Ship ahoy: The analysis found that the number of large, fast-moving vessels travelling faster than 15 knots — considered the greatest ship-strike
threat to whales — has quadrupled since late 2023. Photo: University of Pretoria’s Whale Unit at the Mammal Research Institute
As conflict in the Middle East redraws global shipping routes, scientists are warning that South Africa’s west coast is becoming an emerging danger zone for whales.
Thousands of commercial vessels avoiding instability in the Red Sea and Strait of Hormuz are rerouting around the Cape of Good Hope, dramatically increasing maritime traffic through some of the world’s richest whale habitats.
New preliminary research presented to the International Whaling Commission by researchers from the University of Pretoria’s Whale Unit at the Mammal Research Institute, Afriseas Solutions and the International Fund for Animal Welfare, found extensive overlap between whale habitats and major shipping corridors around Cape Town, Saldanha Bay and Cape Agulhas.
The analysis found that the number of large, fast-moving vessels travelling faster than 15 knots — considered the greatest ship-strike threat to whales — has quadrupled since late 2023.
The assessment combines species distribution models for six whale species — Bryde’s whale, Southern right whale, Humpback whale, Fin whale, Sei whale, Sperm whale — with vessel traffic data to map areas of heightened exposure risk along South Africa’s southwest coast.
“The risk has increased very simply because there’s more ships,” said professor Els Vermeulen the chief scientist of the whale unit.
“And so we did a risk assessment to kind of pinpoint the areas where that risk is highest but it doesn’t mean we see an increased mortality, it doesn’t mean we see an increase in injuries. If anything, a lot of mortality related to ship strikes in general globally is cryptic.”
Vermeulen explained that whale deaths from ship strikes are often invisible. “It happens offshore, if whales get hit, they will sink – they don’t necessarily strand for us to see so we can’t go around waiting for data to see an increase in mortality.
“When we have an increase in shipping, we know there’s an increased risk and what we did is try to map where that high risk is and how that overlaps with the distribution of different whales.”
South Africa’s recorded data reflects this invisibility. “The numbers we have just shows it’s super low; we have just, in the last 50 years of stranding data and we can see that less than 1% of that relates to ship strike and confirms a lot of what might be happening is cryptic, we don’t see it per se,” she said.
According to the preliminary assessment, the southwestern Cape coast is one of southern Africa’s busiest maritime corridors, anchored by major ports including Cape Town and Saldanha Bay, as well as a key mineral export terminal at Saldanha. It is also an established global shipping route.
In recent years, pressure on this already congested sea space has increased further. Geopolitical instability in the Red Sea has led to about 18 international shipping companies rerouting vessels around the Cape of Good Hope. This diversion has significantly increased vessel traffic along South Africa’s west coast.
Cruise ship activity has also risen, with Cape Town recording 83 cruise ship calls in the 2024/25 season.
At the same time, the region supports productive fisheries, including deep-sea trawl, longline, purse seine and small-scale artisanal fisheries across Cape Town, Saldanha Bay, St Helena Bay and Lamberts Bay.
The authors warned that this convergence of intensive shipping, fishing activity and high densities of recovering whale populations has created an elevated risk of whale ship strikes along the southwest coast.
The International Whaling Commission has identified ship strikes as one of the most serious global threats to large whale populations and has recommended a detailed risk assessment for the approaches to the Port of Cape Town.
The International Maritime Organisation has also been working on measures to reduce strike risk globally. A recent global review further highlights southern Africa as a major gap in ship-strike management and flags the region as requiring urgent attention.
South Africa’s southwestern coast and the Benguela upwelling system support globally significant populations of humpback, southern right, Bryde’s, blue, fin and sei whales, as well as sperm whales.
In recent years, scientists have also documented humpback whale “supergroups” — large feeding aggregations of dozens, sometimes hundreds, of whales in confined areas linked to favourable conditions in the Benguela ecosystem.
According to the preliminary assessment, this convergence has created a zone of elevated strike exposure where whale habitats and shipping corridors overlap most intensely around Cape Town, Saldanha Bay, Cape Point and Cape Agulhas.
The researchers found that ship-strike risk is now a present and pressing issue for both inshore and offshore species. The study also highlights suspected underreporting of whale ship strikes, citing research indicating that only 0.6% of more than 7 000 recorded cetacean mortality and morbidity cases in South Africa over 50 years were attributed to vessel collisions.
“This figure seems implausibly low given the demonstrated overlap between whales and shipping,” the preliminary assessment said. “Limited carcass recovery and observations (reflecting the high current speeds and high energy coastlines of the region), difficulty in assigning cause of death and the near absence of offshore detection all likely contribute to this underestimation.”
As a result, the researchers argue, strike data alone cannot be relied upon to assess risk.
“Consequently, strike incidence records alone cannot serve as a reliable measure of risk and model-based approaches and the drawing on international experiences are essential to identify areas of concern.”
The modelling shows that vessel traffic has increased substantially since December 2023, with the fastest ships — those most likely to cause lethal collisions — rising by a factor of four.
The analysis suggests that relatively modest changes to shipping routes could significantly reduce risk.
Shifting lanes slightly further offshore — by less than 20 nautical miles — could reduce strike exposure by 20% to 50% depending on species, while having minimal impact on voyage distances that often exceed 12000 nautical miles.
The greatest reductions were estimated for Bryde’s whales (50%), humpback whales (40%) and southern right whales (31%). Even offshore species such as fin and sperm whales showed measurable reductions in risk.
However, the researchers caution these results are preliminary. “The risk estimates are based on relative occurrence probabilities from SDMs [species distribution models] rather than absolute density data.”
Offshore whale distributions remain poorly understood and require further research, including dedicated surveys and tracking.
For Vermeulen, the challenge is to balance precaution with data limitations. “We can’t quantify what this means for populations and it’s not something we can even try to attempt to quantify because again if we don’t know what the mortality rate is, we can’t see how that impacts a population, right, so I don’t think we need to wait for that either.
“We have an issue, it’s very simple. There’s more ships and we have a lot of whales in our waters so that means there is an increased risk; it means we need to mitigate and minimise risks.”
Researchers are focusing on what is feasible with current data. “There’s a lot of shipping lanes that come together around the Cape of Good Hope,” she said. “That’s quite a big hotspot because all the different lanes come together, they congregate in that area.
“And then you have something called port approaches. It’s where the ships divert from the lane and they go into port. And so you can’t reroute a port approach because they obviously have to go to port so that’s usually where you look at speed restrictions in certain areas.”
Vermeulen said international discussions are already underway with shipping bodies and regulators. “We’re not pointing fingers at anyone, this is a global issue that has led ships to reroute for human safety.
“A lot of these stakeholders who are shipping companies and the World Shipping Council and the International Maritime Organisation, they’re all sitting with us, they’re all keen to talk about this … A lot of them are very happy to adhere to guidelines, even voluntarily, so once in SA waters it’s going to be up to us and the government now to say, okay these are the guidelines you want you to follow.”
Researchers are particularly concerned about the humpback whale supergroups that form off the west coast during summer months.
“We have these very large aggregations of hundreds of humpback whales and those kinds of concentrations or densities of whales really worry us the most especially with port approaches because these ships then come very close to shore,” Vermeulen said.
“And whales that are feeding might not be very much aware of their surroundings or they’re very much engaged in other types of behaviour.”
Large whales are especially vulnerable because of their size and the limits of ship manoeuvrability. “To divert a ship of that size last minute because now you just saw a whale in front of you is fairly impossible,” she said.
Whales, she warned, may struggle to detect large vessels acoustically. “The whale may not necessarily know where the front of that ship is because they depend so much on acoustic signals,” she said. “That is something that might limit them to avoid large vessels when they’re close by.”
