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Underwater noise affects marine animals’ relationships. But to what extent?

Human activity at sea makes a terrible racket. To what extent does this disturb marine animals? PhD candidate Annebelle Kok studied the effect on harbour porpoises, long-finned pilot whales and their prey, and discovered the sheer complexity of the problem. PhD defence on 12 November.

The days when the sea was the preserve of fish, crabs and dolphins are long gone. In many submarine areas, there is a constant backdrop of noise: ships chugging, the thud of pile driving for offshore wind farms and the boom of seismic oil-reserve surveys. Human influence is omnipresent, and above all audible.

For her dissertation, biologist Annebelle Kok researched the effect of all this human noise on different marine mammals and their prey. Does the noise change the hunting and flight behaviour of these predators and their prey change? And if so, can this disturb the balance of the ecosystem, and as a worst-case scenario, lead to species extinction?

In many submarine areas, there is a constant backdrop of noise: ships chugging, the thud of pile driving for offshore wind farms and the boom of seismic oil-reserve surveys

To answer these you first need to know that long-finned pilot whales, harbour porpoises and other small toothed whales make sounds that enable them to ‘see’ under water. This is how they communicate with each other, for instance to attract a partner or chase away a competitor. But equally important: they use sound to find their prey. Any self-respecting harbour porpoise will gulp down 500 fish per hour, all found with the aid of echolocation.

The answer to Kok’s research questions is nuanced. Background noise does appear to change some interactions between predator and prey. Gobies – the favourite snack of the harbour porpoise – are much less alert to a threat if there is a lot of background noise. Kok and her colleagues discovered this by towing a dummy predator through Grevelingenmeer (see video below). This could mean that gobies in busy areas are an easier prey for harbour porpoises than their peers who live in relatively quiet waters.

Research into gobies in Grevelingenmeer

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Long-finned pilot whales also behave differently in busy areas. These super-social members of the dolphin family communicate with one another by squeaking, whistling and buzzing. Transmitter data shows that long-finned pilot whales make louder sounds in noisy areas to get themselves heard above all that din, but they are still less able to hear one another. Compare it to trying to hold a conversation on the dancefloor at a night club: even if the other person shouts, you still only catch half of what they are saying.

‘It’s clear that the relationships between animal species are changing. But that is not necessarily disadvantageous.’

Noise did not have any visible impact on the hunting behaviour of harbour porpoises in captivity. Kok studied this by playing the ‘shell game’ with these intelligent marine mammals. This involved hiding bait in various parts of the pool. The porpoises were quick to locate this tasty snack, even with loud background noise, so the noise didn’t seem to interfere with their echolocation.

What does this say about the disruption of the underwater ecosystem? ‘It’s clear that the relationships between animal species are changing,’ says Kok. ‘But that is not necessarily disadvantageous. A slow goby is a gift to a harbour porpoise, so the harbour porpoise population might actually prosper. It could be that in this example human disruption has a negative effect on the prey but a positive one on the predator. We’ll have to carry out further research to determine what this does with an entire ecosystem.’

The leading players in the research

The harbour porpoise (Phocoena phocoena) is the most common cetacean in the North Sea. It has a maximum length of 1.80m and a maximum weight of 60 kg, and eats fish, squid and prawns.

Like the harbour porpoise, the long-finned pilot whale (Globicephala melas or G. melaena) is a mammal from the suborder of toothed whales. It can be up to 8.5 m long, making it the largest of the oceanic dolphins after the orca or killer whale. The long-finned pilot whale is seen every now and then in Dutch waters.

Gobies are a family of fish in the ray-finned fish order (Perciformes). They are an alien species in Dutch waters. Many have an abdominal sucker that allows them to attach themselves to the seabed.

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