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Liver-Eating Killer Whales Force Sharks to Flee

Majestic great white sharks glide through the ocean waters.
Certain killer whales prefer to feed on shark livers. Credit: Elias Levy / Wikimedia Commons / CC BY-2.0

Scientists discovered that the number of sharks at their main meeting spots has changed. One of the reasons is that certain killer whales favour shark livers.

South Africa is famous for its big, great white sharks. But there’s some concerning news. In certain spots along the Western Cape province coast, where these sharks usually spend time to eat, be in the company of other sharks, or take a nap, there are now fewer of them.

In Cape Town, folks known as “shark spotters” are really good at keeping an eye on great white sharks. In 2011, they witnessed more than three hundred of these sharks at eight different beaches.

Since 2019, none have been seen. Hence, there is growing concern among people as to the general welfare of these sharks.

Study to discover shark meeting spots

Marine scientists wanted to find out if the drop in shark numbers in the Western Cape meant there were fewer sharks in all of South Africa or if they had just migrated elsewhere. To answer this, scientists did some investigating.

They looked at a ton of information collected by other scientists, tour guides, and people who fish from the shore and determined how many sharks there were and where they were located throughout South Africa.

The study led to some significant findings. The marine scientists found that the number of sharks at their main meeting spots was changing in various ways. While some areas had roughly the same amount of sharks or more, other places had fewer.

However, when when all factors were considered, it seemed that the overall number of white sharks has stayed about the same since they were safeguarded in 1991.

More sharks in the Eastern Cape

When scientists looked at the rate of migration from one area to another, they noticed something interesting. It seemed like there were more interactions between people and sharks now happening in the Eastern Cape instead of the Western Cape.

It’s still uncertain if the sharks that used to be in the Western Cape are the same ones now showing up in the Eastern Cape. Hence, more research is needed to figure this out.

Even though it’s good news that the number of white sharks is remaining steady, this shift in where they are can lead to other newer problems. For example, there may be more risks from fishing, and we might need better plans to keep beaches safe. That’s why it’s important for us to keep a close watch on where these sharks are navigating.

Big changes in shark sightings

The biggest changes in shark sightings happened between 2015 and 2020. At Seal Island in False Bay (Western Cape) in 2005, they used to see about 2.5 sharks every hour. However, in 2017, that dropped down to just 0.6 sharks per hour.

Now, to the east at Algoa Bay, in 2013, people who fished from the shore only caught six sharks. By 2019, however, that number went up to fifty-nine. Figuring out why such changes happen is pretty tricky.

These sharks can live for a rather long time, sometimes surpassing seventy years. As they mature, they act differently. When they’re young, especially the males, they like to hang out near the coast. However, while mature, they head out in deeper waters. This is especially true of the females.

Liver-eating killer whale a factor affecting the movement of sharks

The way these sharks move around is also affected by things in their environment. Stuff like how warm the water is, what phase the moon is in, what time of year it is, and how much food there is can change where they go. Changes in the climate and the ocean over a long time might play a part, too.

These sharks are pretty good at adapting. They like to eat lots of different things, and they can handle water that’s anywhere from 57 to 75 degrees Fahrenheit (14 to 24 degrees Celsius). They’re like smart travelers. When the place they’re in isn’t great, they move to where it’s more suitable for them.

The way these sharks move around gets even more complicated when we talk about some special killer whales that really like shark livers. These top-of-the-food-chain predators have been seen hunting white, sevengill, and bronze whaler sharks.

The first time scientists witnessed this was in 2015 along the South African coast. It just so happened that around the same time, white sharks in places such as Gansbaai and False Bay started acting somewhat differently.

We can’t say for sure that the killer whales are directly causing this change in behavior, but what we’ve seen and the data we’ve collected suggest that the sharks are in a way “running away” from these killer whales after they’ve observed them eating other sharks.

More recently, in Mossel Bay, when a group of killer whales killed at least three white sharks, the rest of the sharks decided it was a good time to leave that area.

Risk landscape for white sharks

A study that came out in 2022 showed that white sharks and fishing gear, such as longlines and gillnets, often end up in the same places. This happens in about 25 percent of South Africa’s Exclusive Economic Zone, and the sharks spend about 15 percent of their time near these fishing areas.

The place where they catch the most white sharks is in KwaZulu-Natal, and they catch around thirty-two of them every year. This shows us that it’s important to look at both where the sharks go and how many of them get caught by fishermen to understand the risks they face.

Now, since the sharks are moving eastward, we must consider the risks and how these might change. If they begin dwelling primarily in places with things such as shark nets, drumlines (which are hooks with bait), and gillnets, there’s a higher chance they might get caught.

Chances of shark bites

While the chances of shark bites at the beach are generally low, the way sharks move around could affect how safe our beaches are. When sharks are around, they can change what people do at the beach, especially in popular spots for swimming and water sports. Hence, we might have to change how we manage sharks as they move to new places.

This could mean putting up more signs, closing the beach for a short time, or teaching folks more about how sharks behave.

For instance, in Cape Town, the folks who watch out for sharks have had to modify the measures they take at certain times. After two deadly shark incidents in 2022, they began keeping an eye on Plettenberg Bay, too. People have also noticed more white sharks in some spots in the Eastern Cape where surfers and divers hang out.

Need for more research

Scientists still need to do more research to figure out why sharks are moving around and how it’s affecting where they go. This study reminds us how important it is to use the same methods to collect data so that we can have accurate numbers for shark populations all over the world. This problem isn’t just in South Africa. It’s a problem in other countries too.

Programs that keep a consistent eye on sharks in the Eastern Cape may also be needed, and we should always strive towards reducing shark deaths.

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