The speed of sound in water is approximately 1498m/s, which is about 5 times more than that in air. This means that sound propagates faster when, in water as a medium of transport. This phenomenon can be very easily understood by understanding a simple mechanism of transportation of sound waves. Sound propagates by vibrating particles of a certain given medium. In case of a gaseous medium, the gas molecules are relatively loosely bound and cannot be that easily vibrated. But in liquid medium the molecules are comparatively very tightly bound, which makes it easier for the sound wave to propagate. This also means that for underwater communication, the frequency needed to propagate the sound waves is also high.Aquatic mammals such as whales and dolphins make use of ultrasonic frequency for intraspecific communication and for locating their position in dark and murky waters. It has been in fact also known that apart from intra-specific communication, aquatic mammals have different dialects. It was recently discovered in a survey that orcas belonging to different regions have differences in call signs which are predominantly considered to be differences in their dialects, which very finely means that orcas belonging to Norwegian waters cannot inter-communicate with orcas belonging to the other side of the pond. It has been also recently surveyed that artificial sounds produced by large vessels create disturbances underwater, which also affect the underwater organism, which primarily use sound waves either for navigation or for interspecies communication. Researchers across the globe have been funded and have widely made use of the sound technology to isolate these harmful frequencies produced by larger vessels and ships and have efficiently worked to eliminate the factors causing the disturbances for the fauna.
Sound propagation underwater
Aquatic mammals exhibit vocal sounds for communication as sound travels faster in water than air and is more reliable especially when one of their kind is separated from the other over a distance of meters to kilometers. Hence, it is essential for us to know how sound wave propagates over such distance and how it can serve useful in case of mammals which use it as a major source of communication. When sound can be described as loud or soft, it is essentially the intensity of the sound wave or its amplitude which is under question. However, when we describe a tone of the sound as sharp or blunt, it is more like the pitch or the frequency of the sound wave which is under question. Amplitude or intensity of the sound wave can be directed towards the fact that the pressure exerted by the sound waves is more or less. However, in this case increasing the intensity of sound directly means increasing the volume of the sound. It can be very easily also explained as the amount of energy that it carries. The amount of energy that is carried per unit time in a specified direction is very widely defined as the intensity of the sound wave. This intensity is mostly defined in terms of the same unit as that of power: Watts. However, unlike the other physical parameters which can be measured in “watts”, sound intensity is measured in decibels or “dB”. While working with underwater acoustics, it is very important to note that the reference intensity level is a direct logarithmic function of the ratio of sound intensity in that medium to its reference intensity in the same medium.
Common underwater sounds
While deep sea diving, there is not a lot of things that a naked human ear can perceive. That simply does not mean that the ocean does not have any sound of its own. While many of us believe that underwater sounds are of little or no significance, recent studies show that the significance is more than important for aquatic fauna. The source of underwater sound can be traced back to various sources, which may include the sound of the waves or that of rain droplets. In addition, these sounds are also generated by many human-made sources such as sea-vessels and military sonars.
The natural sources of generation of these underwater sounds include the sounds produced by volcanoes, earthquakes underneath the ocean surface, scarring of icebergs and from heavy rains2. It has been recorded that the bubbles generated by ocean current and rain droplets may be up to 35dB in intensity and could be of frequency ranging from a few hertz to greater than 20,000Hz2.
The sounds produced by aquatic fauna ranges from a few hertz to over 100,000Hz. Aquatic mammals such as whales, dolphins produce different sounds, over a variety range of constituent frequencies. Several other non-mammalian species such as snapping shrimps and some species of oysters have also been known to generate a variety of sound frequencies. These aquatic animals use these sounds to obtain several types of data parts such that could be used in locating them through dark and murky waters and could also be used for communication. In very succinct words they use sound to navigate, feed and to communicate within their own species. Marine mammals use short pulses of sound waves such as clicks, chucks and then using the data from the echoes, can exhibit directional information as well as information regarding probable food sources. Whales and dolphins use clicking sounds or chuckling sounds to have very specific mode of communication within their own group of species. As I also earlier mentioned that their mode of communication is so specific that at times, anyone outside their specified group cannot even understand their dialect.
Most of the anthropogenic sounds are of utmost important while discussing underwater acoustics. Sounds generated by humans are mainly due to human activities on or underneath the surface of the ocean. Anthropogenic sounds are a product of multiple noises generated by on and off shore industrial activities, by large vessels and also by drilling for oil. These noises are sometimes also considered as something that is responsible for disruption of aquatic ambience.
Toothed aquatic mammals such as whales and dolphins have adapted a very specific set of technique which they use for navigating and for locating their prey called echolocation. They produce a vivid variety of sounds by moving air within their skull or other sinuses in their head. The emitted sound wave is then perceived back to the lower jaw bone of these mammals as echoes, which in turn help them in navigating and locating the presence of prey or predators in the near vicinity. This type of conduction is also sometimes called or referred to as bone conduction. The sounds produced can be achieved in multiple other ways. In very brief terms, these whales and dolphins produce two different sounds: “whistles” and “clicks”3. While whistles are very high frequency sound waves, which also travel to larger distances, are used for identifying members of the pack, clicks are used more like a way to navigate. Every individual group has a very specific call sign for its member and thus whistle sounds generated by whales and dolphins can be very specifically targeted to individual member of the same group. The clicking sounds generated by whales and dolphins are usually lower pitched and has a lower frequency gradient. This means that though the wave does not really propagate over larger distances, it can be used for close quarter navigation. The things are equally complicated with clicking. While single clicks are only used for navigation purposes, multiple clicks are considered to be a way to communicate with closely present other members of the same group.
Anthropogenic Danger to Aquatic Communication
Apart from the few sounds that these aquatic mammals exhibit, there are only few other traits of the species which help them in communication. These traits include physical contact with the other member of the same species. This brings me to a point to say that acoustic communication is absolutely vital in case of the underwater communication. And since sound is so important to these species, a recent research has found that anthropogenic sounds could be a source of beaching some majestic aquatic mammals. It is due to increase in the activity of large ships and vessels in the ocean, that researchers have recently discovered that these high-pitched sounds could be a source that hinders the ability of the marine mammals to echolocate and to communicate. These sounds are suspected to be a reason for brain hemorrhages in most of the sea mammals and hence are of huge concern for ecologists across the globe. The article which was very briefly discussed as a part of the course deals with the exact study of the pattern of migration of whales in flee away from these anthropogenic sounds. In this article it was very succinctly mentioned of the troubles that these mammals have while communicating and echolocating and hence are seen fleecing from their original breeding grounds to locations and depths which are habitable for them in terms of reduced manmade underwater noise. It is thus suggested by many researchers that it is absolutely essential for us to monitor and keep a check on the activities which occur within and upon the ocean surface to preserve the balance within the marine ecology. It is also imperative for us to know that while many marine technologists do not really give any heed to these simple things, it could rightfully serve in deterioration of the aquatic acoustic equilibrium. The noises produced by military sonars are usually of high frequency and high amplitude and while mostly it is believed to not range to a lot of farther distances, the actuality could serve differently while producing a proportionate data. Current research indicates that anthropogenic noises can very adversely affect marine mammals. There could be occurrence of cochlear damage to marine mammals which can be also associated with changes in social behavior, stressing their ability to find a suitable mate and may as well hamper their metabolism4. Below given is a list of species following adverse effects of anthropogenic noises.
Source of noise
Noise due to increased vessel transportation and military grade sonars hinders acoustic communication and mutates ability to perceive sound of the similar frequency. While these are only a few known and researched examples, it is believed that the damages to numerous other species could be more severe. It is also found that the above-mentioned sources of anthropogenic noises can not only hinder the acoustic ability of the marine mammals but in the long run could also be a source of their mutation for losing ability to acoustically communicate. As I have also previously discussed that these noises can be a source for behavioral alterations in lots of sea mammals, I would like to discuss this in further details.
Very typical behavioral alterations noted in some of the creatures can be seen very predominantly as sudden response to disinteresting stimuli or even at times attention inhibition. Multiple other instances such as in the case of killer whales or also popularly known as orcas can be seen when these mammals were exposed to military grade sonars. These creatures lost their ability to communicate due to loss in perception of sound. Apart from the abovementioned ill-effects, another major issue of population migration has duly been noted. The imbalance in regional population has been very severely affected due to immigration of these mammals to a different breeding ground4. This could be a direct impact of the fact that these mammals simply did not find their original region to suffice their needs for having intra-specific communication. Discussion With growing technology and ability to discover deeper regions of the ocean, it is also our duty to duly note the dangers to our ecosystem. Nature has provided us with depths of the oceans which man is yet to dive to but nonetheless, it is equally important for us to know the need of conserving the integrity of the ecosystem which prevails. With the use of very powerful sonars, we have made international waters and sea-travel safer but in the due course, we seem to have forgotten the actual need for conserving the harmony between mankind and nature. Having said that, do we very truly understand the meaning of keeping our ecosystem in balance? The aforementioned dangers to aquatic mammals are indeed true and there are much more than the figures usually talk about.
We usually neglect the facts and only focus on what is of immediate necessity and in doing so, we forget that in this era of technology, we must never turn our back to nature. The problems faced by these aquatic mammals are true and it is up to us to recognize of these problems and thereby give proper care and dedication towards establishing the long-lost equilibrium. We have the known the issues since a long while now and while we still discuss these problems, there are not many solid steps taken in order to resolve these issues. We must therefore work harder towards finding appropriate solution to these problems as they can convert to serious threat to our ecosystem in the upcoming years. There is also report of multiple organizations which have begun taking interest in resorting to create harmony between mankind and nature and it becomes our sole responsibility to support such schemes by contributing in our own ways. These means of contributing do not only correspond to monetary helps. They include using technology in a more efficient way at a reduced cost to the natural habitat of several species. In the end, it only depends on our ability of understanding of the gravity of the situations and thereby taking measures to reduce the severity that could be a result of years of exploitation of our ecosystem and of nature!
Through this very brief essay, I have tried to contemplate several facets of human exploitation by disturbing natural habitat of aquatic mammals. I very briefly touched several aspects of underwater noise pollution and have thereby also discussed the harmful effect that it plays on the well-being of aquatic mammals. In the end, I have also discussed the measures that one could take in order to work towards resolving the aforementioned issues and why it is of importance for us to understand the necessity of resolving them. I would thereby conclude this essay by quoting that we will only reap what we sow, meaning what we sow today will benefit the generations to come by.