What is hunting down the great white shark?

Now, speaking of the underwater realm, Gansbaai in South Africa was renowned as the Great White capital of the world; at least until 5 years ago, when the animals disappeared, almost overnight, leaving researchers and locals puzzled. Since 2017 Alison Towner, from Rhodes University on the Eastern Cape, has been piecing together the different parts of the jigsaw puzzle to work out what’s happened…

Alison- It wasn't as if we saw a decline, you know, they just vanished. And initially it happened for about six to weeks and the sharks started to reappear again. What had happened in that time period was kilo whales had arrived and been here for 15 years. I'd never seen killer whales really in this area. Certainly I'd never seen the type of killer whale that's known to hunt sharks. So this is actually quite a rare morphotype of killer whale that specializes in selectively targeting sharks and their livers and, lo' and behold, here were these two very distinctive killer whales. So they were here in the study site and they proceeded to predate on various great white sharks in the area. We know this because the carcasses of the dead white sharks washed out and, in the wake of that, everything changed. So post 2017, when this happened, you know, the numbers like literally drastically declined and then the white sharks would sort of return again in much lower numbers. So, with the killer whales, and it was just a game of cat of mouse since then, until here we are five years later, the, the sciences finally published and we can show the world the evidence and the data. And it's, it's quite startling.

Harry- Is this the first time that we've witnessed this behaviour, this predation of the killer whale eating the, the great white or white sharks.

Alison- This behaviour has been documented once before off of San Francisco, California, at the Farallon Islands and scientists there, it's a 1999 paper so it's quite a while ago, described observational account of some offshore killer whales killing a great white shark. Biting it at the surface, towing it, pushing it through the water to asphyxiate it, and then sort of rolling on its back and, and extracting its liver. There was a written report, it wasn't direct evidence and it certainly wasn't any carcasses washing out for examination afterwards. So, lo and behold, what happened here was the carcasses were very coastal because these types of killer whales are generally more offshore. So the carcasses washed down, that was the first time in the world. We were able to sort of post mortem them. And I actually led all the necropsies, we've had eight so far, but yeah, having been part of that now and seen so many of them washed out is quite spectacular, but quite sinister at the same time, the killer whales literally rip them open at their fins and then just extract the liver and discard the carcass.

Alison- It's incredible.

Harry- Do we know why they're just taking the liver?

Alison- Well, the liver is a huge organ, very lipid rich, so very fatty nutrient high profile nutrient. It's just, it's right there for the taking in that if they flip the shark over and put it into a trance like state and they tear it, then the liver literally is so accessible, right? We've estimated that male adult killer whales would need to eat about one white shark liver a day to sustain itself. So <laugh> the calculations have been done. I mean, it's not gonna exactly fill them up for very long, but they certainly specialize in extracting this one organ.

Harry- Is it that these sharks then know that this is going on. And so that's why they're leaving the area or there's certain cues that they're taking before they run off, away from Cape town.

Alison- So if a white shark's taken off guard by a killer whale - if all of a sudden it turns around and there's multiple killer whales - I mean, we know there's at least two that are doing it. The sheer trauma of that have being chased of other of the same species in your area within, you know, range of you detecting it being predated on. It's probably enough to drive you out the area for one. But then it can be the rotten carcass. So, once it's discarded and it's laid on the sea bed, let's say it hasn't washed out, we do believe there's more that hasn't washed out. The scent of death, if you will, could keep the sharks away, but that would be momentarily, not long term. So all the questions are there, but somehow these white sharks have this evolutionary built in mechanism to know to avoid these <laugh> these areas where it's high killer whale risk. I'm sure the science will definitely unravel some more about that with regards to vocalizations of killer whales or exactly what mechanisms the sharks use.

Harry- I'm making the assumption that at the moment, you're kind of at the start of this bringing together of data. So with that in mind, you know, everybody will be aware that food webs and ecosystems are very fragile, they're very complicated. If you take away something like the great white shark from an environment where it is an apex predator, what happens to the rest of the sea floor? Do we know what happens to that environment?

Alison- At this point? Of course it's speculation, it's hypotheses, but I mean, removing top predators is never good for ecosystems full stop. And now the coastal bay where these white sharks sort of reign king, if you will, is home to so many other critically endangered species that are actually reliant on the white sharks being there as top predators. So, for example, we have the Greater Dyer island system here in Gaansbai, which has critically endangered African penguins on it and their numbers are right down. They can't sustain any more pressure. And now, because just adjacent to their colony is actually a colony of breeding cape fur seals. Now the seals don't have their natural predators around. So they're actually directly competing with the penguins for food, but also predating on them. So again, it's just that little perturbation at the top tier of the ecosystem in our Marine environment here, that's now out of balance and it has cascading effects right The way through the whole food chain. Yeah, If we leave it too long and you know, this isn't rebalanced, then we could have serious consequences in our hands. But what I will say is that we have a couple of great white sharks that have finally come back to this region. There's about four of them out there today that the boat's seeing, but it's been almost a year since they were here.