Bowhead whales are a highly adapted species, living in the cold waters of the Arctic and subarctic oceans of the northern hemisphere. They are members of the right whale family and are the longest living marine mammals in the world – some have been found to be over 200 years old! Bowhead whales face a variety of threats including ocean pollution, decreasing sea ice, and increasing ocean noise as interest in Arctic shipping and drilling continues to grow. There is concern over how cetaceans respond to anthropogenic noise and how certain sound exposure thresholds might impact biologically important life functions. Scientists working in the Alaskan Beaufort Sea have been studying the potential impacts of underwater noise on bowhead whales, in particular the effects of seismic survey activity on calling behaviour.
Researchers deployed 40 underwater acoustic recorders at five locations to monitor the calling behaviour of bowhead whales in the presence of seismic survey operations. Whale calls were localized within 2 km of each recorder and the call rates were measured. Airgun pulses from the seismic survey boat were also identified and analyzed. Call rates and concurrent sound exposure levels were tallied over 10-minute periods. When first exposed to airgun pulses, the whales increased their calling rates, just as people might do in an increasingly noisy room, but after the initial increase in vocalizations, the calling rates levelled off. Interestingly, once the airgun sounds became too loud (~127 db re 1 P μa2-s), the calling rates began to decrease, and eventually the whales became silent – just as someone might give up on trying to get their friend’s attention at a rock concert.
This study has identified a complex behavioural response to seismic survey noise and is the first to identify two different reactions to the same anthropogenic sound source. These findings are key to understanding the impacts of seismic survey operations on cetaceans and for providing insight into data collected by passive acoustic monitoring systems.