Science Interviews


Tue, 12th Mar 2013

Tidal energy & Turtle Mating Habits

Part of the show Tidal energy, turtle mating habits

Richard Hollingham:I'm Richard Hollingham and this time in a nautically themed Planet Earth podcast. How much of our electricity could be generated by tide energy and the challenges of monitoring turtle mating habits.

David Richardson:It's very hard to observe how many individuals are there, Turtlewhat they are doing and very hard to observe who they are mating with and where and when...

Richard Hollingham:More on turtle conservation efforts later. Well this time I'm on the banks of the River Mersey in Liverpool. Behind me the beautifully restored former warehouses of the Albert Dock, the dramatic new museum of Liverpool and the dome and towers of the Royal Liver Building and the liver birds at the top just about visible through the mist and lapping at the stone quayside just beneath me the Mersey River, a kilometre across there's Birkenhead and the Wirral to the south. Well, I have to confess you can't see a great deal this morning. There's a mist that is hovering over the water, although right above us there's blue sky and I'm with Judith Wolf and Nick Yates from the National Oceanography Centre. They have been investigating the potential of this estuary and others around the UK for generating electricity by exploiting tidal energy.

Now, Judith, what do you mean by tidal energy. I suppose we know what the tides are, but what's tidal energy?

Judith Wolf:Tidal energy is the total amount of kinetic and potential energy in tides. If we consider Liverpool itself we have a ten metre tidal range at spring. That is one of the highest in the UK, almost the highest in the world. That energy can be captured in various ways by running a barrage with turbines. The other kind of energy is the kinetic energy of the flow of the tide and in straights and estuary mouths like here we can get really large flows which could be several knots of water speed and that energy can also be harnessed.

Richard Hollingham:Ten metres here! So between the quay and the water below that rises in a tide by ten metres.

Judith Wolf:That's between low tide and high tide, yes, on the maximum spring tidal range.

Richard Hollingham:So, potentially, a phenomenal amount of energy there?

Judith Wolf:That's right, yes. The amount of energy you can get from a tide in an estuary is related to the area that's behind the mouth of the estuary, the area inside the estuary and the tide range at the mouth.

Richard Hollingham:Now, Nick, you were an engineer, you've been studying the potential of tides around the UK. What do you actually do?

Nick Yates:The study in question was a computer simulation that took a computer model and into that model we can see the tidal wave coming in and then into the model were able to simulate barrages, in particular, across estuaries because as Judith said the energy you get is proportional to the area and also the tidal range, and so the ideal place for a barrage to exploit tidal range, which is a physical structure which allows you to delay tidal motions and get a water level difference like conventional hydro power, and actually behind you - you can't see the other side - but the Mersey you've got geography helping you, it's only a kilometre across. So for a relatively short physical structure you can then place turbines in that and get relatively a large amount of energy. So, just to give you an example for the Mersey, that would be something like a terawatt hour, which is a difficult number to get your head around, but you're probably talking somewhere between half a million and a million houses electricity from that one structure.

Richard Hollingham:So how much energy could you generate in the UK from the tide?

Nick Yates:We think it's going to be at least 20% and 15% from tidal range with barrages over the major estuaries plus 5% from tidal stream which is from the fast motion of currents.

Richard Hollingham:And what sort of structures? Like a conventional hydro-electric plant with turbines? We see these wave generators with almost like bobbing buoys going up and down.

Nick Yates:Yeah, the structure for tidal range - obviously you need something that is impermeable because you are delaying the water. So it's very much like La Rance or if you like it's like a harbour actually, it's very similar technology. So you would have rock and then you would have probably concrete caissons where you have got turbines and sluice gates.

Richard Hollingham:And what about the environmental consequences of putting sometime like that on an estuary like this one here?

Judith Wolf:Well some of the environmental concerns are very much about inter-tidal habitats in estuaries. Estuaries are very productive areas and are important for migratory birds and fish, particularly some of the mud waders and feeding birds on the estuary. Many people are concerned that the habitats that they exploit will disappear. One of the things we did in an earlier study was actually to estimate how we could best minimise that impact. If you run the barrage on ebb and flood generation you can actually modify the amount of habitat that is lost and minimise the amount that is lost.

Richard Hollingham:Now you looked at this across the UK, so different sites. Presumably some are more suitable than others?

Judith Wolf:Yes that is true. I mean for a tidal barrage we're looking for the maximum tidal range and the Mersey is particularly nice because it has a very narrow mouth and therefore you would build the minimum length of dam across it in order to capture a moderately large amount of energy, so the energy would be relatively cheap here.

Richard Hollingham:And so the dam wouldn't have to go right away across the mouth, it would just go across a small amount.

Judith Wolf:It would have to close off the estuary completely but the tide would still flow through the dam, through the sluices and turbines.

Richard Hollingham:And what about shipping, because you are disrupting the flow of the river not just for fish and for wildlife but also so shipping.

Judith Wolf:That's right. Usually in the design you would build a ship lock and also fish ladders so there would be some provision for minimising those impacts.

Richard Hollingham:How does this compare say, Nick, with wind power and other forms of alternative energy generation?

Nick Yates:Well, the key thing with tidal is the renewable energy you can set your watch by, and so that predictability in particular is extremely important. It's funny I was just reading recently from Ofgem some figures about the variability of wind which went from nine megawatts to three gigawatts within the space of three days, but that's not to say that it's an either or. I actually think we're going to need all of them particularly to replace what we get from fossil fuels but also if we move to electric vehicles demand is going to go up. Just point additionally, if I may, to raise about tidal range, you can also get them from so-called lagoons which is a structure not across an estuary and so some of the concerns about environmental modification become less but the economics are not quite so attractive unless subsidies change because you are building along a structure instead of a short structure, essentially half a circle.

Richard Hollingham:Nick, do you think this will happen then?

Nick Yates:There's an interesting question. Certainly I have to declare in a different job there is a project ongoing looking at a lagoon, so there is one being proposed. Whether it will happen or not is very much a matter of political will but in terms of the energy mix I think it is important to have this option. I also think it is important to have a good feel for the size of the resource for planning and after that it is very much down to the planning process and political will.

Richard Hollingham:Nick Yates and Judith Wolf, thank you. And we will put some pictures up of our recording on our Facebook page, although with the mist you won't see much of the Mersey today. We're also on Twitter and do visit Planet Earth Online for the latest news from the natural world and to catch up with a four and a half years worth now of Planet Earth's podcasts. To find us just search for Planet Earth Online.

The hawksbill turtle has, as the name suggests, a bird-like beak, but it is the turtle's high decorative shell that has put in on the critically endangered list. Despite an international ban hawksbill turtle shells continue to be traded on the black market and numbers of these animals continue to decline. The hawksbill lives in tropical waters but because these turtles spend most of their lives at sea it has been difficult to assess exact numbers and very little is known about their mating habits. We do know that females come to lay their eggs on the eco-tourism resort island of Cousine in the Seychelles and recently published research will help conservationists plan for the future. Sue Nelson went to the University of East Anglia which led the study to meet two of those involved in this research and first spoke to molecular ecologist, David Richardson.

David Richardson:The hawksbill turtle is a very beautiful elegant marine turtle. It is probably about half a metre long to a metre. It has got that classic tortoiseshell patterning on its back and that's why the hawksbill turtle was so hunted.


Sue Nelson:This particular island in the Seychelles, how big a population are there, or how important is this site for the turtles?

David Richardson:It is an important site for the turtles in that part of the western Indian Ocean. It does seem to be that that population exists around the Seychelles is unique to that area and within the Seychelles although there is over 100 small islands many of those islands have lots of people on, they're not protected. There's only a few beaches where it has got the right kind of beach, the right kind of sand, and where there is protection so the turtles can lay their eggs without being hindered and Cousine Island is one of those islands and it has quite a healthy population of between 50 and 100 females come up to lay every year and produce offspring that way.

Sue Nelson:Now it's been on the endangered list since 1996, so I was quite surprised considering that's the case that apparently so little is known about its mating habits. Why is that?

David Richardson:Well, the real problem with turtles is that they spend most of their life widely dispersed throughout the oceans living mainly under water and so it is very hard to observe how many individuals are there, what they are doing and very hard to observe who they are mating with and where and when. It's unlike looking at birds where you can see them in a tree. It's very hard to observe what they are doing at any point.

Sue Nelson:Is this why then you decided to look at the genetic material of the turtles because it would be an easier way of finding out what's going on?

David Richardson:Exactly. Because we only ever see the females come up to lay eggs on the beaches every couple of years we never see the males, so by looking at the DNA of the females and also then looking at the DNA of the offspring, the nestlings that come out, we can take that DNA in the nestlings, compare the DNA that must have come from the mother but also work out which DNA that's in the offspring must have come from the father and by doing that we can reconstruct the DNA that must have been in the father and we can reconstruct how many different males must have been fathering the offspring.

Sue Nelson:Now that research is done here at the laboratory by one of your PhD students.

David Richardson:Karl Phillips has been running this project, he's been doing the genetics and the field work for this. He has been looking at that DNA, matching up the offspring with their mother and then reconstructing the DNA of the father and determining which male mated up with each female.

Sue Nelson:In that case I think my next port of call is definitely the labs then to find out from Carl himself what the results have been.

David Richardson:Indeed, yes.

Karl Phillips:Welcome to the lab. You will have to forgive the sound of the air conditioning and the smell as we pass through of the drosophila breeding laboratories. This is the business end of it, as it were.

Sue Nelson:You've got a large refrigerator here - open it up and lots of colourful plastic boxes. There's a blue one.

Karl Phillips:So inside each of these plastic boxes are 100 tubes of ethanol and each tube containing a very small piece of turtle tissue. If I can find the box I'm looking for and show you the kind of samples that we are collecting from these animals in the field.

Sue Nelson:Which part of the turtle is the sample actually from?

Karl Phillips:So from the adult females we collect the sample from the trailing edge of the front flippers and we try to do that when the adult is laying its nest, when an adult female turtle is laying her nest she goes into a kind of trance and she really becomes completely oblivious, so the procedure of collecting a tissue is going to be a bit uncomfortable for the animal but if its done at a time when she's in that kind of trance it's almost like she is under an anesthetic she really doesn't react at all.

Sue Nelson:She's too busy focusing on what she is actually doing.

Karl Phillips:Her eggs and her nest, so we do have to wait until she is actually starting laying the eggs, prior to that point when she is digging the nest chamber, when she is prospecting for a nest site she is extremely vulnerable to disturbance so we stay very well hidden or very far back.

Sue Nelson:So the sample itself is incredibly small isn't it?

Karl Phillips:Yes.

Sue Nelson:You're just getting a pair of tweezers to pop in there.

Karl Phillips:So here is a sample from an adult female.

Sue Nelson:Oh my goodness that is just a couple of millimetres square effectively isn't it?

Karl Phillips:Yeah, and from that I will then take a razor blade and take a very small slither of that and that will yield enough DNA to do a DNA profile of that female. The hatching ones are even smaller. These really are tiny and these are taken -

Sue Nelson:In goes the tweezers again, a different tube this time - oh, yes, this is almost one millimetre square effectively.

Karl Phillips:Yes, that really is tiny. So that's taken from above the right back leg of the hatchling just when it's on its way to the sea. So the flesh of the shell is still soft at this time and we take the biopsy tool, take a little semicircle of flesh and there's good published evidence that this kind of sampling doesn't cause any longterm harm to the hatchlings. So the female turtles lay their nest, we monitor that nest as it is hatching, we take all the little hatchlings, we put them into a bucket, we pick 20 hatchlings at random from that bucket, take a small flesh sample from each of them and then the hatchlings are released to the sea. They are allowed to finish their scampering down the beach.

Sue Nelson:And what have you found then from examining the DNA from these samples?

Karl Phillips:What we've found is that the typical female hawksbill in our population tends to mate with just a single male. Now within a nesting season a female hawksbill will lay four to five clutches of about 160 or so eggs. A bit of variation in that going from as low as 70 up to 220, 230 eggs but five of these clutches, about two weeks apart. Now if we look at the sequential clutches of a particular female, so she has laid five clutches and we look at one, two, three, four, we find that it is the same male for each female who has fathered all of her offspring.

Sue Nelson:So she has the same partner each time. She is being monogamous effectively?

Karl Phillips:She is being monogamous but we don't think she's going out there faithfully to the same gentleman waiting romantically out in the warm tropical waters for her, we think there's no indication that that's going on from what people have seen of turtles in the sea, so it's far, far more likely that she is mating once at the beginning of the season, storing the sperm from that single mating and then using it to fertilise all of the sequential eggs, and when you think of that that's quite an impressive feat. She's laying five clutches of 160 eggs, so 800 eggs all fertilised by a single mating. That's pretty special.

Sue Nelson:Gosh - so with gaps in between. So storing that sperm and then however is weeks or months later?

Karl Phillips:It's typically about two weeks between clutches. So after five clutches, it could be five or six clutches, we're talking 75 days she might have stored that sperm. It has remained viable all that time. To us as mammals this is quite remarkable but sperm storage is an extremely prevalent tactic in many animal groups.

Sue Nelson:Has this ever been discovered before for this turtle?

Karl Phillips:Yes and no. A small study had looked at the nests of ten females on the other side of the Indian Ocean but prior to that there had been nothing on the hawksbill. One other thing I should point out is that a very small number of our females, about 10%, had mated with a second male but what was particularly striking about this was that just as a female had been singly mated and we see the same one father across all five of her clutches, if she were mated twice it would be the same two males across all of her clutches and what's more those two males would have roughly the same proportions of paternity consistently across their nests.

Sue Nelson:So she has stored two male sperm?

Karl Phillips:Yes.

Sue Nelson:In equal proportions almost.

Karl Phillips:Not necessarily equal proportions. One male often has a larger share but he then has the consistently larger share, but if he has 70% of paternity in the first nest he tends to have roughly 70% in the last nest. It doesn't look like she is mating with one male, packing his sperm into the back of her sperm storage reproductive organs then putting another males sperm in on top of that and then using them sequentially, it does look like they are mixed up and that 10% of multiple-mated hawksbills is quite low compared with what we have seen in other marine turtle species.

Sue Nelson:So can this information then that you have discovered, can this help with the conservation of these endangered turtles?

Karl Phillips:Yes it can. One of the most important ways is, as I see it, that it gives a genetic census of the number of breeding turtles that are out there. Without doing this kind of study you wouldn't know how many breeding males are attributing to your population, you might see 50 females nesting of your beach but what if they've all been mated by the same male. We don't see that. In fact every single female in our sample had been mated with a different individual male. Each female, it was the same male for all of her offspring but every single female had mated with a different individual male. That to us is highly indicative that there's a large number of males out there and that's very good from a conservation genetics perspective. It suggests there's a large population and if there's a large breeding population then the population is less susceptible to negative processes such as inbreeding, processes that could further damage the conservation future.

Sue Nelson:So for once this sounds like quite good news from a conservation point of view.

Karl Phillips:It's very good news and coupled with the fact that on some of these protected islands in the Seychelles the hawksbill breeding population is now going up and so as things stand based on this that the future looks quite promising.

Richard Hollingham:Karl Phillips and David Richardson from the University of East Anglia with good news for the endangered hawksbill sea turtle, they were talking to Sue Nelson. And that's the Planet Earth Podcast from the Natural Environment Research Council. Do visit Planet Earth Online for features, news and comments from the natural world. I'm Richard Hollingham from the foggy banks of the River Mersey in Liverpool. Thanks for listening.


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