Fisheries

In media coverage of South Africa’s white sharks, demersal shark longline fisheries have been portrayed as a central culprit for their disappearance — but many white shark scientists are quick to point out the lack of data.

The longline fisheries target small sharks that are important prey for juvenile white sharks (mature sharks tend to eat marine mammals, such as seals). Scientists from South Africa’s Department of Environment, Forestry and Fisheries recommend catch control based on their data, but there are currently no limits in place and there are concerns about the impact on the ecosystem of overfishing sharks such as soupfins and common smooth-hounds. Additionally, the monitoring of South Africa’s coastlines is notoriously weak and some boats still fish in the no-take zones of marine protected areas.

One interesting finding by Ph.D. student Dylan Irion, supported by observations of some fishermen in and around Algoa Bay, is that as white shark sightings in the Western Cape have dropped off, sightings in the Eastern Cape have spiked — suggesting the sharks have relocated eastward, even though there is more demersal longline fishing of sharks there.

This weakens the argument that fishing of smaller sharks is denying juvenile sharks their prey — though overfishing in the open ocean could be a reason some orcas have moved from deeper waters to the coast.

Kock cautions against speculating before there is data to confirm these theories. “You have to be careful, because you can have unintended consequences … for people’s livelihoods. It’s really important, particularly for people in the decision-making sphere, to have evidence-based information, so that they can make the right decision. And at the moment, in terms of the white sharks disappearing, that needs a lot more work.”

Andreotti says she believes the fisheries should be more strictly regulated in any case, as sustainable fishing will have positive effects for people, sharks and the whole ecosystem in the long run. “They’re dealing with the livelihood of people,” she says. “I respect that. But I would appreciate it if they could try and see … the long-term goal.”

Shark nets and drumlines (baited hooks specifically targeting sharks) pose additional risks to white sharks along the South African coastline. Although the species has been protected in South Africa since 1991, the province of KwaZulu-Natal (KZN) uses baited hooks to cull white sharks as a method of preventing them swimming close to the shore. Between 2013 and 2017, an average of nearly 17 white sharks died on KZN’s drumlines each year, as well as scores of other marine animals: turtles, dolphins, and other shark species.

Image courtesy Marine Dynamics/Dyer Island Trust
There are currently no limits in place to prevent the overfishing of sharks. Image courtesy Marine Dynamics/Dyer Island Trust

Uncertainties and COVID-19

Kock emphasizes the importance of taking a wider view. “If you talk to people in the Western Cape … the shark spotters aren’t seeing [the white sharks], the surfers aren’t seeing them, the fishermen aren’t seeing them, the cage diving operators aren’t seeing them, our science shows that we’re not seeing them. But if you talk to fishermen in Algoa Bay and the Eastern Cape, they’ll tell you the opposite — [they’ve] never seen so many white sharks,” she says. “When people focus on just one aggregation area, they’re missing the bigger picture.”

Irion is currently using available data sets to look at the whole South African coastline. The information is relatively new; white sharks live an estimated 70 years, but the earliest data only starts from the early 1990s, recorded by Gansbaai cage-diving operators; in False Bay, recorded observations began in 1996. In addition, white sharks, especially the larger individuals, spend so much time far offshore — and satellite tags are extremely expensive — that there is a great deal of movement and behavior that scientists never see.

“We always concentrate on the things that we do know because obviously, they’re the facts that we have at hand,” Kock says, “but I think it’s equally important to realise [what we don’t know] … so much could be happening that we’re just not aware of.”

COVID-19 means there will be a gap in the data. Towner, who works for Dyer Island Conservation Trust in Gansbaai and alongside a local cage-diving operator, says the blue NGOs engaged in white shark research in South Africa are “heavily reliant” on international tourism, partly for the funding, but also with the cage-diving boats affording continuous monitoring of the sharks. Towner says the emergence of bronze whaler sharks in Gansbaai saved the industry when the white sharks dropped off in 2017.

“We’ve got the largest database,” Towner says. “It’s been 15 years and still going, with no seasonal interruptions in the data. So, we notice if something’s wrong very quickly.”

But with South Africa on a strict lockdown, tourism — even domestic — has ground to a standstill and, at the time of writing, researchers aren’t allowed out to sea.

Perhaps what has captivated people so much about the white shark population in South Africa is all these unknowns and how an array of possible factors — the orcas, a decline of prey species, climate change, culling — may be impacting these elusive apex predators. What is clear is that without data, we cannot draw any firm conclusions.

Kock laughs when asked about the expected publication date of the paper on the orca impacts she’s been working on with Towner, which is in the process of being submitted, after which it will undergo peer review.

“The slowness of science is frustrating for everyone — but it is slow for a reason,” she says. “It has to be verified.”


Citations

Engelbrecht, T. M., Kock, A. A., & O’Riain, M. J. (2019). Running scared: When predators become prey. Ecosphere, 10(1). doi:10.1002/ecs2.2531

Jorgensen, S. J., Anderson, S., Ferretti, F., Tietz, J. R., Chapple, T., Kanive, P., . . . Block, B. A. (2019). Killer whales redistribute white shark foraging pressure on seals. Scientific Reports, 9(1). doi:10.1038/s41598-019-39356-2

Andreotti, S., Rutzen, M., Walt, S. V., Heyden, S. V., Henriques, R., Meÿer, M., . . . Matthee, C. (2016). An integrated mark-recapture and genetic approach to estimate the population size of white sharks in South Africa. Marine Ecology Progress Series, 552, 241-253. doi:10.3354/meps11744

Towner, A. V., Wcisel, M. A., Reisinger, R. R., Edwards, D., & Jewell, O. J. (2013). Gauging the threat: The first population estimate for white sharks in South Africa using photo identification and automated software. PLOS ONE, 8(6). doi:10.1371/journal.pone.0066035

Banner image: Great white carcass, Gansbaai. Image courtesy Hennie Otto/Marine Dynamics.

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Article published by terna gyuse