With human activity increasing both in coastal and deep ocean, the world’s ocean has become quite a noisy environment. Concerned researchers have investigated how this may be impacting marine life, including marine mammals.
One study has investigated the potential effects of pile driving on the dynamics of bottlenose dolphin populations.
Importance of Sound to Bottlenose Dolphins
Bottlenose dolphins use sound to:
- investigate and understand their environment
- find and capture prey
- transmit information to other members or conspecifics
Depending on the surrounding contexts, dolphins will utilize different sound types and project the soundwave at varying frequencies and sound levels (Table 1). Approximately, unmodulated whistles within 3.5-10 kHz have an effective range of 14-25 km. However, when the whistle frequency increases to 12 kHz, the effective range decrease to 1.5-4 km. Pulsed sounds, such as clicks, rarely travel further than a few kilometers.
|Sound Type||Frequency Range (kHz)||Dominant Frequencies (kHz)||Source Level (dB re μPa at 1m)|
Sound Generated by Pile Driving
On average, an individual pile driving pulse generates a sound source level of 151 dB re 1 μPa. Dr. Mciwem (2006) summarized potential masking of dolphin call types (i.e., whistles, clicks) in the figure below.
The figure depicts how 3 different frequencies (9, 50, 115 kHz), commonly used by dolphins, dissipates as the sound travels further from the source. Sounds that lie under the lines of the pile driving hammers (i.e., diesel, drop) potentially mask dolphins sounds.
At 9kHz (top panel), the drop hammer could mask vocalizations at a radius of 1.3 km; while the diesel hammer potentially masks vocals over 40 km away.
At 50 kHz, the diesel hammer could mask echolocation clicks up to 6 km away, and the drop hammer up to 0.2 km.
At 115 kHz, the diesel hammer could mask echolocation clicks up to 1.2 km away, and the drop hammer 0 km.
What does this mean?
Here, we see that depending on the loudness of a sound received by and call type – dolphin vocalizations have the potential to be masked or go unheard by a conspecific. Masking may cause dolphins to change the frequency range, loudness or even how often they emit signals. This can cause additional excursion of energy, or possibly change vocal composition of specific social groups. Masking could also influence missed opportunities to mate or receiving vital information from another individual because signals were not heard.
It is important to keep in mind that this information comes a single review study and only goes over potential sound interference from pile driving. However, there are multiple sounds, both natural and human-made, occurring all at once. Therefore, the influence of masking may be underestimated. Scientists continue to do work on understanding the full spectra of masking.