Keep the Noise Down!

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?

Blue shark under a boat (Image Credit: JCP van Uffelen , Source: www.divephotoguide.com)

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 and are able to detect sounds from several kilometers away. Some species of sharks even use sound to communicate; using “tail slaps” on the surface of the water to compete for food resources.

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”, delicate sensory cells (known as “cilia”) sway, which is interpreted by the brain as sound detection (Casper et al, 2012). 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).

To understand how noise pollution might affect sharks, a research team baited sharks towards their boat and tested how they behaved when they played them different sounds through an underwater microphone.

The experimental setup (Chapuis et al, 2018).

A: MP3 player

B: buoy

C: anchoring chain

D: video cameras

E: underwater speaker

F: bait bag

G: weights (Chapuis et al, 2018).

They compared the reaction to artificial sounds of mixed frequencies and intensities, against the calls of orca (Orcinus orca), which are experienced naturally in the marine environment. In Western Australia, they tested seven different species: sicklefin lemon sharks (Negaprion acutidens), bronze whalers (Carcharhinus brachyurus), grey reef sharks (C. amblyrhynchos), dusky sharks (C. obscurus), sandbar sharks (C. plumbeus), scalloped hammerheads (Sphyrna lewini) and zebra sharks (Stegostoma fasciatum), and they additionally tested great white sharks (Carcharodon carcharias) with the same experimental set-up in South Africa (Chapuis et al, 2018). The Australian species were all coastal and/or reef dwelling, whereas great whites are nomadic and wide-ranging.

Effects of noise pollution on sharks (Chapuis et al, 2018)

The scientists discovered that all the coastal sharks approached the bait much less often when both different sounds were played to them, compared to “control” conditions (when no sounds were played). They recorded significantly fewer inquisitive interactions when sounds were being played and the sharks did not make attempts to bite the bait. Whatsmore, when playing the artificial sounds, the sharks were much slower to arrive at the food source. The great whites also arrived late and left quickly when artificial sounds were being played. Basically, despite being tempted in with tasty baits, the sharks were still deterred by unnatural noises. These findings confirm that underwater sounds do alter shark behaviour across many different species, but that the responses can vary between 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.