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Dolphins bully porpoises, researcher discovers | Greater training

April 3, 2023
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It's a cold, sunny spring day as our boat heads out of the little town of New Quay on Wales's west coast. We're on the lookout for dolphins and porpoises – at least, I am.

Bangor University PhD student Hanna Nuuttila, however, isn't really interested in getting a sighting of these cetaceans – instead, she wants to hear them as they chat to each other beneath the waves.

By using submersible underwater hydrophones called C-Pods to collect acoustic data on their vocal behaviour – ie, where and when they do their characteristic "clicking" – Nuuttila is hoping to discover more about how bottlenose dolphins and their smaller and more shy relative, the harbour porpoise, use the vast sweep of water encompassed by Cardigan Bay.

We're chugging along the coast towards a 1km-square "grid" she created just before Christmas, when she sank 44 C-Pods every 200 metres. Each of their locations is marked by a buoy, and for the moment, that 1km square is Nuuttila's own little patch of sea.

"All the fishermen have been great and agreed not to put their (crab and lobster) pots in it," she says. "The lifeboat has agreed to avoid it, too, and the coastguard is broadcasting twice a day and warning sea-goers not to go there."

Donated by Cornwall-based manufacturer Chelonia, C-Pod devices are worth around £3,000 each, so she's keeping close tabs on them – during the recent high spring tides several became completely submerged and for a few anxious days it looked as if they'd gone for good.

Cardigan Bay is a Special Area of Conservation (SAC), so designated partly because of its importance for bottlenose dolphins as they feed, calve and raise their young. Understanding where and when dolphins frequent different parts of the bay is vital to ensuring that the effects of human activities are mitigated in areas crucial to their survival. But harbour porpoises also use Cardigan Bay, and it is this species that could benefit the most from Nuuttila's research.

This carries on from a previous acoustic monitoring study she did for The Seawatch Foundation, prompted by the discovery that Cardigan Bay dolphins were killing porpoises.

"When dead porpoises stranded, you could tell dolphins were involved from dentition marks in their flesh," she explains.

"The porpoises were also suffering massive haemorrhages and their internal organs were all mushed up. They were being battered to death, really."

Wanting to know more about the interactions between the two species, 10 submerged "click loggers" – a less sophisticated version of the C-Pod kit she's using now – were monitored over a three-year period.

These devices made it possible for Nuuttila to carry out her survey work far more accurately than would be possible by simply looking from a boat or cliff. As she puts it, "we can't be out there at night and we can't be out in winter. These devices can be listening all the time."

She discovered that although dolphins use the area in summer to breed and feed, "harbour porpoises use this area all year round.

"The extent of this activity simply wasn't understood before because monitoring porpoises visually is so hard to do – they're tiny, you only ever see them for a second or so before they dive down, and in choppy winter weather, you just can't do surveys the traditional way, by looking," Nuuttila explains.

She also found that porpoises were using the bay at night and at times when dolphins had made themselves scarce.

"At night, porpoises would move nearer to the coast as dolphins travelled out, so it looks like there might be some avoidance behaviour going on," she says.

Her team's discovery demonstrated that the habitat provided by Cardigan Bay may be just as important for porpoises as for dolphins, "because it shows they use the area over a longer period each year than had previously been thought".

This means that human activities that could harm porpoises may need to be re-evaluated and action taken to minimise their effects.

Gaps in knowledge from her past work include, she says, "not knowing how environmental variables affect the way the hydrophones work; not knowing the number of cetaceans that are echolocating at any particular time; and whether in groups they click more than when they are alone. Precise numbers of dolphins and porpoises using the bay are also hard to define. These questions are the subject of her current research.

To gather the raw data, during the three months before the summer season gets under way and she has to haul up her C-Pods to allow leisure and fishing access, Nuuttila is placing visual observers on clifftops. Her assistants spend their days peering through powerful binoculars to see how many dolphins and porpoises are swimming in and out of squares on the grid.

"That data is all noted and we match it against the time of clicks registered on the nearest C-Pods," Nuuttila explains.

If she can prove that porpoises use the bay in significant numbers all year round, then the SAC designation could be amended to give the species special protection.

"Despite the SAC designation, Cardigan Bay is a multiple-use area," says Nuuttila, "and scallop dredgers are permitted in some parts of it.

"If the fishing industry says, 'the dolphins go away in the winter, so we can dredge for scallops' – and remember, scallop dredging is highly destructive of the sea bed and the creatures that live there – then up till now, that's been seen as OK. But if we show that the bay is used as a feeding ground by porpoises in winter as well as summer, those activities may have to be curtailed."

Similarly, she points out, if it can be shown that there are areas not used by cetaceans and that don't support other important habitats, "maybe those areas could made available".

However, getting more accurate information and using the precautionary principle until that knowledge is available, is the only safe way forward, she says, "before allowing destructive practices, not just in the SAC, but in all marine areas."

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