-called focusing event very essential for bringing regulatory provisions driven by strong public outcry does not get created due to the slow rise in the low-frequency ambient noise caused by URN [25, 26].

e.It is interesting to note that the acoustic stealth requirements for the naval vessels drives the same technologies and techniques required for managing acoustics habitat degradation. However, the absence of any regulatory provisions the profit hungry merchant marine has avoided implementing any such provisions. The humans being terrestrial animals do not see a direct impact of acoustic habitat degradation of the marine ecosystem on their well-being.

The global merchant marine fleet is directly connected to the economic engines, and so the enhanced global economic growth has translated to rise in the shipping traffic. The term "Noiseonomics" has been coined by Frisk to describe the relationship between ambient noise levels in the sea and global economic trends. His work is based on the assumption that distant shipping is the single ubiquitous source of ambient noise in the ocean, and these assumptions lead to the following hypothesis [27].


The plots in **Figure 4**, as given by Frisk in his work, confirm that the rate of growth in all the three parameters, namely the world GDP, world fleet gross tonnage and the low-frequency ambient noise in the oceans. This closely matches with actual underwater recordings presented by Ross given above.

"Measurements of ambient noise levels, world fleet gross tonnage, and world gross domestic product are plotted as decibel (dB) quantities for the period 1950- 2007. Linear fits to the data for all three quantities show similar slopes of 3.3 dB per decade with high goodness of fit (R2 ) factors."

**Figure 4.** *Long-term trends in ambient noise levels, gross tonnage of the world fleet, and world gross domestic product [27].*

#### **Figure 5.**

*Recent marine mammal stranding along the Indian coast [10]. Top left: 42 ft. Blue whale stranding off the Alibag coast in June 2015. Top left: 50 ft. Bryde whale stranding off the Mumbai coast in January 2016. Below: Over 90 short-finned pilot whales stranding off Tuticorin beach in January 2016.*

The massive maritime infrastructure push in the IOR is creating unregulated activities both within and also on a regional level, thereby causing sustainability concerns in the IOR. The growing global consciousness on environmental degradation is bringing uniform regulatory frameworks across regions, and now India being a signatory to global norms may get constrained by these regulations. Acoustic habitat degradation is a major fallout of the rising maritime activities without comprehensive regulatory framework. The increasing maritime activities are also accompanied by higher noise levels in the ocean. The frequent stranding of marine mammals along the Indian coast is a manifestation of the catastrophic acoustic habitat degradation. **Figure 5** presents recent incidents of stranding that is a manifestation of the severe acoustic habitat degradation. Such stranding is attributable to the navigation failure due to high ambient noise leading to disorientation [10].
