During our whale watching trips, guests on board often ask us: “Why don’t you ´just´ use sonar to look for whales and dolphins?” In this article we attempt to explain what effect this can have on whales and dolphins.
So, what is sonar?
Active sonar is a mechanism where pulses of sounds are emitted by a device, which then listens for its echoes. This may be used to estimate distance and location of an object. In figure 1 you can see an example of sonar from a vessel. Since technology is very advanced, there are different kinds of sonar that humans can make. Navy sonar, for example, can be used to locate submarines and can be heard up to a 1000 kilometres afar, which means that it can be very loud; it could even cause hearing damage to humans (Cummings, 2008).
What is a bio-sonar?
Bio-sonar, used by marine mammals such as whales and dolphins, essentially works the same way as man-made “active sonar” does. Whales and dolphins emit calls and listen for the echoes of those calls that bounce back from objects. The difference is that marine mammals use this mechanism to communicate, navigate and forage, which means that they rely on sound to survive (Schevill & McBride 1956).
Sperm whales (Physeter macrocephalus), for example, which are toothed whales, use echolocation to localize their prey and make the distance estimation accurate. They have the most powerful bio-sonar of the animal kingdom! Particularly when they dive to great depths, below the photic zone where the hunting grounds are preferred (Isojunno & Miller, 2018), they rely on echolocation to localize their prey as there is no light at greater depths.
Effects of active sonar on marine mammals
So, hearing is very important for the survival of marine mammals, and thus, sound coming from elsewhere can disrupt their behaviour. They can be disoriented, completely shocked, or driven away from an important area such a breeding or foraging ground (Cummings, 2008). Additionally, sound that does not come naturally from an ecosystem can confuse them or can prevent them from communicating amongst each other. This phenomenon is called “auditory masking” and “occurs when noise interferes with an animal’s ability to perceive (detect, interpret and/or discriminate) a sound” (DOSITS, 2017). These kinds of effects may seem minor, however these behavioural changes can stress the marine mammals (Mountain, 2012) and may affect their ability to reproduce and avoid predators, because they are too busy avoiding the sound and using up their energy reserves on avoiding the sounds (Doksæter, 2016).
But there are also some other fatal effects of man-made sonar on marine mammals. Diving behaviour has been documented as being disrupted. Beaked whales (Ziphiidae), for example, are deep divers and can reach depths up to 2992 meters (Tyack et al., 2006). When their dives are disrupted by a loud noise, they may be forced to surface too rapidly, which may cause them to have fatal or non-fatal injuries (Cummings, 2008).Consequently, they are frequently affected by the decompression sickness (or symptoms of it), also called diver’s disease, which is caused by the bubbles of gas that enter the blood vessels or any other organ under pressure, i.e. when rapidly ascending from deep waters (with higher pressures) to shallower depths (where pressure is less).
Sometimes whales or dolphins can be found stranded, dead or alive, on a beach. Although the exact reasons of stranding events are not fully understood, multiple stranding events of marine mammals have been related to navy sonar. This may be explained by the intensity of the sound of the man-made sonar. The whale or dolphin may be too close to the sonar, which is then so loud that it causes temporary or permanent hearing damage. This hearing-loss can stress them and disorient them forcing them to strand themselves on the coast. In the Canary Islands, for example, six stranding events related to navy sonar have occurred (Faerber & Baird, 2010). After a stranding event, a necropsy can show damage in the ear canals, becoming evidence of hearing loss.
So, to sum up, using sonar to encounter marine mammals may in theory work to “find them” but, this does more harm than good. The further away we want to search for animals, the louder the noise must be to detect them, and the more damaging it is to the behaviour and even the survival of the animals. So this is why we use the method of look-outs, which is a non invasive and non-disruptive technique of searching for the animals.
If you are interested on learning more about this topic, you can have a look at the following links: SONIC SEA is a documentary explaining in detail what the worldwide effects of sound are on marine mammals
Cummings, J. 2008. AEI FactCheck Navy/NRDC Sonar Debate. Accessed online in February, 2019. http://www.acousticecology.org/srSonarFactCheck.html
Doksæter, L. (2016). Behavioural effects of naval sonars on fish and cetaceans. University of Bergen.
DOSITS, (2017). Discovery of sounds in the sea. Accessed online in February, 2019. https://dosits.org/
Faerber, M.M., Baird, R.W., (2010). Beaked whale strandings in relation to military exercises: a comparison between the Canary and Hawaiian Islands. Marine Mammal Science 26(3):602-613.
Isojunno, S. & Miller, PJ.O. “Movement and Biosonar Behavior During Prey Encounters Indicate That Male Sperm Whales Switch Foraging Strategy With Depth”. Front. Ecol. Evol., (11): 28.
Mooney, A. (2016). Deafening Silence: The Impact of Naval Sonar Activity on Cetaceans.
Mountain, M., 2012. “Navy sonar will impact more sea mammals”. Earth in transition. https://www.earthintransition.org/2012/05/navy-sonar-will-impact-more-sea-mammals/
Schevill, W.E.; McBride, A.F. (1956). “Evidence for echolocation by cetaceans”. Deep-Sea Research. 3(2): 153–154
Tyack, P.L., Mark Johnson, M., Soto, N.A., Sturlese, A., Madsen, P.T. (2006). “Extreme diving of beaked whales”. The Journal of Experimental Biology. 8(209): 4238-4253.