Sperm whales use echolocation to search for prey such as squid in the deep, dark ocean – so it makes sense that competing sounds down there could screw that process up. A recent study now indicates that even a new-and-improved type of manmade sonar does indeed cause problems.
The research is being led by Prof. Patrick Miller, of Scotland's University of St. Andrews.
Based on a previous study that he conducted in Norway, Miller already knew that sperm whales cease foraging when exposed to pulsed sonar – that's the traditional type of sonar that we've seen in movies, consisting of a series of acoustic pulses with pauses in between. Users such as submarine crews listen for echoes of the pulses, during the pauses.
A relatively new technology known as continuous sonar is different, in that it sends out one long acoustic signal while simultaneously listening for that signal's echo. This allows for quicker detection of objects such as enemy subs, while also not requiring the signal to be as loud as the traditional pulses.
In order to test if sperm whales weren't as troubled by the new system, Miller and colleagues returned to Norway. There, they tagged a number of whales with suction cup-attached data loggers, then followed them in a boat that was towing a continuous sonar-emitting device behind it. The scientists maintained a distance that allowed the animals to hear the sonar, but not have their hearing injured by it.
When the logger data was analyzed, it turned out that as was the case with the pulsed sonar, the continuous version likewise caused the whales to stop searching for prey. Interestingly, it was found that greater amounts of sound energy – which is the total energy of a signal, including its duration – caused more of a problem than the sound amplitude, which is a signal's instantaneous loudness, regardless of its duration.
The scientists have therefore suggested that navies reduce the energy content of their sonar signals, in order not to affect sperm whale feeding.
A paper on the research was recently published in the Journal of Experimental Biology.
Source: University of St. Andrews
