Summer season is kicking in with full speed and, surely, the one thing that is becoming wider and wider is the boat traffic belt near the coastal areas. It is almost impossible to ignore this type of pressure, since it is so powerful that you can experience it first hand when jumping into the petrol polluted ocean. But is this the only problem, or does it extend further? – Let’s dig a little deeper to find out more together!
Recently, I have been focusing on the acoustic aspects of cetacean communication and predatory strategies that they have adopted to forage food in the deepest parts of the oceans. I stumbled across numerous articles that were addressing the problem of vessel noise pollution, and how it disrupted the cetaceans’ socialization. “Now, wait a minute,” I said to myself and continued “how would I feel if someone tried to constantly interrupt a conversation between me and my friend?”
Cetaceans are aquatic mammals divided into two parvorders: Odontocetes and Mysticetes. The first group is characterised by toothed whales, including dolphins, porpoises and sperm whales; while the second group is represented by whales with a filter-feeder system, such as the right whale, gray whale and bowhead whale. Mysticetes also have a tactile system, called vibrissae, of short hair around their mouth. Cetaceans produce a wide array of vocalizations, from moaning songs of the humpback whale to the clicks and whistles of dolphins.
These vocal patterns are species-specific, and each species has evolved a wide range of frequencies to use either as foraging or as socializing and navigational tools. Furthermore, these frequencies also differ in different populations of the same species. For instance, sperm whales (Physeter macrocephalus) off northern Norway make three types of clicks: usual and creak clicks have short duration, are profoundly directional and produced for echolocation of meso- and bathy-pelagic fish and squids, whereas slow clicks are low-frequency and more suitable for communication with conspecifics that are up to 60 km away (long-range sound communication).
As for the man-made noise, the most common is that of motorized vessels that are increasing with globalization, which implies a fast-growing demand for material and food. In addition, recreational and whale-watching boats are also becoming devastatingly innumerable. The biggest source of underwater noise pollution is not generated only by mechanical vibrations of the engine, but from cavitation, a phenomenon whereby air bubbles form and collapse on the edge of fast-moving propeller blades (Ross, 1976). The faster the vessel goes, the faster the blades propel and, therefore, the noise from cavitation increases.
In a study concerning the masking impacts of vessel noise on both bottlenose dolphin (Tursiops sp.), in a shallow area, and pilot whale (Globicephala macrorhynchus), in a deep-water area, communication, they have discovered that vessels moving less than 2,5 knots did not significantly increase the ambient noise within the bottlenose dolphin frequencies. Increased cavitation (faster-moving boats), on the other hand, increased the ambient noise at ranges beyond 50 m, masking the acoustic cues necessary for mediating the social structure of Tursiops sp. and Globicephala macrorhynchus.
Whale watching, for example, has become an important tourist attraction in more than 120 countries in the past few decades and, surely, it has economically replaced the more environmentally harmful activities and increased public awareness. Unfortunately, it has also evoked unusual behaviours, such as those studied in killer whales (Orcinus orca) that include the avoidance of boats, as well as attraction, longer dives and shortened surfacing, interruption and termination of feeding and travelling behaviour. Since this observational activity is usually performed by many whale-watching boats around a single aggregation of a whale population, this human behaviour could further contribute to the disturbance of their natural habitat. The most obvious solution would be to establish whale-watching regulations and RESPECT them.
On the 19th of June, 2019, a humpback whale (Megaptera novaeangliae) was sighted for the first time between Capraia and Corsica. The natural habitat of this species is the Pacific ocean. It is speculated that this individual has entered the Gibraltar passage as an immature during the seasonal migrations, destined to die due to the lack of krill (their favourite food), but it survived and found a krill abundant area – Tyrrhenian Sea. Even though we do not have any proof of more than one individual being present in this area, management implementation should be applied, since it hosts many other marine mammal species.
In fact, it has been observed that humpback whales increase their vocal sound level and switch from using primary vocal sounds to surface-generated sounds, such as pectoral and tail slapping, in response to increased wind-generated noise; whereas the response to increased vessel noise is none. Another study, however, has shown that males lower the frequency of their songs, which they use for attracting a female, in order to escape the frequencies of anthropogenic underwater noise and be heard for potential mating.
Finally, the humming noise of a mosquito that we hear during a hot summer night could basically be compared to the man-made underwater noise, except that the last one is far more intense and continuous. Would you be willing to hear it? Ask this question next time you think of getting yourself a nice fancy boat. Meanwhile, I will do my best to keep dreaming of inventing something more silent and peaceful.