Bottlenose Dolphins’ Adaptations to Noise in Urban Harbours: Shouting, Eavesdropping & Fleeing

Health risks of urban noise to Bottlenose dolphins

Recently, urban sightings of cetaceans have become increasingly prevalent, which can be linked to higher water quality and healthier environments in general. Recent examples include dolphin sightings in the Baltimore (US), and Bridlington (UK); killer whale sightings in California (US); and multiple cetacean sightings in and around Reykjavík (Iceland). Despite the apparent increase in water quality, many of these urban harbours are still inadequate habitats for dolphins due to the high degree of industrialization and associated levels of noise. Dolphins living in these harbours can suffer massively from noise and need adequate adaptations to survive.

Recent acoustic analyses in the urbanized Estuaries of the Swan River (Perth, Australia) and Lagoa dos Patos (Rio Grande, Brazil) have shown that there can be major variations in the soundscape of habitat of urban Bottlenose dolphins, both in time and space. With regards to time, there are much higher levels of man-made (or anthropogenic) noise in the warmer months of summer and autumn as well as the morning and afternoon, aligning with so-called “vessel rush hours”. Spatially, there may be noise sources, such as industrial sites or long-term construction.

Bottlenose (Tursiops) dolphins produce a combination of whistles and clicks to communicate with each other and locate their prey. Urban noise sources provide a challenge to the ecology of dolphins, as research has shown that even noise levels lower than those commonly encountered in urban harbours impedes cooperation between dolphins. This pattern, if left unchecked, can lead to increased habitat fragmentation, massive reductions in population numbers, and eventually risk of extinction, such as with the Chinese white dolphin (Sousa chinensis) in the Pearl River Estuary of Hong Kong (China).

Bottlenose dolphins’ adaptations

Dolphins’ adaptations to noise in urban harbours are three-fold: “shouting”, “eavesdropping” and “fleeing”. First, “shouting” is the direct adaptation of whistle frequency. This tactic could increase communication range and potency and has been shown in populations in the Gulf of Mexico. However, it appears to only be present in the oceanic ecotype of the species, suggesting the coastal ecotype has lost the flexiblity in its vocalization due to consistent exposure to anthropogenic noise. Their second strategy is a method called “eavesdropping”, which involves passive listening to the noise produced by prey, as it is less affected by anthropogenic noise than the dolphins’ echolocation.

Finally, bottlenose dolphins tend to avoid noise, or “flee”, by hunting outside vessel rush hours and avoiding areas with a high level of noise. This last tactic is only possible when there are sufficient areas of refuge for the dolphins. Consistent exposure to noise without options for refuges can lead to severe health issues for dolphins, something apparent in the currently endangered Chinese white dolphin.

Appointing refuge areas in/near urban harbours is a measure that was shown to be effective at recovering population numbers of bottlenose dolphins in the Port River of Adelaide (Australia). In addition, global lockdowns and the resulting decreases in the numbers of recreation vessels during the COVID-19 pandemic also resulted in a recovery in the estuaries of India, New Zealand, and Thailand, among others. So while bottlenose dolphins’ adaptations to noise seem to be decently effective, refuge areas with reduced vessel traffic are needed for urban populations to thrive. Thus, conservation efforts for dolphins should focus on providing these areas to circumvent the risks provided by anthropogenic noise.