What’s the deal with eels? – Lucy Porter, David Righton and Caroline Durif

0:28Hello, I'm Robin Ince. show, I'm afraid to tell you. Well, mustard seeds are fairy, not an elephant. You have got a little bit of knowledge there. So if we just drop the round about Legolas, Elrond and Hawk the Slayer, which is a pity because that means we won't have all the questions about Bernard Breslau, who was an actor, younger people, you should know. Anyway, so... He's very odd, isn't he? You're odd. Careful, I'm not averse to using a gullship spell. No one here knows about the gold ship spell i cannot believe totally the wrong i do do you think you can do it because you will get the verbal and somatic components required to make a water vessel fly oh i'm going to choose my own adventure anyway so uh today what are we discussing bro today we're discussing eels why is their life cycle so complex how do they navigate on their long migrations and where do they sit on the tree of life and why are jellied eels not as popular as they used to be to help us untangle the complexity of the eel we are joined by a fish behaviourist, a fish ecologist and taking to the surface of the river someone who is the regular hook-a-duck lady at her school faith.

1:37It's true. And they are. I'm David Wrighton and I'm a scientist at the UK Centre for Environment, Fisheries and Aquaculture Science. I've been studying European eels for nearly 20 years, including tracking them across the Atlantic to the Azores and beyond. And one of my most memorable journeys was going for a short walk on a misty Welsh mountainside with my children and turning that into quite a long and lost walk emerging from the mist a couple of hours later than originally intended and I've never been so glad to see a pay and display car park. See Brian that's like the kind of adventure Legolas would have.

2:21I'm Caroline De Reef and I work at the Institute of Marine research in Norway. But I grew up in France and studied eels in France. And I'm interested in fish migration, especially my favorite fish. That's eels and not electric eels. Those are very different. And my weirdest voyage was when I was traveling with my family in Myanmar. And I lost my family and was trying to catch up with them and asked a kid. We were in a village and I I asked a kid along the side of the road if he had seen a sort of European-looking family. And he took me to his home and showed me his family, which was very sweet.

3:09Very awkward.

3:14Adorable. I'm Lucy Porter. I am a comedian. And I was trying to think of a long journey that I've done related to water, given that we're talking about eels and stuff. So the longest water journey I ever did was on the River Thames. And it was when I was in my 20s and my friend was marrying an American woman. And we got on a boat at the Thames Embankment at midday. And they set sail. The couple were going to get married on the boat. And we were going to sail up and down until about 7pm. And they started handing out the champagne. And we all went, oh, this tastes a bit funny. We're all British people in our 20s.

3:51and they said oh yeah because the americans had organized it and they said oh yeah yeah yeah no we're just having elderflower champagne for now because we thought we'll wait to have alcohol till we get back now if you know british people in their 20s spent seven hours people one person attempted to jump off when they spotted a pub and we we i mean i'd be fine with it now but at that Asia we all crawled off like we were sort of returning from an arctic adventure and it felt like a very long time and this is our panel I love that story it reminds me of I went to a Jewish wedding in Philadelphia and the waiters had all been told uh once all the toasts have been done uh you we don't need any more wine apart from that table they're all from Europe

4:50Carol, because I realised we actually weren't going to deal with it in the show but because you've mentioned it, I want to know the difference between electric eel and the eels that we're going to be talking about so what is that, apart from obviously the charge, what is the difference? They're completely different species, families so they have nothing to do actually with anguillid eels which is what we're going to talk about and those are also called freshwater eels and they live in both habitats, so in freshwater and saltwater, whereas electric eels, it's actually not really an eel. Just completely different.

5:30The question has to be then, what is an eel? We might as well start with a definition. So you mentioned they're fish already, but what actually specifically is it? How is it related to a salmon or something? Where do you go? where's the common ancestor? Why do you always ask that? Every show you do on nature because you're a physicist. That one we did about chimpanzees you said how are they related to salmon? The salmon seems to be whatever the last sandwiches you've had What I mean is it's a non-fish looking fish Yeah well the name of the eels is anguiliforms which is not really very helpful because it means something in the shape of an eel So that's That's probably a bad place to start.

6:12But there are hundreds of species of eels, many of which remain within the marine environment all the time. You'd think of things like moray eels or conga eels, or perhaps if any of you scuba dived, you might have seen some things called garden eels and that sort of thing. But the anguillid eels, which we're talking about today, are those that are also called freshwater eels, and there are about 20 species of those. If you were to try and go back and find a common ancestor with us, you'd need to go back about 500 million years, something like that, to what would be a jawless fish, a very primitive fish, similar to the hagfish.

6:47So that would be our common ancestor from all those years ago. But the anguillid eels that we're talking about are relatively recent in that context. They emerged only about 50 to 70 million years ago. And the European eel, which Caroline and I both study, emerged even more recently than that probably about three to five million years ago i love that what is an eel something that looks like an eel is the most what's the gives an overview of them what's the smallest one what's the biggest one um well the the biggest anguillid eel um is the mottled eel which can reach about 200 centimeters in length two meters um and weigh up to about eight kilograms in weight so that's quite a sizable um individual it's taller than me but it weighs less yeah i just put that out there but i would try and fight one yeah i like the fact this is what we're going to use now for every other show we do about any form of living creature we shall use the measurement of lucy porter now of course this is merely a half a lucy porter but full grown and it can be three Lucy Porters.

7:58Yeah, so the smallest, it's difficult to say. Probably, Caroline, you might know this one, but one of the tropical eels, I would imagine. But the temperate eels that we work on tend to be relatively large compared to some of the tropical species. Yeah, so you would think the maximum of the smallest eels, which is the maximum length, would be one metre. So, I mean, because the smallest would be the juveniles. So it's sort of... That's true. It's a silly question. It's one that's just been born. It's a small as well. But, yeah, the maximum size of, like, the eels we study would be one metre, and you rarely encounter those.

8:37So they're, like, usually 60 centimetres. This might sound like a silly question, but what's the defining characteristic? I mean, is it just like it's this thing that's basically a fish with no fins? Is that... What's the specification? Well, they do have fins. They have a long fin all along the top of their body, which joins the tail and then comes around under the belly. So they have this fin all the way along the top and along the bottom. And that actually enables them to swim backwards as well as forwards. They just change the wave of swimming that they're undertaking to do that. So they swim like a snake?

9:09Yes, yes. It's a particular form of swimming called anguiliform swimming. Which sounds very much like an eel. But they also have sort of pectoral fins. Those are the fins at the side of the body which are used for changing their sort of angle to move upwards or move downwards or break or whatever. So they're not finless. They have very, very tiny scales and they do produce a lot of mucus to cover those scales, which means that they have this sort of characteristic slipperiness. I read that some of them use the mucus to sort of protect themselves. from scratches. So I've tried that tonight. I apologise if Trevor gets this chair afterwards.

9:55But is it a sort of protective layer or what's the function of the mucus? One very special characteristic about eels is that they can live out of water for many hours. Is Michael Gove one? I just always have my suspicions. But that's amazing. So, yeah, and the mucus helps. So that they don't dry up. And also it allows them to breathe through their skin somehow to at least, yeah, to get some oxygen. So how long can they be out of the water for? Well, if it's cold, if it's not, you know, in the sun, right in the sun, then they could stay. I mean, I've sometimes, because they also have this ability to escape, which is crazy.

10:42If you have a tank, you really have to block all of the little holes. otherwise they'll find the tiniest one and then just like climb up the wall of the tank and then the next day you just find them on the floor and I've found eels on the floor up to 24 hours I guess after I've left them and that also there are many stories when they go to reproduce at the time of their migration then they will sometimes if they're in a landlocked lake And then they're able to cross over land in wet grass to reach the stream or to reach the sea, actually. It's like British people going to Magaluf. We like to reproduce by the sea.

11:31This is why they're so fascinating as well, because people have found them in the grass trying to reach another water body. and maybe that's why also they're kind of scary and remind people of snakes in addition to their anguillid form. Yeah, it's one of the many unique characteristics of eel but coming back to this form actually, one of the things that's worth saying is that eels exist in different forms. So when they are in their larval phase, they don't even look like eels at all. They're transparent, they have very tiny heads with very pointy teeth, they have a tiny little tail and they drift.

12:07It is my drift. They drift through the ocean on ocean currents. They're sort of transparent as a predator avoidance mechanism. But this difference between the larval eel and the juvenile and the more adult forms is one of the reasons why it took such a long time to address the eel question as to where eels came from, because no one could connect where the larvae were found in the ocean and with the existence of the juvenile glass eels and then the yellow eels in rivers and streams and so on. What is the life cycle? I suppose there are so many species, there's a typical life cycle, but in terms of how long are they in the larval form typically, when do they mature?

12:49Well, in European eaul, a larval form might last up to two or three years, so it's another sort of unique feature, another exceptional sort of aspect to them. So when they hatch as larval, they're probably about six millimetres long, and they might grow up to about 100 millimetres long in the course of that three-year journey across the Atlantic. and then they metamorphose into what are called glass eels. So that's a tiny replica of an eel, but transparent, and they migrate in their hundreds and thousands and millions up to the coast and then populate coastlines, estuaries and streams and rivers.

13:21By transform, how abrupt is that transformation? How does it happen? It's a matter of days, isn't it, I think, Caroline? It was an accidental discovery. So the connection David is talking about between There were these larvae that are called leptocephalus. Lepto means leaf, and cephalus means head. So they look like leaf. They're transparent, very fragile. And they were thought to be another species. And then it was in the 1800s, two Italian scientists caught some leptocephalus larvae and put them in an aquarium. And they saw this metamorphosis into little glass seals. and so that was, for them, it was a complete surprise, and they made the connection that this wasn't another species, but it was the eel larvae, because before that, and this is like a 2 ,000-year-old mystery, Aristotle thought that eels were the example of spontaneous generation, meaning that they did not reproduce, but just that glass eels would come out of, he called it putrefaction of seaweed.

14:33Mice were meant to form in sawdust, weren't they? That was the thing, because if you had sawdust at the bottom of a barrel, you'd go, oh, there's mice there now. They must be made out of sawdust. Can I just talk about genitals for a minute? Yeah. I've been so good. Do you know what? Every show you've ever been on, you have eventually. It's just a bit earlier tonight. It's just to be Aristotle's or the eel's. Just everybody's. If you're going with eel genitalia, I'm happy. But did he not check and couldn't find any genitals? and then kind of went, oh, well, then that's... Yes, that was the other part of the story, is that he never found any eggs in eels like you would find in cod or salmon.

15:15And the reason is that when they go to reproduce, so they spawn in the Sargasso Sea, so they migrate down the rivers and then disappear into the ocean, but they're still pre-adolescent, so they're not sexually mature, so they hardly have any developed gonads. So Aristotle never found any genitalia. So they leave the rivers of Europe as adolescents, basically. Go to that Bermuda, that kind of area. Yeah. And then never come back. It's only the little larva that come back. Right. They die, probably. Most likely because they do not eat during that long journey, which can be up to 8 ,000 kilometers.

16:00so they have sex and then die so they better not have a headache yes that's it it's this incredible level of just going this is the only purpose is to make sure more of you come forward that is biology it's a whole of biology and it can seem from a human perspective not having a hobby or all those other things where it's just like It can feel very unfair, as far as I'm concerned. It raises so many questions, Dave, because that migration, for a start, that's a monarch butterfly-style migration. So what do you say, it's 8 ,000 kilometres? Yeah, at its maximum it would be 8 ,000 kilometres or so from the easternmost point in which Eels are found.

16:47The obvious question is how. Well, yeah, that's one of the things that people have been trying to work out for a long time, ever since the Danish scientist Johan Schmidt discovered that the spawning area was in the Sargasso Sea. And that journey across the Atlantic Ocean is something that's motivated generations of scientists, possibly since Aristotle, including myself and Caroline, to try and understand a little bit more about that and how they do it. Because they must transform into this final version of an eel in this part of their life cycle. They start as a leptocephalus. They turn into a glass eel.

17:24They then become elvers and then yellow eels, which is where they spend the majority of their lives, which can be decades long in the case of the European eel. The oldest ever recorded was about 90 years old. Wow. But the sort of average age of eels in the UK is probably sort of 15 or so, or perhaps a little bit even higher than that. But silver eels are the migratory form, so-called because of their silvery belly, but they also have a dark upper side, the dorsal side, and they have special adaptations for migrating across those ocean distances. so that when they an individual sort of chooses let's say it's the wrong word but when it begins its migration so that varies it's not just when they are 10 years old or when they are 15 years old it can be when they are 30 years old yes yes or even older and this is one of the one of the challenges in understanding eel population dynamics because the the relationship between the age of the eels leaving and the larvae that then come back is very very difficult to untangle what we call the stock recruitment relationship.

18:25So how do they migrate? You mean how do they navigate? Navigate, yeah. It's a huge challenge when you think about they are distributed all over Europe and North Africa. So eels from northern Norway will have to congregate with eels swimming from Morocco, for example, and find a common place. And because we see that the genetic structure is just a big mix, So this is what happens. It's a big orgy in the Sargasso Sea. They all meet. And so they have to meet each other at the right time, at the right location, at the right depth. And it's just amazing. And so there's a series of cues that they have, both in time.

19:13So generally, they migrate during the fall. But that will be different according to where they are because the Spanish eels will take a shorter time. And magically, they leave later. Yes. And then the Norwegian ones leave earlier in the season. And it's regulated by, we think, by photoperiod. So there's more light in Spain versus Norway. And so that kind of triggers the timing of their migration. And after that, to find their way to the spawning area, then there aren't many signposts. They do have huge, what we call olfactory bulbs. So they have a tiny brain, but the part of the brain that is responsible for smell is huge.

20:02So that probably plays a role, and it increases also at the reproductive stage. But what we think is the major signpost or cue for them to navigate is the Earth's magnetic field. And that's what I've been working on for 20 years. partly because we know that many other organisms like turtles, birds, butterflies use the Earth's magnetic field. In the magnetic field you have three characteristics that you can use. So you have the direction of the field which is magnetic north so that's your basic compass but you also have the intensity, the pull of the magnet which changes between the pole and the equator.

20:51So it's very strong at the poles, and it's weak at the equator. So if you can sense the intensity of the magnetic field, then you know where you are more or less on a latitude. And if you also can sense the magnetic north, then you can more or less navigate. So this is probably the only model that makes sense with regards to finding the sargassism. It's amazing how mysterious it all is, isn't it? So what for you was that first story that you thought, this is something I have to unravel, this is something I have to know more about? Well, for one, we've never found the spawning area. I mean, literally, we...

21:31That was going to be my question. How accurate are they? The only reason we know is that because this Danish oceanographer trawled the Atlantic Ocean in the 19-something, and he measured Leptocephalus larvae for 20 years and he saw that the smallest ones were in the Sargasso Sea. But up to very recently, that was all we had and then now we're able to track eels and this is where David's studies come in. And so we know pretty much where they are migrating, where they're spawning, but we've never been able to observe it. And I think that's fascinating. Like, we've never found eggs. So it's still a question, and some scientists believe that there are spawning areas in the Mediterranean.

22:21So this, I think for me, that was a species where there's so many mysteries. Also, we don't know what triggers sexual maturation. So there's been experiments where we inject them with hormones, but we've never really found the natural triggers for sexual maturation. So if they stay in continental waters, then they're eternal adolescence in a way. It's unbelievable to me because it's such a common thing. You know, when you say, well, we don't really understand the life cycle of this, you think of some rare animal or plant. There aren't many of them around. But these just everybody knows these things.

23:03And have, you know, historically... I was reading about Ely in the Fens, which is called Ely because it's like Eel Island, and they used to just sort of scoop them out the water, and the Bishop of Ely was paid in eels. He used to get 16 ,000 eels a year. It was pretty good going for him. But no, 80 ,000. He used to get 80 ,000 eels a year, the Bishop of Ely. What year was that? Because I'm thinking of inflation and stuff. It feels like... And I've eaten, I hate to say it, because you're not allowed to, but I have eaten jelly deals, which was about 25 years ago, and it remains the worst thing I've ever eaten, which is quite impressive.

23:47This is a big question that we have, which is around Europe and in Japan. They are eating eels, or they did eat eels. They had an enormous number of really delicious recipes. But again, we seem to focus very much on the mucus element in the UK, which is a very common part of our English cuisine, obviously. How can we make this a bit more mucusy? Yes. So what was it like? I've never had jellied eels. Oh, completely foul. It was in a pie and mash restaurant and I actually had some American friends over. And I mean, the only thing I can say, I think they serve the jellied eels to make the pie and mash seem comparatively appealing.

24:23So Lucy was just saying that you're not allowed to eat eels anymore. Is that correct? Well, it's not quite, no. There are still some fisheries for eels, but it's now highly regulated because the European eel is classified as critically endangered. The eel is probably the most widely distributed fish in Europe. Any country with a coastline will have eels within its borders. It used to be obviously one of the most common fish that people would encounter if it's in every river, every stream and so on. That's why the Bishop of Ely was receiving 80 ,000 eels a year as payment. and this was a common thing, for lords of the manor to be paid by their serbs in sticks of eels.

25:02So why are they endangered? If you were to sort of put together a blueprint, if you like, or a design plan for a fish species that would be impacted by the various challenges of the Anthropocene, the age in which we live, the eel would probably be it. It's a long-lived species that reproduces only once, so it can bioaccumulate pollutants, for example, which can be damaging the risk of disease, obviously the risk of encountering disease or parasites. It lives in a habitat that is now highly modified in which there are many barriers, dams, power stations and so on. And it has a life stage in its larval form, which could be impacted by, for example, climate change and changes in ocean currents, but also in its adult and migratory form, which can also be impacted.

25:51And of course there is the big issue of fishing and fishing rates as well, which can impact upon population sizes. So it is critically endangered, but there are management plans in place. The difficulty is, you know, for the eel recovery plan, the eel management plan, is that eels are so long-lived, their generation time is so long, that actually it will take quite a long time for that plan to come into action. So the people who are designing the plan and putting the plan into action may not actually be around to see the recovery of the eel stocks to their former high levels. but there are some encouraging signs.

26:25Just wanted to add also that we don't know how to farm or do aquaculture of eels because we manage to fertilise them and to get larvae, but we don't know what to feed them. So they die, and that's a big problem. So everybody relies on fishing class eels and having on-growing farms' aquaculture. We don't know what to feed them. We don't know what to feed the larvae. Wow. We think that in nature, in the wild, they feed on what we call marine snow. And they have these, like David was describing, these very long teeth. And they don't really swim, so they drift with the Gulf Stream after they hatch.

27:11And they probably latch on to marine snow and feed on that. And also since their drift takes two to three years to reach the coastline, then their diet probably changes and we're not able to to reproduce that so there's a lot of fishing pressure also on the glass seals to have some kind of aquaculture you mentioned in your introduction that you've been involved in an expedition that went so trying to track them essentially across to the azores and then onwards so could you describe how we do that and how much success we've had yeah so there's there's been a number of different phases um in um of technology really that's that's helped that and the very early work was done with tags that will make a little sound a little ping what's called acoustic tags and people would track eels that were um tagged with an acoustic tag and they would have a hydrophone at the sea surface they'd let the eel go and they would try and track the eel as you follow it so basically follow it in a boat you'd have a couple of operators on the boat listening for the ping of the of the tag and then they would navigate and maneuver the boat to follow the eel sometimes for days i think the longest um the longest track is about a week long um you know a few tens of kilometers but that technology obviously has its limits i love the fact you always called it the longest track because that sounds like they were recording it and then took it to abbey road and released it here is a new christmas number one it's eel ping and um well i do have some ideas about how that could be possible but getting back to the...

28:44Oh, if you can... No, no, no, no. This sounds like there's money here. Let's work out how we can turn these eels into recording artists because once they start making money, they'll definitely survive. That's the way the world works. Well, what we do now is we use a type of tag called an archival tag. That's a tag that records the environment that the eel is in, the depth and the temperature and the light, and those data are stored within an internal memory in the tag. And then after a particular period of time, some six months or a year, that tag is then released from the eel and the data can be transmitted to us by satellite, so communicated through satellite and then beamed down into our laboratories and so on, and we can reconstruct the life of that eel.

29:22So the tag's acting a bit like a sort of black box in an aeroplane, and we can try and understand where it is and how it's been behaving. There are other types of tags that we've used. I've got a little example here, if you're interested. So this is an example of another tag we use, which is an archival tag. It doesn't communicate, but it drifts. So when the eel dies, this tag will come out of the eel, and that will then drift back. And in some cases, they'll drift back and land up on beaches, on shores, and they'll be recovered by people who are walking on beaches. And this particular example recorded the migration of an eel for about 1 ,000 kilometres from the west of Ireland in about 2010.

30:04and at about 600 metres one day in that eel's life, it was eaten. It was eaten by a long-finned pilot whale. And we know that because the pattern of data that we get from the tag allows us to look at the depths and the temperatures. Obviously, the temperature increased above water temperature as soon as the tag was eaten to 37 degrees. And then the depth sensor was recording then surface-based dives of the mammal that had eaten it. Here is an example of predation, 600 metres. There's no light really at 600 metres that we could ever distinguish. So really it's a fantastic example of what goes on in the deep ocean and how you can find these things out.