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Source: Greenpeace New Zealand

Solène Derville is an award-winning scientist based in New Caledonia.

Here she answers supporters’ questions on her work studying Humpback whales’ behaviour around seamounts in the South Pacific.

You can watch the video version, plus her full presentation, below.

Q: Can you please briefly summarise your recent work on the impacts of climate change on Pacific whales?

Yes, so this work on climate change is related to the map I showed earlier, this work has been done as a collaboration with many other scientists who have been collecting data in different archipelagos – so Tonga, French Polynesia and so on. 

In this publication what we did is we looked at encounter rates and tried to get an understanding of the density of whales and the habitats they use in Oceania. And tried to understand how temperature affects their distribution over time because we have 19 years of data. Throughout these years the temperature has fluctuated – you all know about the el nino and le nina phenomenon, which results in warmer or colder waters. So the idea was to try to understand how whale presence is affected by temperature, and if it is affected by temperature what is the current range they use and how will this be affected in the future. 

One of the main takeaways from this study was that the whales in Oceania use a range roughly between 21 and 28 degrees for the surface temperature, and we predicted where this range of temperature would be at the end of the century. What we discovered is that a lot of the current breeding grounds will be in very warm waters by the end of the century. Far above 28 degrees. Which means places like America Samoa or Tongo or Society Islands in French Polynesia, which are currently occupied every year by a lot of whales, might be too warm for them by the end of the century. 

The question that remains is whether this is going to be a problem – whether this will affect mating and calving of whales and if it does will they be able to move in response. Of course there are many other breeding sites further south that will be the right temperature but we do not know whether the whales will be capable of moving to shift their distribution.

Q: Do you know if the seamounts and the behaviour of the humpbacks is mirrored in the northern hemisphere?

I think now that many people around the world are using satellite tags and these tags are getting better at staying for longer on the whales, we are able to see what they do offshore. I think certainly whales use seamounts elsewhere around the world; it’s just a case of finding these places. 

Around Hawaii certainly there are seamounts that whales tend to use, we’ve seen this on recordings. Scientists have heard songs on seamounts north of Hawaii, and we’ve seen whales stopping on other seamounts around the world.

Q: How are you going to verify your hypotheses around deep diving and the whales?

This is really tricky, for the feeding hypothesis it’s a case of – until we see a whale feeding we don’t know if they are. And to see one feeding at depth is very difficult. One method would be to deploy another sort of tag, one that you stick on the back of the whale and you retrieve a few hours later – these can measure acceleration and movements through the water column much more precisely, and this way we could measure feeding launches at depth. But for now what we’ve been doing is that we’ve collaborated with other teams and people who are working on the prey – so all the little fish and crustaceans – and we’re trying to look at the density and composition of prey around seamounts to see if there is some potential for feeding.

Q: Of the 18 whales that have been tracked have you named them?

We name them for sure, I can’t remember all of their names, but we try to have themes each year. We had a Zealandia and an Oceania based on where the whales went.

Q: On a practical level can you explain how deep sea fishing and some of the threats to the seamounts are negatively impacting them?

In New Caledonia we are fortunate, there is a bit of fishing but it’s a very small fisheries that are not harmful for whales.  I think what is harmful in other places is the bottom trawling. Obviously this isn’t harmful directly to whales but as I said – the organisms that live on these seamounts and the benthic species and corals and sponges that live at the bottom are usually very slow growing organisms and very fragile. Bottom trawling mechanically destroys these that have taken hundreds of years to form.

Q: Are all of the significant seamounts in the South Pacific chartered?

A really good question. Working on seamounts my main nightmare has been to simply map them and know where they are and what their height is. Because the answer is no they’re not all mapped. We often know their position but there is a lot of uncertainty on how big and how high they are. 

That’s a problem because in the case of Humpback whales we have determined that the depth of the summit is very important for the whales – this is really what determines whether it is suitable for them or not, and this limit seems to be around 100 – 200m. All seamounts that are deeper than that are not suitable for whales but sometimes it’s hard to determine if a seamount is at that height or not. It’s also tricky to count them because it depends on how you define them, in terms of height and shape. Something might be a seamount or more a hill. It depends on how you classify it.

Q: Have you ever been diving with whales and how close did you get to them?

It happened to me once to freedive with whales, not in New Caledonia but somewhere else, it was amazing. For our work we do not need to go underwater with them. It would be useless and do more harm than anything else as we really try to affect them as little as possible. We take pictures, we take biopsy samples, but we aim our interaction to be as short as possible.

Q: Could you say that as the water gets warmer we will see more humpbacks moving to temperate waters?

We shall see. I think that’s a conclusion of our work on climate change and how it may affect whales. We’re able to say that the temperature is going to rise, and it’s going to rise in a way that makes some waters unsuitable, but we are incapable to say if the whales will move. In the past we had examples of whales shifting distribution, in response to a changing environment. 

They seem to be plastic enough in their behaviour to be able to shift their distribution by a few 100 kms, it would be very interesting so see in coming years if there is more breeding activity on the migratory corridors – in places like Norfolk, New Zealand, Kermadecs. To see if the calves are younger, if we hear songs, if we see competition between animals.

I think this highlights the value of long term monitoring because to detect that sort of change it is essential to have long term data. To be recording data in an area over many many years and that’s the sort of data that we often lack. 

Q: Was there a specific event that inspired you to start your research on Humpback whales?

Not necessarily, I’ve worked a bit on other marine species including sea turtles and Māui dolphin in New Zealand, and I found a lot of interest in studying these highly social intelligent animals, but I was just very lucky to meet with with Claire Garrige in New Caledonia who was looking for someone to do spatial ecology with her data, so just a lot of luck. 

Q: Would a protected area around those seamounts prevent scientific whaling?

These protected areas would be created would be within the EEZ – within the natural park of the coral sea so this park covers almost the whole economic zone. Scientific whaling does not occur within that, so in any case that is not a threat that they are facing there. 

Q: What is it that you’ll do next with this research?

In terms of research the latest stuff that I didn’t have time to get to today is that we conducted four research cruises this last winter in collaboration with other scientists who look at currents, who look at prey in water, we counted Humpback whales but also other cetacean species and seabirds to really try to understand seamounts at an ecosystem scale. And understand why whales select certain seamounts. 

All of this is feeding in trying to understand the hypothesis that I have spoken about, and just understanding why they regroup over certain seamounts. I’m currently analysing this data and hoping it will bring some answers. I guess in terms of protection everything has been brought to a halt these past weeks, and we don’t really know what the plan is, but the Government in New Caledonia has committed to marine protected areas around seamounts, and we have participated in different scientific meetings with people from Government in the hope that this will be put in place over the next year or so.