As if endangered southern resident killer whales didn’t have it tough enough already, new research shows the noise from shipping could make it harder for them to catch the already dwindling stocks of salmon on which they prey.
The ships turn out to broadcast the same high frequency sounds they use to hunt.
We’ve known for years that propellers on ships produce low-frequency rumblings that overlap with those used by baleen whales, such as the humpback. But a new study, published in the journal PeerJ today, show ships also generate high-frequency noises, which are used by toothed whales such as orcas.
Like bats, killer whales send out clicks to detect prey, and listen for their echoes. This works particularly well in murky waters, such as estuaries.
In the open ocean, high-frequency sounds made by ships dissipate within a few kilometres. But within bays and straits, those sounds bounce around.
Such is the lot for southern resident killer whales.
Their core habitat is the Haro Strait in Washington State, part of the international boundary between Canada and the United States, which also happens to be a busy shipping lane.
So the researchers, led by Scott Veirs from the Beam Research Marine Science and Sustainability School in Seattle, measured the noises emitted by around 1,600 ships as they passed through the strait over 28 months – the biggest such study yet.
They found that ships are not only responsible for low frequency noise pollution, but also medium and high frequencies – including at 20,000 hertz, where killer whales hear best.
It’s important to note the researchers only measured ship noise – they didn’t see how killer whales hunted with or without the noise, and it may be they have adapted to get around the problem.
Other marine mammals have adjusted their clicks and whistles in response to boats. Indo-Pacific bottlenose dolphins, for example, lower their own clicks when boat noise encroaches on their frequencies, like changing the radio station.
To give killer whales more of a chance of survival, the researchers suggest silencing technology used on military vessels – the quietest craft they measured – could be applied to container ships, which were the noisiest.
The other option is to simply slow down. Dropping speed by one knot can reduce noise by a decibel.