In recent years, there has been increasing concern surrounding the issue of "noise pollution" in our oceans. This refers to the plethora of sounds which are pumped into marine environments by human activity. You might think this does not sound like a serious issue, but, in fact, when you consider underwater drilling, shipping lanes, sonar, the use of explosives, seismic surveys, blast fishing… you start to realise that we expose marine life to an enormous amount of unnatural sound (known as “anthropogenic sound”). This issue received notoriety after several mass stranding of whales and dolphins were linked to noise pollution, but what about other animals? Could noise pollution also be damaging to fish like sharks?
I Hear You
It might surprise you to learn that sharks can hear! But it’s true. In fact, they have a very well-developed sense of hearing. In fact, as sounds travel further in water than in air, sharks can often hear sounds over a longer distance compared to humans - from several kilometers away. It is thought that sharks can perceive sounds with a frequency between 20 Hz and 1.5 kHz, depending on the species. They are especially sensitive to low-frequency sounds, with a peak in perception between 200 and 600 Hz (Casper et al, 2012; Chapuis et al, 2018).
Sharks ears are almost invisible - tiny pores that open on the tops of their heads - but internally, the structure of their ears is very siilar to our's. The bodies of sharks are equal in density to the surrounding water. Therefore, sound waves travel throughout their entire body, until they come into contact with a structure of a different density. This is known as being “acoustically transparent”. In sharks, when sound waves come into contact with a dense organ known as the “otoconia” inside the inner ear, delicate sensory cells (known as “cilia”) sway, which is interpreted by the brain as sound detection (Casper et al, 2012; Chapuis et al, 2018).
Many sharks use sound to forage for food. To learn more, you can check out Move to the Music. Some species of sharks even use sound to communicate. Unlike many terrestrial animals, sharks do not have an organ (aka voicebox) for making sounds, but they use their body-parts to make sounds. For example, great white sharks (Carcharodon carcharias) use “tail slaps” on the surface of the water to compete for food resources (Casper et al, 2012).
What's That Sound?
To understand how noise pollution might affect sharks, scientists have tested their reactions to natural versus artificial sounds played to them in the wild via an underwater microphone. These studies have shown that coastal sharks (including ...............), approach baits in the water much more slowly and less commonly when man-made sounds are being played into their habitat. Similarly, great whites leave the baits more quickly when these noises are playing. These findings confirm that underwater sounds do alter shark behaviour across many different species (Chapuis et al, 2018).
In the natural environment, continuous rhythmic sounds, like wind, waves, bubbles, the flow of schooling fish and the calls of other animals fall within the low-frequency range perceptible to sharks. These sounds are a part of their natural “soundscape” and so do not elicit a startle response. However, anthropogenic sounds are often chaotic and lacking rhythm, with quick variations in intensity and frequency. The researchers hypothesised that these unfamiliar sounds likely acted as a “cue”, which triggered sharks to exhibit “aversion behaviours”. This tells us that anthropogenic noise might repel sharks from food-rich habitats. In the long-run, this could shift their natural ranges and seriously impact upon their health (Chapuis et al, 2018).
Realistically, anthropogenic noise pollution is not going to stop any time soon - international shipping, drilling for oil and military exercises are all seriously big business. Therefore, the only weapon that we have to fight it will be to perform more scientific research into the effects of noise pollution. If we are able to determine that specific anthropogenic sounds adversely affect marine life, then we can petition parliaments to control these activities in some way.
References
Casper BM, Halvorsen MB & Popper AN (2012). Are sharks even bothered by a noisy environment? Advances in Experimental Medicine and Biology, 730. Access online.
Chapuis L, Collin SP, Yopak KE, McCauley RD, Kempster RM, Ryan LA, Schmidt C, Kerr CC, Gennari E, Egeberg CA & Hart NS (2018). The effect of underwater sounds on shark behaviour. Scientific Reports, 9:6924. Access online.
By Sophie A. Maycock for SharkSpeak
