People tend to think of sharks  as “fearsome”, “powerful”, “untouchable” predators.

They’re  definitely scary and powerful. But untouchable?

Maybe not  as much as you might imagine. Even the white shark, often considered  the largest predatory fish on the planet, is not immune to the  unforgiving nature of, well, nature. These iconic predators of the  sea face all kinds of threats.

And in two different places,  for two apparently different reasons, great whites are dying in large numbers. And scientists aren’t totally sure why. [♪INTRO] First, let’s introduce the  great white shark, also known as the white pointer or  just simply the white shark. They were named for their white undersides, despite the fact that most of  the shark is actually grey.

White sharks are common in popular media. Why wouldn’t they be? They grow to around 4 and a half kilometers long and roughly 1800 kilograms, and who doesn’t love a massive predator?

They also make pretty compelling  subjects for lots of scientific research, and they can be easier  to study than other sharks. See, studying migratory species is extremely hard. They spend time far offshore and in deep waters that are really hard to reach.

But, even though white sharks are  also migratory, their patterns can be pretty predictable, and they tend  to spend a lot of their time close to beaches and in shallower waters  when they’re not crossing oceans. Bad news if you’re a seal. But if you’re a researcher,  this makes your job way easier.

And the truth is, sharks,  including the white shark, are critical to the health  of our ocean ecosystems. They help regulate the  population sizes and behaviors of animals at lower trophic levels,  such as sea lions and otters. So if we want a healthy ocean, and therefore a healthy planet, we want healthy sharks.

Which is where our problem lies.  Let’s start with South Africa. White sharks are ubiquitous  in South African waters, aggregating in several regions around the coast. Or at least, they used to be.

Over the past decade, people  have seen fewer white sharks swimming in those waters, and more  of them washing up on beaches. Beached shark carcasses,  shark-casses if you will, were often found with missing livers  and perfect slashes on them, so precise and clean they almost seemed surgical. Which probably raised more  questions than it answered.

And some people were worried that swordfish had evolved into scalpel-fish. But the discovery of tooth  marks on the bodies lead scientists to a more feasible hypothesis: orcas. Although we often describe  white sharks as if they’re the top of the food chain,  that’s not always the case.

Orcas are known to hunt white sharks, and they’re not interested in a filet o’ shark. They really just want the sharks’ liver. Shark livers are massive, making up approximately a third of their body weight.

They’re filled with lipids  and, since sharks don’t have gas-filled swim bladders  like their bony fish cousins, this fatty liver is what they  rely on for buoyancy regulation. Because of how fatty and nutrient dense shark livers are, it’s the  perfect meal for a fast-paced, warm blooded predator like the killer whale. Orcas have been known to hunt  white sharks in other regions, like California.

And when an orca  pod rolls through, the sharks start to roll out, avoiding their  predators like any sensible prey would. However, while the new snacking  habits of South African orcas sounds like a tidy answer,  it may not be the only reason the number of white sharks has  dipped in the last few years. In addition to orca gangs, South  African white sharks are also dealing with fishing pressure, and shark  deterrent efforts on local beaches.

While experts really aren’t sure  which factor tipped the shark scales, it all adds up to a pretty hostile environment. Experts hope that addressing  as many of these concerns as possible may bring back the  iconic South African sharks. Not sure how they’ll convince the orcas to lay off the liver, but we’ll see what happens.

Our next group of endangered white sharks is found off the coast of North America. And what’s disrupting the sharks in Canada and the US is even more confusing and less visible than it is in South Africa. Allow me to explain: In 2023, a dead white shark washed ashore on Prince Edward Island in Canada.

It didn’t look injured, and  seemed otherwise pretty healthy. But when researchers did a necropsy,  which is just an animal autopsy, they found that the shark had  inflammation of the brain and surrounding tissues, otherwise  known as meningoencephalitis. Meanwhile, more seemingly healthy sharks were washing up dead, many with brain swelling.

One shark was seen displaying some odd behavior, like swimming erratically and bumping into things, before dying and washing onto a beach. Surely this was another case  of brain inflammation, right? Shockingly, no!

Just when we  think we’ve figured out a pattern, good ol’ mother nature throws a curve ball. And it’s not just the great whites. Dead sharks with swollen  brains seemed to be on the rise in several species, like sandbar  sharks and thresher sharks.

Even stranger, this is happening  in the Atlantic and the Pacific, meaning whatever the problem  is, it’s really widespread. So some mysterious thing is causing sharks’ brains to swell up and die, and we  still don’t know exactly what. But as it turns out, the answer may have been hidden in a bunch of decades-old  frozen salmon sharks.

In the early 2000s, eighteen  beached salmon sharks were found to have died of meningoencephalitis, and in 2012, researchers analyzed some of them  and found that a type of bacterium called Carnobacterium maltaromaticum  was the likeliest culprit. Which was weird, because this  bacterium has been found in other fish species and in the water they swim in, and those guys all have  perfectly un-swollen brains. But for some unknown reason, this bacterium may be becoming pathogenic in some  fish, including sharks.

And we have an idea of how it  could be reaching their brains: through the endolymphatic system, which aids in hearing, balance, and orientation. Sharks have small pores on  their heads that lead to their endolymphatic ducts, which  provide access to the cranium. And we know that when these structures get invaded by pathogens in the water it can cause disease and even death in several shark and ray species.

So what do we know? We know that, for a decade, sharks have been washing up on beaches. We know that their deaths are often caused or accompanied by meningoencephalitis.

And we know that this disease is sometimes attributed to a particular bacterium. What we don’t know is probably  the biggest piece of this medically mysterious marine puzzle,  which is why this is happening, and how concerned we all should be about it. Scientists are worried for some  reasons and not so worried for others.

Seeing sharks wash up on shore  doesn’t seem like it could be positive in any way, but it could be a sign  that shark populations are recovering. A bigger population of sharks  that are living also means that there’s a bigger population  of sharks that are dying, hence an increase in washed up carcasses. And if this thing is contagious,  more sharks also means more of a chance for diseases to be passed around, simply because the population  is bigger and might be more densely packed into an area.

So  while we don’t love a disease, if it means that shark populations  are rebounding, it’s not all bad. The concern comes in, though,  when we start to think about a bacteria that's been  living on other fish species without a problem suddenly becoming  a deadly pathogen in sharks. If this truly is what is happening,  we need to understand why.

Is it climate change? Pollution?  A secret third option? Knowing what causes the bacterium to go rogue can help us figure out how to  stop it from spreading and prevent it from happening with  other bacteria in the future.

These two shark-killing mysteries  have two different causes, and therefore, two different solutions, at least. But that also means they’re teaching us two different lessons about our planet, our oceans, and all the big and not-so-big creatures in them, from the sharks and orcas down to tiny bacteria. And maybe the biggest one  is that the driving force behind ecosystem shifts is  never really black and white. [♪OUTRO]