Noise pollution, a rising threat
Increasing noise pollution is now becoming a global threat affecting both terrestrial and aquatic ecosystems. Aquatic animals are especially vulnerable to noise pollution. This is because visual cues are absent or limited in aquatic environments and animals mostly depend on sounds to process information to survive and sound travels faster underwater.
Whales and dolphins are especially vulnerable to underwater noise as they depend almost entirely on pulsed and tonal sounds for long-distance communication, finding food, navigation, and sensing their environment. Low-frequency sounds emitted by these fish are prone to interference from similar low-frequency noise produced by ship engines.
Noise can have a negative impact on fish species
Underwater noise from vessels and cavitation noise from propellers can have a negative impact on fish species and force fish to move away from their preferred habitats, lead to changes in their behaviour in terms of their eating and diving habits, can result in temporary hearing loss, high metabolic energy expenditure, and increases in stress hormone levels. Noise interference can force fish to modify the sounds they generate to be heard above the high noise levels. This can lead to energy loss, loss of opportunities for feeding and resting, reduced fitness and disorientation making fish come in close proximity to fishing nets or vessel propellers increasing the risk of injury or mortality.
A number of studies have been conducted on the impact of noise on marine dolphins that stay in unrestricted environments in terms of availability of space. This is not the case with freshwater dolphins that live in rivers, lakes and estuaries where space is restricted and is not very deep. The impact of noise pollution on freshwater fish living in such restricted environments is poorly understood as of now.
This study titled “Interacting effects of vessel noise and shallow river depth elevate metabolic stress in Ganges river dolphins” published in the journal Nature Scientific Reports aims at understanding the impact of underwater noise on dolphins in the Ganga river at Bihar, India.
Gangetic dolphins, the unique, but threatened freshwater fish of India
Gangetic dolphins are one of the important fresh water fish found in the shallow sediment rich waters of the Indus-Ganges-Brahmaputra river basins and have evolved about 24–26 million years ago (Ma). They are blind due to the evolutionary degeneration of the crystalline lens of their eyes and optic nerves due to lack of need to use them in murky river waters.
They have very long ear canals and show the ability of side swimming, which helps them to adapt better to shallow river waters. Dolphins cannot make sounds through their mouths. Rather, they generate clicks through the nasal sacs located above their heads as a type of sonar to locate other type of fish and objects and respond by listening to the returning echoes of these clicks from fish and various objects located in their vicinity.
Gangetic dolphins provide a unique context to study effects of anthropogenic noise in shallow riverine environments with seasonal flow variability. This is even more important in the context of the recently proposed expansion of industrial transportation and recreational waterways in the Ganga-Brahmaputra river basins, which are already under the threat due to declining river flows, fisheries bycatch mortality, targeted killing, poaching, and pollution.
Underwater noise deeply affects the dolphins
The study found that increase in underwater noise due to motorised vessels resulted in major behavioural changes in the Gangetic dolphins and led to alterations in their responses to sounds, strong masking of the communication modes achieved through echolocation clicks, and high metabolic stress. Vessel noise impacts were the strongest at low water depths in the dry-season due to increase in vessel traffic and dolphins suffered from the dual impacts of high underwater noise and declining river discharge leading to declines in fish prey availability.
When vessel traffic and ambient noise levels were low (<4–5 vessels per hour), dolphins compensated for masking of clicks and lost echo perception by enhancing the sounds they produced. However, when vessel traffic and noise levels were above this limit, dolphins suppressed the sounds they made and did not significantly alter their clicks. The dolphins thus lost a lot of energy in enhancing or altering their acoustic activity and click properties when noise remained high. This had a negative impact on their feeding habits and social behaviours.
While waterways are being planned on the Ganga, maintenance dredging is conducted in the dry-season to deepen channels. River dolphins need a combination of deep and shallow water areas in the river for resting, feeding, and other activities. Dredging and underwater noise can affect dolphin habitats both physically and acoustically. While noise levels due to dredging are relatively low and localised, they can still lead to stress among river dolphins by physical and bio geochemically disturbing riverbed sediment and impacting fish prey. For example, field observations reveal that dolphins show a three-fold increase in dive times during dredging indicating stress levels and the tendency to avoid dredged channels.
Saving the river to save the dolphins
The study argues that maintenance of natural river channel depths by providing ‘ecological flow regimes’ will help in sustaining river dolphin populations. This would mean maintaining adequate flows in the river right from upstream irrigation dams and barrages to inflows from tributaries and the need to plan for sustainable water demand management strategies across sectors.
The study argues for the need to minimise the impacts of noise pollution through precautionary conservation approaches and mitigation measures such as reducing waterways intensification through preventing dredging, downscaling vessel traffic to limit underwater noise and technological improvements such as improving propeller efficiency to cut down cavitation noise and improve fuel efficiency for vessels. This would involve assessing trade-offs between efficiency, vessel capacity, and technological improvements on the one hand, and background maintenance costs on the other to reduce and mitigate risks to dolphins from vessel traffic.
The paper ends by arguing that the endangered Ganges river dolphin is a flagship species for river conservation and also India’s “National Aquatic Animal”. Conservation plans need to acknowledge underwater noise as an important and emerging threat, and work towards scientific monitoring and mitigation of noise impacts on the dolphins.
The paper can be accessed here