Killer Whales Hear Ship Noise
The growth in commercial shipping has raised the intensity of low-frequency noise almost 10-fold since the 1960s. Because this noise occurs at the low frequencies used by baleen whales, there is growing evidence it may impact their ability to communicate, and therefore their survival.
A group of scientists, led by Dr Scott Veirs of the Beam Reach Marine Science and Sustainability School in the U.S., have now determined that ship noise extends to the higher frequencies used by toothed whales, such as killer whales.
The scientists measured approximately 1,600 unique ships as they passed through Haro Strait in Washington State. Around 20 container, cargo and military ships pass through Haro Strait each day, most headed for the Port of Vancouver in British Columbia.
The area is the core critical habitat for the endangered Southern Resident killer whales, salmon-eating orcas which are iconic in the Pacific Northwest and which support a multi-million dollar ecotourism industry in the U.S. and Canada.
Because these orcas, like other toothed whales, use mid-and high-frequencies to communicate and find their prey, the study measured a wide range of frequencies (10 Hz to 40,000 Hz). The results show that ships are responsible for elevated background noise levels not only at low frequencies as expected, but also at medium and higher frequencies (including at 20,000 Hz where killer whales hear best).
This means that in coastal environments where marine mammals live within a few kilometers of shipping lanes, ship noise has the potential to interfere with both communication and echolocation.
The study is unique because it estimates the source levels of larger populations and more classes of ships than in previous studies. Overall, container ships exhibited the highest median source levels (at all frequencies below 20,000 Hz). Military vessels had some of the lowest levels, suggesting that transfer of quieting technology to the commercial sector could be a successful noise mitigation strategy, say the scientists.
The study shows that another potential way to reduce noise pollution is to simply slow down. The data suggest that, on average, each reduction in a ship's speed by one knot could reduce broadband noise levels by one dB.
Further research will be undertaken to better understand how the noise levels documented in the study are impacting marine life in the Pacific Northwest, Veirs said. “As an endangered species, the killer whales will be at the top of our list, but we also want to look at fish, invertebrates and the many other marine mammals we have. Some of them are also high-frequency specialists, such as white-sided dolphins and Dall’s porpoise.”