Most of us have been there. You’re in a pub or a club trying to have a conversation but the music…it’s just too loud to hear what the other person is saying. You shout louder and louder, the listener has their ear up close to your mouth but alas, the conversation doesn’t flow as it would do if you could both hear each other easily. Now imagine that sound wasn’t just important for having a conversation, but for seeing. And imagine that the loud noise preventing you from hearing properly wasn’t just in the pub, but occurred throughout your day-to-day activities.
Noise pollution is a problem for cetaceans because they use echolocation to ‘see’ and hear. It’s quite a nifty technique because often the ocean is too murky or too dark for your eyes to see very far in, but sound can still travel. Thanks to evolution, cetaceans have echolocation down to a fine art. Not only can they figure out that something is there, but they can work out what it is. But when it’s too noisy, the echolocation process can be disrupted and activities like hunting, navigation, and pod communication can become difficult to impossible. Noise has even been linked to stress, and increased energy expenditure in our aquatic brethren. One of the problems with figuring out just how noise pollution is affecting cetaceans is a lack of baseline data – to a large extent we don’t know the status of cetacean populations inhabiting different areas. When we do get around to taking measurements of noise, we don’t have a good handle on how noisy different areas were in the first place to know if the noise has increased. This lack of baseline data includes in conservation areas designated as important marine mammal habitat – just like the Moray Firth up in Scotland.
The Moray Firth is home to a well-studied population of bottlenose dolphins (Tursiops truncates), but it also has strategic importance, forming a base for North Sea oil and gas exploration and potentially in the future, a base for an offshore wind farm. Noise is likely to increase but to figure out by just how much Nathan Merchant of the University of Bath, alongside Enrico Pirotta, Tim Baron, and Paul Thompson of the University of Aberdeen decided to get some baseline data before developments begin. Once that data is in place, they argue, more accurate correlations between noise and effects of marine mammals can be determined.
During the summer of 2012, Nathan and his team placed two underwater noise monitors – both in deep narrow channels popular with the dolphins for foraging, as well as prime shipping traffic routes. They then monitored the noise on a cycle of 1 minute every 10 minutes and tied that data up with Automatic Identification System (AIS) ship-tracking data. For the other 9 minutes recordings still took place, primarily to provide more analysis of noise events of interest. And of course, this sound recordings also picked up the bottlenose dolphins as well as other marine mammals, but the team also deployed C-Pods – recorders dedicated for marine mammal noise – at the sites. Conditions like rain and wind can also create noise in the Firth so meteorological data was also collected.
The acoustic data confirmed that the dolphins were using the two site quite heavily, with recordings of their clicks at both sites being made every day. The two sites differed a fair bit in their baseline noise levels, with one generally much noisy than the other, with shipping traffic appeared to be the main source of noise pollution. The researchers hypothesise that increase in noise levels at the already noisy site may be less damaging to the dolphins than increases at the quieter site, because the noisy site has already suffered noise-related habitat degradation to which the dolphins have already become accustomed. Indeed the Moray Firth population size is showing signs of being stable, and is perhaps even increasing which is a positive sign. However, the dolphin vocalizations overlapped both in frequency and amplitude with the shipping traffic. This is concerning because it means that there is a higher risk of the dolphin’s vocalization being masked out by increases in shipping traffic. Just how much shipping noise is too much is still unclear.
The paper is published in the journal Marine Pollution Bulletin and has been made open access. You can read it here: http://dx.doi.org/10.1016/j.marpolbul.2013.10.058
There are also a couple videos up on YouTube where you can listen to “short real-time clips” of the ship noise monitoring in the Moray Firth, accompanied by ship tracking data, underwater recorders, and time-lapse cameras. Check them out here Ship noise monitoring in the Moray Firth – The Sutors and here Ship noise monitoring in the Moray Firth – Chanonry.
Image: Adult female Bottlenose Dolphin with two young at side, Inner Moray Firth, Scotland May 2005. Photographer Peter Asprey/Wikipedia and cropped by Clayoquot/Wikipedia (CC BY-SA 3.0)