Underwater shipping noise in the Salish Sea is likely making it difficult for endangered southern resident killer whales to find food and could threaten their survival, according to a team of U.S. scientists.
A new, two-year study, published in the academic journal Peer J, used underwater microphones to take 3,000 noise measurements as 1,600 individual ships passed through the Washington State side of Haro Strait.
The study site is in the middle of critical habitat for the fish-eating southern resident killer whales and researchers found shipping noise extended to middle and high frequencies used by killer whales to echo-locate prey. Killer whales emit a series of clicking sounds and then listen for the bounce-back echoes in order to find fish.
The researchers found the growth in commercial shipping has raised the intensity of low-frequency noise almost 10-fold since the 1960s and there is growing evidence that it is affecting the communication ability of baleen whales, such as humpbacks, gray whales and right whales.
The paper was authored by Scott Veirs from Beam Reach Marine Science and Sustainability School in Seattle, Val Veirs from Colorado College physics department and Jason Wood of SMRU Consulting in Friday Harbor, Washington.
The question the team set out to answer was whether ship noise extended to higher frequencies used by toothed whales, such as killer whales, and could therefore pose similar threats to them and other marine life such as dolphins and porpoises.
“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. 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,” Scott Veirs said.
“This means that in coastal environments where marine mammals live within a few kilometres of shipping lanes, ship noise has the potential to interfere with both communication and echolocation.”
The study looked at a wide range of ship types and found container ships had the highest median noise levels and military vessels, most of which use noise-suppression technology, had the lowest levels.
“We saw a lot of variability in noise levels, both between and within the 12 classes of ships we studied,” Veirs said in an interview
“That variability suggests that investments in quieting technologies may efficiently lower the median noise levels for many types of ships.”
The researchers also found that, if ships slow down, the noise level is substantially reduced.
“For example, a three knot decrease in speed could cut in half the acoustic power emitted by the ship,” he said.
The three pods of southern resident killer whales, which spend much of their time in the Salish Sea, are the stars of a multi-million dollar ecotourism and whale-watching industry on both sides of the border.
The southern residents eat mainly chinook salmon — unlike transient/Biggs killer whales, which eat marine mammals and offshore populations which rely on a shark diet — and it is essential for their survival that they be able to locate sufficient salmon, which are already in short supply.
Government studies in the U.S. and Canada previously identified noise, pollution and lack of salmon as the main threats facing the southern residents.
The three pods struggled to recover after decades of whaling, followed by captures for marine parks in the 1960s and 1970s and, despite a nine-birth baby boom over the last year, the population stands at only 85 individuals. The first-year survival rate for calves is about 50 per cent.
The study by Veirs and his colleagues is raising questions not only about current noise levels, but what will happen if the dozen oil and coal terminal expansion projects, now proposed for areas around the Salish Sea, get the go-ahead.
The proposals include controversial plans to twin Kinder Morgan’s Trans Mountain pipeline, which would mean a massive increase in oil tankers travelling through the area. It is estimated the number of tankers could increase to about 400 a year from 70 a year.
“The main effect of the proposed development within the Salish Sea may be to further reduce the amount of quiet time our local species enjoy — those periods when ship and boat noise are not part of their environment,” Veirs said.
“Our results confirm that currently about 20 ships a day transit the critical habitat of the southern resident killer whales. Adding just four ships per day would mean that, on average, there would be a ship every hour of the day throughout the year.”