Sea Turtle Stories – Episode 5

Episode Transcript

Dr Minnie Liddell: Welcome to another episode of Sea Turtle Stories, a podcast by the Olive Ridley Project where we take a deep dive into all things sea turtles.

I am Dr Minnie, sea turtle veterinarian and the host of this podcast

So without further ado, I’m really excited to introduce our guest today, Sean Williamson. Hi, Sean.

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Sean: Hi, Minnie.

Minnie: Thank you so much for joining us. I’ll go through a little bit about your background. So Sean is a research fellow at Monash University in Melbourne, Australia. With a strong passion for wildlife and conservation, Sean’s research spans ecophysiology, spatial ecology and conservation science. His academic journey began with a Bachelor of Science at the University of Wollongong in Australia, and the University College Cork in Ireland. Sean then completed his PhD in Monash in, with his research on the embryonic development of turtles and crocodiles.

Following the PhD, Sean’s commitment to marine conservation led him to Upwell Turtles, an NGO where he served as a research scientist for a year. Since 2019, he has been a postdoctoral researcher at both Monash and Florida Atlantic Universities, delving into the conservation, spatial ecology and physiology of marine megafauna such as turtles, sharks and crocodiles. So thank you so much, Sean, for being here today.

Sean: Thanks for having me and it’s a pleasure to join you on the podcast series.

Minnie: We’re really excited to pick your brains a bit about a really interesting part of a sea turtle’s life cycle, something that we call the lost years, as you’ve done a bit of research into that, which we’d really love to hear about. Could you tell our listeners exactly what the lost years of a sea turtle are?

Sean: So lost years is essentially a term that’s used within sea turtle biology that refers to the early life history period of sea turtles. There are a lot of researchers and conservationists who were a bit flabbergasted figuring out where on earth these hatchlings that were coming off nesting beaches were ending up, because there was not much of a record of them being found frequently, at least within the oceanic environment. So the period between a hatchling entering the water and then subsequently recruiting as a juvenile or a sub-adult to the main foraging areas was termed the lost years. We’re still very limited in our knowledge of what happens to these tiny turtles when they enter the ocean.

Minnie: Yeah, sure. Okay. Could you tell us a little bit about the difference between the hatchlings and also juveniles, just for our audience to kind of clarify these terms for us?

Sean: Yeah, it’s a bit confusing even for myself as a sea turtle biologist, and it does change between species. You know, without a doubt, we could definitely all agree that hatchling is a small sea turtle that’s recently emerged from the egg and then crawled from the nest down to the water’s edge.
Now, that’s definitely what we would call a hatchling. Some people then refer to the moment they enter the ocean as post hatchlings, and once they hit usually one year of age, you can also see people referring to them in the literature as yearlings. So sort of denoting that they’re no longer a hatchling at all. They’re closer to a yearling, but it’s probably still a bit ambiguous as to when we start calling them juveniles, depending on the species. These sea turtles all have varying growth rates, and that is probably a function of the resources and the water temperature that they’re exposed to in different regions of the world. So turtles in one area may grow faster than other areas.

Minnie: That’s really cool. It just goes to show, though, how everything you do with sea turtles just has to be really slow. Everything just takes a lot of time to figure out, and you have to really have some patience when you’re working with them. I suppose that kind of follows on to my next question, which is probably going to be quite hard to answer really. How long do these lost years last? Is there a year’s sort of stretch that we can kind of say that there is, or really do we not actually have a clue of how many years some of these guys are out there?

Sean: Yeah, it’s a good question, and again, it’s kind of contingent on us getting a better idea of the age of a lot of these turtles. When they do first show up on foraging grounds, we go off size, which is not necessarily a good indicator of age. So we don’t really have an answer to that necessarily at the moment. In some regions, we certainly have a much better idea of, I would say, for example, for green sea turtles that there’s much more knowledge around what age we think they are when they go through these different stages of development. So whether they’re a hatchling or a juvenile, yearling or sub-adult, and that’s because a lot of the green turtle populations have been well studied.

Minnie: So yeah, I used to say to people, for example, my experience is in the Maldives, and the Maldives is quite a well-known sort of foraging site for juvenile hawksbill turtles. And so at that point, we would always say, obviously, when we started seeing these turtles back in coastal waters, we would consider that to be when the lost years had ended.They’ve gone out to sort of deep waters, and then they have kind of recruited back into the more near shore areas. And so therefore, they’re now found. I guess they’re not lost.

We’re seeing them regularly, but some of them were tiny. They were sort of maybe 25 centimeters, 30 centimeters carapace length, straight carapace estimated. We couldn’t measure a lot of them, although we have recently done a little bit of research into that. So we will have more data on specific sizing and things like that, but they were just minuscule. And so it was interesting to see that some of these really, really tiny guys had seemingly kind of recruited back. And I always wondered, people would ask me how old I estimated they were, and I obviously didn’t really have any particularly good answer. I think I used to say that they were maybe five, maybe they were six. We didn’t really know. I was totally guessing and probably not very educated to guess, come to think of it now, after all that you’ve said.

But yeah, it must be quite variable. Potentially could they be spending longer than like a decade out in the lost years? Or do we think it’s under that kind of time?

Sean: We think it’s under that time, especially for the hard shell species. And I suppose if that’s, you know, how we’re sort of thinking about it in terms of recruiting to a particular area, like that example you just gave with the hawksbill sea turtle, and it’s similar as well for the green sea turtle, where they will recruit to reef structures as juveniles. And you could consider that to be the end of the lost years period for those two species.

But for other species, maybe we can’t, maybe there isn’t so much a thing as the end of the lost years, for example, for leatherbacks, which are essentially roaming the open oceans throughout their life. They’re not recruiting to a particular or inhabiting a particular area of the ocean, really, at any point throughout their life history, except for when they get to the adult stage, and then they’re heading to their natal nesting beaches to reproduce.

Minnie: I guess that might be interesting about, so obviously of the seven species, maybe I could ask you, as you are from Australia, you might be able to talk to us a little bit about the flatback, because the flatback turtle is quite interesting in that, as far as we understand, they don’t really have this oceanic phase. So they don’t go off into deep waters and then return, like the greens or the hawksbills that we just mentioned. They seem to kind of stay around the coastal waters, is that right?

Sean: Yeah, that’s as far as we’re aware, that’s correct.

Minnie: Do you know if you can find little, very small flatbacks kind of mooching around the coast of Australia, or do we still not quite know where they’re going? They’re just not going as far out.

Sean: Yeah, I would say that we still actually don’t have that good of an idea, would be my sort of take on it at the moment. In saying that, there was a really great publication that came out just recently this month from the Australian Institute of Marine Science, and the study essentially looked at flatback sea turtles in Western Australia. The majority of these hatchlings would be retained within what we call the neritic environment, so the more shallow environment of Australia’s oceans. So relatively close to the coast still, still expanding quite some distance, but in comparison to some of the other species, they’re certainly staying relatively close to shore. As opposed to loggerhead sea turtles, which we know a lot more about in the Atlantic Ocean that nest on the east coast of the US in places like Florida and the Carolinas.

We know that they, as hatchlings, will drift in the Gulf Stream all the way across to the other side of the Atlantic Ocean until they get around in line with kind of France and then start heading further south down towards the Azores. So, you know, very different strategies.

And the team in Western Australia have been using acoustic tags. So these are tags that are very, very small, much, much smaller than what a satellite tag is, but they place these acoustic tags on hatchlings as they’re leaving the beach, flatback hatchlings in Western Australia. And so these sorts of studies that are trying to understand the capabilities, the dispersal capabilities, the swimming behavior of these small turtles can then inform researchers to be able to say, okay, to what extent do we think these hatchlings are able to control their own destiny, so to speak, or influence their dispersal? Are they completely passive? Are they just drifting with the currents without any influence on what path they take through the ocean? Or do they actually have the ability to swim strongly enough and I suppose for long enough to be able to end up in an area of the ocean that we wouldn’t have predicted if they were just passively drifting?

So this sort of passive versus active dispersal is something that’s still debated or looked at a lot within the literature on lost year sea turtles. And over the last probably 10 years or so, we’re starting to move a lot more towards developing models that incorporate some amount of active dispersal from these hatchlings so that we’re not just treating them as passive drifters, so to speak.

Because from a lot of this early preliminary work that, for example, the team in Western Australia have done and then myself in collaboration with Upwell Turtles have done in places like Costa Rica with leatherback sea turtles, we’ve definitely found that hatchlings are capable of swimming in patterns and directions offshore that would mean that they’re not necessarily going to end up in the place that we would predict if they were just passively drifting. They certainly will be largely influenced by oceanographic currents, but they have the ability to move around quite substantially and where we would predict they end up becomes a little bit more complicated to work out.

Minnie: We did not account for free will.

Sean: Yeah, or the actual endurance and the swimming ability of these little turtles.

Minnie: The sheer dedication. I guess hatchlings, they have that, I guess what we call the swimming frenzy when they first hit the water. And I guess they’re powerful little things, actually. They can really make a move.

So I suppose it’s not surprising to suggest actually that they’re not just passively floating around in the ocean.

AD: You are listening to Sea Turtle Stories, a podcast about all things sea turtles, brought to you by the Olive Ridley Project.

Minnie: Do we know how much navigational capacity these little guys have? We know from research how these guys find their nesting beaches, we know that hatchlings imprint on the magnetic signature of the earth from where they were laid, so they can return to that area in 20, 30 years or whenever when they need to lay their own eggs. But do they have that capacity as well, I guess, from very early on? Can they, do we think that they can navigate the oceans or are they sort of just trying to stay where the food is?

Sean: It’s a slightly complicated answer. The most researched species with regard to the ability to navigate in the ocean as small turtles probably comes from the loggerhead sea turtle. The loggerhead sea turtle has been researched by someone called Ken Loman, who’s a professor based in the United States working on the east coast of America and the nests are laid on the east coast of the US in places like Florida through up into North Carolina. The hatchlings then enter the water and the Gulf Stream is pretty strong off the east coast of the US there and they get convected into the Gulf Stream and then they head all the way across the Atlantic until they get to roughly in line with the north of France, and at that point, instead of continuing on up towards Britain or in between the channel between Britain and France, they actually, we find, start to swim south and they end up heading down to past Portugal and towards regions like the Azores and they will very reliably show up at about what we think one year of age in the Azores or give or take a couple of months either side of that.

Ken’s group have pretty reliably shown now with a lot of really great experiments and some really solid evidence that these loggerhead sea turtles are definitely changing their swimming behavior and the directionality of their swimming when they experience various different geomagnetic signatures around the Atlantic Ocean. It’s the signature of the Earth’s magnetic field that then indicates to these small turtles, oh no, I’ve gone too far north, I need to start swimming south, sort of this instinct that kicks in once they start to receive a certain geomagnetic signature. So that is some really great evidence that yes, these little turtles are responding to cues in the environment which then tell them to behave in a certain way.

I would really be cautious of applying that knowledge to every population or every species of sea turtle. For example, the Eastern Pacific leatherback population off the west coast of the Americas, so places like Mexico and Costa Rica, the modeling work on the dispersal of those turtles which has been done by George Schillinger has shown that those sea turtles, the little leatherbacks disperse through quite wide expanses of the Pacific Ocean, but they’re not necessarily following a really cyclical pattern around the Pacific Ocean, so to speak. What’s probably more important to the leatherback is the ocean’s temperature.

So these turtles would be trying to avoid areas that are outside of the thermal optimum. These small individuals are much more constrained to lower latitudes, so they’d be needing to stay closer to the equator than what the subadult or the adults are. So it is much more contingent probably on making sure that they’re not straying into waters where they’re going to get too cold. But also likewise, there are regions of the ocean that can be too hot at various times and they want to avoid those. For the different species, there’s going to be different cues that exist within the environment that they pick up on and that behave in a responsive way to that cue in the environment.

So for some populations or species that might be geomagnetic cues, temperature cues, foraging cues, so chemical cues within the water where they’re picking up on prey availability in particular areas of good food patches and then they’re heading towards those areas. So yeah, I suppose those are the kind of different things that might be dictating where in the ocean these little turtles are heading to.

Minnie: hat sounds really interesting. People sort of talk about how these baby turtles are just kind of washing around in the ocean waiting to get big. And actually maybe they’re not doing quite as much washing around as we previously said. So they’re actually taking really active steps to maintain themselves in certain areas, which is super interesting for something that can be literally hours old that’s trying to figure out where to go.

Sean: Yeah, I would say that I don’t want to paint the picture obviously that they’re completely in control of their own destiny.

Minnie: No, no, they’re too small for that.

Sean: I think a good example would be like through Upwell, I had the absolute honor of being able to go out to the Azores in the middle of the Atlantic Ocean, which is Portuguese territory. And we were putting microsatellite tags on lost years loggerhead sea turtles.

What we find with those loggerheads that once they’ve reached the Azores, they are often sort of passively drifting within ocean currents because there’ll be these eddies that exist. So they’re kind of circular ocean current patterns that exist around the Azores and they’ll kind of get into one eddy and stay within a particular temperature. And if that eddy then kind of pushes into some particularly cold water, they may end up trying to disperse away from that colder area.
So it’s sort of a combination of passive drifting with active swimming at various points. So they certainly will, especially these little loggerheads are very cute when they rest at the surface with the flippers all tucked up, looking like a floating leaf essentially. They will certainly passively drift for extensive periods of time. But if they encounter habitats that aren’t ideal for them, they will definitely try and do their best to get to somewhere better.

Minnie: That’s really cool. I just think of them out there. There’s so many ways in which this is really important to figure out for lots of different reasons. But what would you say, I guess, why do you feel it is so important to know this? What can it help to do? And I guess, how can it guide our conservation efforts by having more information on these lost years?

Sean: Yeah, it’s a great question. A lot of people around the world were probably aware that sea turtles face a lot of threats throughout their life, whether that be on nesting beaches or in the ocean. If we can’t even find these lost year turtles and understand where they go during their development, we have very little hope in trying to understand what threat they face in that area from something, for example, like plastic pollution. And obviously, being a much smaller turtle, we would imagine that ingestion of plastic or other threats that might occur within that area could be potentially worse or they could be potentially less impactful depending on what the threat is.

So we really do need to find out more about where these turtles are heading so we can understand what threats are currently overlapping in those regions where they develop to try and better understand how those threats might be impacting them and then how we might be able to mitigate those threats that exist within those regions.

Minnie: Yeah, for sure. I guess if we don’t know where they are, we can’t work to protect the areas that they occupy and implement the changes. I was going to ask you actually about the sort of risks that these guys face during their lost years. Is that obviously predation, I guess being eaten by things must be a pretty big risk, but also kind of other main threats you mentioned, pollution, bycatch, I guess that’s a bit of a question mark. Then we don’t know how affected they are by that, but plastic pollution then I assume we think is quite a significant risk to these guys during their lost years. But also, what eats them when they’re out there?

Sean: Anything will have a go with a little hatchling, that’s for sure. Whether it be a small crab that carries them off the beach down into the little crab hole, it’s really quite a confronting sight to see at a lot of nesting beaches.

But I think we have to remember that these sea turtles are part of an ecosystem, and these ecological interactions have been occurring for millennia and they’re really important for those predator species as well. So lots of things will have a go at a small sea turtle, essentially anything that can fit them inside their mouth. Sometimes even things that can’t, I’ve seen, for example, seagulls predate upon hatchling green sea turtles a lot at Heron Island on the Great Barrier Reef in Australia. And they can’t swallow the hatchling because they just don’t have a gape that’s wide enough to accomplish the feat. But they’ll still very inhumanely behead them and pick out bits from within.

So they’re definitely on the menu for a lot of different animals out there, different species of fish, snapper, for example, or dolphin fish, or lots of reef sharks that hang around close to the nesting beaches and will try and pick off hatchlings. And then once they get more out into that pelagic environment, they still face lots of threats there, like whether it be large predatory fish like marlin or sharks still. And they will also run the risk of being predated upon by foraging seabirds, also they’re still small.

During our sort of last year’s research, working on leatherback sea turtles on the Caribbean coast of Costa Rica, we were following little leatherback sea turtles in a dinghy boat. Essentially, we’d put acoustic tags onto these turtles. So we were following these hatchlings and we could only really do one at a time. So we followed them fairly extensively over quite a few weeks and would release one hatchling after the other and see where they ended up in the ocean. And during that time, we actually didn’t see that many of them get predated, which was something that was quite surprising to us.

A lot of the literature sort of suggests that predation upon turtles when they enter the ocean is greatest within the surf zone. So the wave action zone between the beach and the open ocean. And we were expecting to see a lot more predation events occur while we were tracking these turtles. We certainly did see some, like I saw a frigatebird take one and then we definitely saw a dolphin fish get one. So they do face a variety of predators as they’re moving about through the ocean.

Minnie: Well, I think you mentioned actually, you kind of touched upon that fact that, yeah, sea turtles obviously have been around for a very long time and they have quite a high volume sort of reproductive strategy of having as many babies as they can fit into a nest, 120, 130 eggs at a time. Because actually, baby sea turtles do represent quite an important part of the marine food web. So I guess it’s, although it’s quite hard for people sometimes to maybe accept that these little guys are, to an extent, fish food for a lot of other predatory fish.

Minnie: I suppose, when pollution gets involved or when sort of human-induced threats start to limit their survivability, that’s when we sort of have to take action. But actually, unfortunately, to an extent, baby sea turtles in their lost years are important parts of the food chain.

Sean: Yeah, for sure.

Minnie: Just, I guess then actually, just to maybe kind of finish this really interesting topic, we’ve discussed a bit about the difficulties that are faced in researching this particular life stage of turtles because of things like tagging being both, I guess, there’s ethical considerations, there’s financial considerations, there are size considerations. All of these things can make it quite difficult to research these guys. What are your sort of hopes for the research into this area now that tags are becoming more sophisticated? Do you think we would be in a position to ever tag hatchlings? Would that ever be something that is possible with the size of tags? Or what do you sort of hope the next steps might be in this particular field?

Sean: Yeah, it’s a great question. And I think technology is constantly amazing us, right? And I’m aware of some really exciting projects that are in development in places like in the Middle East, in Saudi Arabia, that they have some research teams at Cal State University that are working on flexible satellite tags, which are, that would be a really exciting development.

At the moment, a lot of satellite tags are quite rigid. So when you place them onto a turtle using an adhesive like epoxy or something like this, as the turtle grows and expands, eventually that epoxy has to break off and then you lose the tag and you’re no longer able to track that individual. The really exciting component of introducing flexible tags would be that as the animal grows, the tag could potentially grow with the turtle. So we may be able to get much longer durations of tracking completed. So it might be feasible in the future to track an individual turtle from very early life history stage through until they’re much older.

And likewise, miniaturization of technology is constantly improving and increasing. So I wouldn’t be surprised if in the future that, yeah, we can tag hatchlings with really small, perhaps flexible tags. The tags that we’re using at the moment are really lightweight. And, you know, just for context, we were putting these small tags on leatherback turtles that were around four to six months of age. So they’re really young turtles. And I don’t see it being too far of a stretch in the future that the tagging technology could become small enough to be able to actually place these sort of satellite tags on hatchlings. That possibly with improvements in circuitry and especially with battery design, we might be able to see, you know, really tiny satellite tags being placed on hatchling turtles.

Minnie: Yeah, that’d be amazing, I guess, to be able to definitively tag the youngest and smallest of the lot and see if we can follow them through for as long as possible. I guess that would give us as much insight as we would hope to achieve to see what they’re doing. So it sounds like a really exciting area to pursue and obviously also a really important one from a conservation perspective.

So it’s been super, super fascinating to pick your brains on this. But unfortunately, I think we’re gonna have to bring it to a close there. Thank you so much for your time and all the information you’ve shared with us and all the work that you’ve done.

It’s been really, really interesting and I’m sure our listeners have left with a really incredible insight into this really crucial development phase and interesting part of a sea turtle’s lifestyle. So thank you so much, Sean.

Sean: Oh, it was my pleasure, Minnie. Thanks for having me on and it was great talking to you.

Thank you all to everyone who’s listening. We would love to hear your thoughts. So please do leave us a review and let us know. If you’d like to learn more about sea turtles and the Olive Ridley Project’s work, please visit our website where you can also support it by naming and adopting sea turtles or adopting one of our sea turtle patients. And lastly, you can follow us on Instagram, Facebook, Twitter, and even TikTok and stay up to date with the world of sea turtles.

So we will see you for our next episode. And until then, I hope you have a totally awesome week.