Sharks washed up on Cape shore after orca feeding frenzy less of a threat than knock-on ecosystem damage caused by humans - marine biologists

A sevengill shark who had its liver removed by the killer whale pair in Pearly Beach in February 2023. Whale expert Simon Elwen said one adult killer whale needs the equivalent of one white shark liver a day for its dietary requirement. (Photo: Marine Dynamics Conservation Trust)

By Julia Evans

19 Mar 2023

Despite often being cemented in our minds as the kings of the ocean, sharks are not the apex predators. A shocking predation event, likely from two orca killer whales last month at Pearly Beach that left 20 sevengill sharks washed up on the shore, has garnered public attention. But marine biologists say that this is a drop in the ocean compared with the complex modern threats both sharks and whales face – which have been created by humans.

Last month, 20 sevengill shark carcasses washed up on Pearly Beach, catching the attention of the public.

The sharks were likely killed by Cape’s iconic orca or killer whale pair, Port and Starboard, in a predation event.

The Cape’s infamous killer whale pair, dubbed Port and Starboard for their bent dorsal fins which point left and right. (Photo: Marine Dynamics Conservation Trust)

Ralph Watson, a marine biologist at the Marine Dynamics Conservation Trust, part of the team that picked up the first batch of sevengill carcasses at Pearly Beach after receiving reports from the community on 21 February, said, “this situation grabs people’s attention because all of a sudden 20 sharks wash up, most of them mangled".

“This grabs our attention because we see it. But there are literally fisheries out there that catch hundreds, if not thousands of sharks a day. And if we don’t see that, it doesn’t grab our attention, and we don’t really care, which is problematic.”

Scientists started noticing these two orcas predating on sevengill and white sharks around 2015 in the Cape, and they have contributed to the short-term abandonment of these species from their typical aggregation sites around False Bay and Gansbaai and in the decline of the white shark population. But marine biologists say this isn’t the threat we should be concerned about.

Watson said we don’t know the population size of sevengill sharks in this area, so we don’t know the impact this predation event has had, but even so, the number is a drop in the ocean to what fisheries catch out at sea.

“Then you’ve got to question, how impactful is this in comparison to what we don’t see?”

Is this an unprecedented event?

The marine biologists explained that it’s not necessarily unusual for a predation event like this to occur.

“There have been reports from around the world of killer whales hunting sharks,” explained Watson, saying in South America 17 sevengill sharks were found after a predation event by killer whales, but the weather circumstances in Pearly Beach that week were just right for the majority of the carcasses to wash up.

Dr Enrico Gennari, a marine ecologist and one of SA’s leading shark scientists, emphasised that while this is a record event of carcasses on the beach after an orca predation event, sharks don’t float, “so it is likely that this event has not been the biggest of its magnitude, but the fact they all reached the beach for us to see, that is unusual.”

Dr Simon Elwen, a marine biologist with Sea Search whose research specialises in whales and dolphins, told Daily Maverick, “I think the only reason they were on the beach is because of the weather conditions – there were big swells and onshore wind.

“Like every time they go hunting, they probably kill that many every day.”

“It’s almost like the orcas won the jackpot by finding all these animals close to each other,” said Gennari. “However, that doesn’t mean it is a rare event. [It’s just] we don’t see it.”

“We don’t know if they [orcas] eat 15 sharks around South Africa a day,” said Watson, “it was just an opportune moment for us to actually see the effect.”

Elwen explained that what is unusual about this event is that killer whales are pretty rare around South Africa, and that the vast majority around SA eat mammals, particularly dolphins – and they tend to do it away from shore.

Elwen emphasised that this is not a shift in killer whale diet in South Africa, but that this pair likely grew up on that diet, and by arriving brought a new hunting strategy to our ecosystem.

“But because those individuals are eight tonnes each – they can do a lot of damage,” said Elwen, explaining that one adult killer whale needs the equivalent of one white shark liver a day for its dietary needs.

“But you know, 20 sharks killed, even white sharks, is not a massive number if you put it into perspective,” said Gennnari, who is with the Oceans Research Institute in Mossel Bay.

“When you look at coastal fisheries, those sharks are killed in those numbers within 10 minutes by a longliner,” said Gennari, “it is not a really massive worry compared to what happens because of us humans.”

White sharks, as well as Sevengill sharks, are classified as Vulnerable by the IUCN (International Union for Conservation of Nature) and Gennari estimates that white sharks in SA have slowly declined over the past 20 years, and have declined even more after orcas started killing white sharks in Gansbaai in 2017.

So conserving this species should be a concern, but which threats are most important to focus on?

The knock-on effect on ecosystems of orcas taking out sharks

In May 2022 a hobbyist drone pilot recorded the first direct evidence of orcas killing white sharks in South Africa, at Hartenbos Beach and river mouth in Mossel Bay.

Marine biologists have been tracking these two specific killer whales – called Port and Starboard – for years, but his was the first footage of a predation event like this.

Elwen said this video was a real “smoking gun, providing insight into social dynamics. And we got some insight into the techniques of how they’re [orcas] hunting animals,” he said.

“We first observed the flight responses of sevengills and white sharks to the presence of killer whales Port and Starboard in False Bay in 2015 and 2017,” said marine biologist and South African National Parks’ shark expert Dr Alison Kock, when the study about the drone footage came out last year.

“The sharks ultimately abandoned former key habitats, which has had significant knock-on effects for both the ecosystem and shark-related tourism.”

The 2019 study, Running scared: when predators become prey, showcases how once those two orcas turned up in False Bay (the largest known aggregation site for this species globally) and began preying on sevengill sharks, it led to the prolonged absence of sevengill sharks from False Bay.

And the 2022 study, Fear at the top: killer whale predation drives white shark absence at South Africa’s largest aggregation site, illustrates how white sharks have emigrated from their aggregation site in Gansbaai after the orca predation events. And the loss of an apex predator – like white sharks – is known to have profound impacts on ecosystems and trigger trophic cascades (when the disappearance of a species often results in a complete rearrangement of the food web).

“However, that doesn’t explain why the sharks do not come back – like in any other parts of the world,” emphasised Gennari. “Or that the decline of white sharks in Gansbaai started before those orcas were spotted killing white sharks.

“It is a compound effect – shark nets killing sharks (which started more than 60 years ago) started the decline, compounded by the removal of the prey from badly managed fisheries. Together with the orcas, it is the nail in the coffin,” said Gennari.

“We should focus on where we have the power; removing nets and drumlines and better managing the coastal fisheries.”

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Impact of coastal fisheries

Forestry, Fisheries and Environment Minister Barbara Creecy revealed in a written reply to parliamentary questions in July 2019 that on average every year just two or three demersal shark longline boats catch 20,000 smoothhound sharks (which are an endangered species), 6,000 soupfin sharks (critically endangered – similar to a Black Rhino), 2,000 bronze whalers and 2,000 skates.

“If we average two operating vessels fishing every single day of a year (which we know is not possible), a single boat would catch in a day at least over 30 sharks, which is double the number killed (for the first time we know of) by orcas,” said Gennari in regard to these figures.

“So how is the Minister allowing a critically endangered and endangered species to be targeted?”

While Gennari agreed that most soupfin sharks are caught as bycatch by trawlers and not demersal shark longliners, setting a maximum quota for bycatch – as many countries in the world do – could be an option. For smoothhound sharks, which are targeted by demersal shark longliners and line fishing boats, yearly target quota would be an important proactive step to conserve South Africa’s natural heritage.

Watson agreed, saying that while bycatch of sharks has decreased in the last few years, “there’s still bycatch of sharks that likely outnumbers what these killer whales did in this situation by a huge factor.”

Gennari said improving the fisheries doesn’t mean stopping fishing, “it just means that all the fisheries in South Africa are sustainable” – and at the moment, that is not the case.

A recent study that came out in the East coast of the US, showed that sharks and rays – a third of which face extinction – can rebound, their decline can be reversed with improved fisheries management through government enforcing science-based fishing limitations.

“They saw that as soon as fishery approached change, and they started to look at sustainability, that numbers bounced back,” said Gennari regarding the study. “Even though South Africa was the first country that protected the white shark, it might be the first country that is going to lose the white shark, many lesser known shark and ray species.”

Gennari emphasised that he is not against fisheries, but in favour of sustainable fishing practices, which are beneficial for the longevity of the fishing industry itself.

To find out more of the impact of shark nets – read in Daily Maverick: “No need to panic, say experts after fatal Plett shark bite incident”

We should focus on the issues we can control

“At the moment, there’s big changes happening in South Africa – white sharks, orcas, other species of sharks…” reflected Gennari.

Read more in Daily Maverick: “The harpoons are gone, but whales face more complex modern threats

Elwen agreed, saying that, as we’ve seen with the orcas, one change can have such cascadal effects on the ecosystem.

“The environment is changing rapidly – direct human activities re fishing, indirect human activities, climate change – but also because of natural cycles like El Niño and super-effective top predators in the mix.”

Before 2015 orcas killing sharks was not regarded as a big deal in SA. Port and Starboard “brought a new hunting strategy into South Africa,” said Elwen, “and what’s amazing about it is – it’s just literally two individual animals.

“It’s not like a shift in the population or anything, but because they’re so big and such effective hunters it had massive knock-on effects for the entire ecosystem.”

Gennari said, “Instead of trying to figure out which is a bigger problem – orcas as the natural predator, or us with the fisheries – I believe we should focus on the things we have control of. Otherwise, those animals won’t have too much time left. We have a responsibility toward the future generations to do better than this.” DM/OBP

https://www.youtube.com/watch?v=REeWvTRUpMk

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South Africa

Is this an unprecedented event?

The knock-on effect on ecosystems of orcas taking out sharks

Impact of coastal fisheries

We should focus on the issues we can control