Secret Lives of Sharks

If you were watching the Euros a week ago on Saturday, you’ll have been shocked by the sight of Danish midfielder Christian Eriksen suddenly collapsing to the ground. According to their team doctor, it was a cardiac arrest - and Eriksen very briefly died right there on the pitch. It’s only thanks to some quick CPR and defibrillation that doctors managed to get him back - he went immediately to a hospital to recover. Strangely, they’ve since claimed they don’t have an explanation for the incident, and that examinations of him - in their words - ‘look fine’! So what might cause a person to drop like that, seemingly out of the blue? Phil Sansom went to Rohin Francis, a cardiologist from the Essex Cardiothoracic Centre, to find out whether he’s seen what’s happened...

Rohin - I wasn't watching live. It was my wedding anniversary, and my wife told me that I should be paying attention to her for some reason... but from watching back the replays, Christian Eriksen appeared to just collapse to the ground unprovoked. He wasn't moving, they lost a pulse, and that's when chest compressions started and he also was defibrillated.

Phil - This can't be a usual thing to happen, right? Someone at the peak of their fitness, quite young, even someone who's been screened for heart problems... so what generally could cause something like this?

Rohin - In any 29-year old a cardiac arrest would be a very rare event, but in a very, very fit 29-year old with no medical problems it's a very unusual event. It's about - something in the region of 1 in 50,000 sportspeople will suffer something like this. But because professional athletes do push their bodies much harder than the rest of us, these problems can manifest whilst engaged in competitive sports, so that's sometimes why we see these rather high profile occasions. And no medical test is a hundred percent accurate, and unfortunately there is a false negative rate, and sometimes abnormalities are just not detectable.

Phil - If you had to go into this as a cardiologist and give, for example, three possible explanations for what this might be, what would they be?

Rohin - If we took somebody competing in competitive sport in general, we know that the most common cause of sudden cardiac arrest in athletes is something called hypertrophic cardiomyopathy. And that's an abnormal thickening of the heart muscle itself. The heart is mostly just a big muscle that that's continually beating, and when the muscle starts thickening that can cause problems. However, one would hope that would have been detected with scans. Again, they're not perfect, and sometimes it can manifest a bit later, but at Christian Eriksen's age, you'd imagine that that would have shown up on a scan. It could be any number of other structural abnormalities to the heart; or it could be an electrical abnormality, and that's probably more likely to be the case here, because electrical abnormalities often don't show up on screening tests because the heart can look completely normal. I suppose another possibility is whether this is an inflammation of the heart muscle, which may not have been present when he'd had previous testing. There's no evidence that Christian Eriksen had COVID, but other viruses as well can cause something called myocarditis, which is an inflammation in the muscle of the heart. But again, this is all pure speculation at the moment.

Phil - I mean, presumably all of these are still pretty rare.

Rohin - Yeah. So I think that's an important point to take home - that these are very unusual. It's obviously really shocking to see someone in peak physical fitness suddenly suffer a life-threatening medical emergency, but these are rare things that happen. Perhaps a more positive message to take away is, if this were to occur, you would know what to do. If somebody collapses, we don't typically recommend checking for a pulse unless you are trained in that, and if somebody is not responsive and not breathing normally, and not breathing at all for 10 seconds, to start chest compressions. And I think there's a lot of hesitancy because people worry that that might be uncomfortable or it's not the right thing to do, but it definitely is in this kind of situation, because you can save a life. So chest compressions, and then early defibrillation. And with every minute that passes, someone's chance of having a full good recovery drops by about 3-7% per minute. So every minute counts, and the sooner you start chest compressions and attach a defibrillator, the higher the chance of that person making a full recovery.

Medications that we use can end up in the environment, where they can alter the behaviour of wild animals, potentially upsetting the ecosystem. That’s the finding of a new study from the University of Florida. Eva Higginbotham has been hearing from Lindsey Reisinger what happens when crayfish are exposed to the antidepressant citalopram, which can pass from wastewater treatment plants into the streams where they live. So why crayfish?

Lindsey - They tend to be really abundant so they can have large impacts. They're major processors of things like algae and leaf litter at the bottom of the stream, so dead leaves that fall in the stream, and also are predators on other small aquatic organisms like aquatic insects. And then they're a major connection to higher level predators. So they're eaten often by predatory fish or by wading birds or mammals like raccoon or mink. So we were really interested in their behavior because they're so ecologically important, and we thought that changes in their behaviour might change aspects of the ecosystem. We specifically wanted to know how an antidepressant, in this case citalopram, which is one of the most commonly prescribed antidepressants, affected the behaviour of crayfish. And we were also interested in how that might alter the impacts of crayfish on the ecosystem.

Eva - So how did you go about studying the effects of antidepressants on crayfish?

Lindsey - We set up a study in artificial streams. So these are tanks that have water circulation, and we put natural components of streams into the tanks. We collected rocks, we put packs of leaves that had been set out in streams and were colonised by invertebrates living in the stream, so small aquatic insects and crustaceans. Then in half of the streams, we put three crayfish that we also collected from the same stream where we got all the other organisms and we added about five milligrams per litre of this common antidepressant, citalopram. So we're probably on the high end of what's common, but definitely in the range of what you would find in the natural environment.

Eva - And did you see a difference in the crayfish that had been exposed to the antidepressant stream versus the non-antidepressant stream?

Lindsey - Yes we did. So after two weeks in the experimental streams, we removed the crayfish and we tested their behaviour. And we found that crayfish that had been exposed to the antidepressant were bolder. So we put them inside a shelter and they came out of the shelter into a novel environment almost twice as fast as the crayfish that had not been exposed. And then we also offered them a choice of water that had different scents in the experiment. So on one side of our tank, we had water coming in that smelled like another crayfish of the same species, and on the other side, we had water coming in that smelled like a really great food source. And when crayfish had been exposed to the drug, they spent almost three times more time in the section that smelled like a food source. And the other crayfish really just walked around the tank and spend an equal amount of time in both sections.

Eva - And do you think that this change in behaviour in the crayfish, do you think that could have consequences if that were happening in the wild?

Lindsey - Yes, we do think it could. So we did measure the ecological effects of crayfish in these tanks, but we didn't see that crayfish that were exposed to the drug had different ecological effects than those that were not exposed, but this might be because we didn't run it for long enough. So in retrospect, we've thought that it probably takes some time for the drug to accumulate in the crayfish and change the crayfish's behaviour. And then it probably takes more time for those changes in behavior to affect the ecosystem. We would try to run this over a longer time period and look for if these changes in behaviour could have larger effects.

Eva - Is there anything we need to do? Is there anything that we should be changing to prevent that from happening?

Lindsey - So I think the easiest thing that we could change would be to make sure we're properly disposing of medications. So some ways that you can do that is there are lots of programmes that will take back medications that are unused. So that would be most ideal thing to do. And if that's not possible, the best thing to do is to put the medication in a container with something absorbent that would be unpalatable to animals. So coffee grounds is a great example and then enclose it in that container before you throw it away.

Far from being solitary hunters, some shark species are social. In fact, some sharks like to ‘hang out’ with the same group of animals for years in a relatively small location. Adam Murphy had a chat with Yannis Papastamatiou to find out what happens when sharks socialise, and if they actually make friends.

Adam - For some, the stereotypical image of a shark is of a lone predator sneaking its way through the ocean with no company, not at all like a pack of wolves. But science is starting to challenge that assumption.

Yannis - At least for some species we have evidence that they do like other individuals would like to spend time with other individuals, and how stable that is can vary. It may be that they just like to hang out with a certain individual for, let's say, one evening. And then the next evening they'll be hanging out with somebody else. But in some cases we have found some sharks which seem to hang out with each other, not continuously on and off, for up to four years.

Adam - That's Yannis Papastamatiou, who's been studying sharks off the coast of the Palmyra Atoll, which is a tiny 12 kilometre long island in the middle of the Pacific ocean. They've been tagging them to see where they go around the ocean and it seems like around this Atoll at least, there are little shark gangs forming.

Yannis - So for example, we know that they show strong residency to the Atoll, and all of them seem to be living there for multiple years, if not much, much longer. We also unexpectedly found that sharks only use small portions of the Atoll. So the Atoll is not very long, it's about 12 kilometres in length, so you're not talking about a very big place. But we found that sharks hang out in small sections. So a shark, for example, that lives on the west side of the Atoll only really stays on the west side of the Atoll. Whereas one that's on the east side of the Atoll only stays on the east side. And that's surprising because a shark could pretty much navigate the Atoll in a day, if it wanted to, that's not a great distance for a shark, but they choose not to. And we also then found out they kind of hang out in these groups together within their various regions and hang out with other individuals within those groups. And sometimes, as I was saying before, those social associations could last many years.

Adam - So why might such long sharp partnerships form up? What's in it for the sharks?

Yannis - Animals sharing information is quite common, and information could be things such as the location of prey. It could also be the arrival of a predator, things like that. The information isn't something that the animal may necessarily want to share - not saying that they purposefully share it with each other, but it's shared nonetheless. And in the case of sea birds, for example, you may have birds that head out to sea to forage, and you may have some birds that will follow other birds because they have a good idea that that bird knows where the good prey can be found. This may be beneficial for everybody in the group because overall your foraging success or hunting success goes up.

Adam - Because this hasn't been studied in detail, we don't know a lot about why sharks do this. And most of our best guesses have to be based off seabirds. But it does have implications for where the concept of being sociable came from in the first place.

Yannis - Sharks in some ways simplify things because they don't have parental care, for example. So a lot of the more sort of complex social interactions you might see with other animals like mammals, you don't have. So it means that we can sort of narrow down the potential drivers of sociality in these animals. There also are some conservation implications because if you have, for example, animals that have evolved behaviours that involve, let's say group foraging, what happens if those groups are reduced in size? So as we know many shark populations are facing threats from overfishing, and it may be more detrimental than we realise because when you make these groups smaller, now these animals that may have evolved mechanisms that involve group hunting can no longer do that. So what does that mean for the forging success for those individuals that are still around?

Adam - And this partnering up? It's not just a good offensive strategy. It's a pretty good defensive one too.

Yannis - Sometimes it may be related to hunting. Sometimes it's going to be related to defence. Again, we like to make the claim that sharks are top of the food chain, top predators, but that's only true for a few species. Most sharks are not at the top of the food chain and they actually have predators. Being in a group can actually have a defensive function as well, almost like a fish school. So that's something we see off our coast here in Florida, where we have, in the spring, tens of thousands of Blacktip sharks, which aren't small, they're about 1.5 - 1.6 metres long, but they are hunted by Great Hammerheads, which are also quite abundant around here. And so you can see Great Hammerheads trying to stalk the group, but once individuals in the Blacktip group have caught wind of a Great Hammerhead, you could see that information spread because suddenly everybody starts to flee as that hammerhead tries to catch its prey.

Adam - So why does it matter then if our view of sharks isn't quite the one we thought it was?

Yannis - We tend to think of sharks as just mindless predators. And the more we find out about them, the more we realise that that's really not the case. In terms of sociality, I like to kind of use the term that this is the secret social lives of sharks. And it's not because they want to keep it a secret, it's more that up until recently, we didn't really know it was there.

With many shark species living far from shore and in the depths of our oceans, it can be a case of out-of-sight, out-of-mind. We may think that since sharks have been around since before the rings of Saturn were formed that they can cope with any changes that come their way, but that is not the case. Adam Murphy had a chat with David Sims to discover what's going on...

David - Well, sharks of the open ocean - these are sharks that swim far from land, they live most of the time in the top layers of a few hundred metres, perhaps down to a thousand metres - it's incredible to think that over the last 50 years they've been exposed to high rates of exploitation from commercial, industrialised fishing activity. There's been, over the last 50 years, about an 18 fold increase in the relative fishing pressure. So far and away fishing is the main threat facing sharks. There are other things - habitat loss and pollution - but fishing is the principal thing that we've seen driving down populations.

Adam - And what kind of impact does it have? What numbers of sharks are being pulled out of the ocean by fishing?

David - The numbers are colossal actually. There have been relatively conservative estimates ranging from 63-273 million sharks pulled out of the ocean, so about a hundred million sharks roughly every year. And about 50% of those are pelagic sharks, oceanic sharks. And since about the 1950s, when this industrial scale fishing started to escalate... large vessels, a few hundred tonnes, with refrigeration, navigation, wayfinding equipment as well as sonars, and also very powerful winches and gears. Some of these longliners, for example - they can deploy a longline with a thousand hooks that's a hundred kilometres long, and do that on a daily basis. And there are several thousands of these plying the oceans. So over the last 50 years, we've seen about a 70% decline in oceanic species of sharks and rays. And that's largely because there's a huge overlap between where the sharks like to hang out - where they prefer to be, their preferred habitats - and where the fishers want to be to catch not only sharks, but also tunas and swordfish.

Adam - Let's say the worst happens and all the sharks and their relatives go away. What impact would that have? Why is this such an important issue other than - it's sad that there are no more sharks?

David - I guess people say, "well, hold on a minute, so what if the sharks aren't there anymore?" Well, clearly sharks have been around a very long time, as we've been hearing in the programme, and they're important components of the ecosystem. They're not all top predators; they have really important roles as mesopredators in stabilising the diversity of food webs. And so it's crucial that some controls, more controls than we have had up to now, are put in place to try to allow these populations to recover. So for example, the shortfin mako shark is the fastest shark in the sea, it can swim up to 40 miles an hour when it's hunting and chasing down these different fish species. And what we found with those is that their population in the North Atlantic at least has declined by about 40% over the last 50 years. And as Jenna was saying, the longevity of these sharks, the late age to maturity, is a problem. Shortfin mako sharks don't reach maturity for a decade or more, and they don't have very many young - a female might only have 12 young every three years. So a species like that just finds it a real problem to replace itself, if indeed the exploitation is happening fast enough. There's only a 60% chance of stock recovery by 2070...

Adam - Wow. That is really coming up. And then speaking of going into the future, what about climate change? That thing that always rears its head - what's that going to do for sharks?

David - Change is going to have an impact; not just perhaps directly through changes in their thermal habitat, which may increase ranges of some sharks, but also the interaction with fisheries. So for example, climate change is a major driver of ocean deoxygenation - so this is the ocean losing its oxygen. And this is happening due to the warming of the ocean, changes in the stratification, the temperature profile of water, but also the ventilation. And what we found in our study satellite tracking sharks is that in areas that are losing oxygen fastest, that's reducing the habitat of sharks in the top surface layer as that low oxygen blob of water starts to rise towards the surface. And the fishers actually are able to catch more sharks in these areas because there less water, less volume for the sharks to be in. And so catches a higher in these areas. So in the future, as deoxygenation climate-driven change is happening, it'll interact with fisheries, perhaps to exacerbate some of the population declines we've already seen.

Adam - And this is all very doom and gloom. So is there any cause for optimism to be had?

David - There is, absolutely, there is cause for optimism, because ultimately fisheries and the control of fishing is within our hands. This is something we can solve. And some management actions for these pelagic oceanic sharks have been put in place, but much more needs to be done. And the problem with a lot of pelagic sharks is they're caught incidentally. It could be that the fishing boats aren't really after the sharks; they're after tuna or swordfish, but sharks get caught anyway on these baited hooks. So we've got to reduce the overlap of sharks and these hooked lines if we're to have any chance of recovery, because of that incidental capture. It's very difficult to be specific about which sharks and where you're catching. And there are negotiations at the moment between the UN - or parties in the UN - to have a treaty for the high seas which better protects biodiversity. And spatial management for marine protected areas in certain places where sharks, for example, like to hang out has got to be part of the future solution.

Adam - What about me sitting in here inland in the middle of Cambridge? Is there anything I can do, or other people can do to get involved and help them out?

David - Yeah, there is, there's a number of things on a number of levels. For example, one can be political and start registering views with policymakers, whether that be in this country or for international bodies, the EU and the United Nations. More people thinking about sharks and being concerned about the population declines we've seen is a force for good; that can often lead to change. But I think one of the most important things is education, learning more about sharks. Why is it that many people are frightened of them? And as we've heard in the programme, there are many reasons why we shouldn't have that point of view. And I think also there's a greater opportunity for social engagement, for citizen science. So it's political, it's educational, but it's also scientific; and we're actually launching a project next year as part of our European Research Council funded work which hopefully should allow citizens to actually start to record data from sharks swimming a thousand metres below the surface in the twilight zone, using a small, four centimetre long video camera. So we're asking citizens to look at images that are taken by these sharks as they swim around to see what it is they're encountering way down there, perhaps below where light actually reaches.