How South Africa's sardine run is changing

enomics at the University of Johannesburg, South Africa. "They don't like this hot water – and move up the coastline in a relatively shallow area of cooler water. At Waterfall Bluff, in Lusikisiki Eastern Cape, the Agulhas Current is so close to the coast, they bottleneck."

By its peak in June, the sardine run is a feeding frenzy. As many as 18,000 common and bottlenose dolphins round the sardines up into bait balls, stretching 20m (66ft) in diameter. Now, they are easy pickings for other predators too – and sharks, whales, seals and birds join in this most plenteous of ocean banquets. (Watch the BBC Earth video below to see predators' tactics during a sardine run feeding frenzy – and the defensive behaviours of the sardines).

"The sardine run is an exceptional event and an important part of the food web," says Teske. During an upwelling event, he says – when cold, nutrient-rich water comes to the surface – phytoplankton blooms, zooplankton feeds on the phytoplankton, fish feed on the zooplankton, "and essentially everything else" feeds on the fish. "Suddenly, there is this huge source of protein."

For the few sardines that survive the onslaught, the future remains bleak. The shoal continues north, moving with the cold water – until the upwelling ends. "They just keep moving into warmer water – the worst possible habitat," says Teske.

These cold-water sardines find themselves trapped in a subtropical habitat, too hot for them to survive. The epic thousand-kilometre journey of this little fish ends in an "ecological trap" – making the sardine run a rare example of a mass migration that has no apparent benefits for the survival of the species.

"We don't know the ultimate fate of the majority of them," says Teske. "But, once the run is complete, it's very likely a lot of them do die."

But one thing is certain, an entire food web – including endangered species such as the African penguin, Cape cormorant, Cape gannet, and school shark – relies on the sardines to make the run. Now, over-fishing and warming oceans are causing sardine stocks to collapse and the world-famous sardine run could be at risk of ending within a few decades.

Humpback whales, for example, travel from the depths of the Southern Ocean, timing their 8,000km (5,000 miles) round-trip to arrive in time for the event. There they meet the year-round inhabitants such as Indian Ocean humpback dolphins and Bryde's whales. Ocean aggregation sites such as this can leave species vulnerable to exploitation. And overfishing of sardines has led to what experts are describing as the "collapse of the sardine run", leaving "predatory species left stranded" without the prey they depend on to survive.

Add climate change and warming oceans into the mix, and the future of the sardine run looks uncertain. By 2018, the run's total biomass had fallen to less than a quarter of its 2002 high of more than 4m tonnes. Now, sardines are appearing progressively later in the year – if at all. A 2019 study found, since the sardine run was first documented in newspapers almost a hundred years ago, the first arrival dates of sardines has been delayed by 1.3 days per decade over the period 1946 to 2012, coinciding with a poleward shift of warm ocean water.

Plön says: "Since 2000, oceanographers have been noticing a change in the sardine run. Previously, it was big, with lots of animals, and very temporally defined. Now, it's more like little pulses going through, that might last five to 10 minutes. And the timing of the run is not as predictable anymore."

Changes in phenological events like this – recurrent natural events such as bird migrations or the flowering of plants – can act as a bioindicator of climate change, says Plön.

Plön, who works in Stellenbosch University's department of pathology, studies the health of cetaceans by looking at the contents of their stomachs. "There were about a hundred stomachs in the freezer when I arrived at Port Elizabeth Museum [in South Africa]," she says. The stomachs – defrosted and examined by Plön's PhD student, Shannon Ambrose – belonged to common dolphins which had been accidentally caught in shark nets along the KwaZulu-Natal coastline between 2000 and 2009. "The common dolphin is the species known for the sardine run. It's the species that forms super-pods of thousands of animals," says Plön. "You have to open them up and look at the fish and squid that's in there, at various stages of digestion."

The ear bones of the fish, or otoliths, says Plön, remain intact after digestion, as do the mouth parts of squid. "The cool thing is they're species specific," she says. "So, I can tell a mackerel from a sardine. You can also measure them and calculate the size of the animal that was eaten. So, it gives you a lot of information about the last meal."

"It's like we're just documenting extinction" – Stephanie Plön

The team compared their findings with data collected between 1972 and 1992, and discovered a huge shift in the dolphins' diet. "The animals had previously fed mainly on sardine – it's the sardine run, that's what you expect," says Plön. "But they weren't feeding on sardines anymore. They were feeding on mackerel," she says.

The study, says Plön, illustrates how we can trace changes in the Earth's patterns by looking at "sentinel" species – or harbingers of danger. Like canaries in a mine used to warn of toxic gases, "cetaceans are indicators of ocean health", she says. However, she adds, as over a third of all cetacean populations are of conservation concern, this concept is "problematic". "We have increasing numbers of species disappearing – you can't really call them sentinels anymore if the sentinels are gone," she says.

Now, changes in the sardine run are resulting in a "phenological mismatch", meaning predators and their prey are no longer in the same place at the same time. This is already showing serious ramifications for top predators, including Cape gannets and African penguins, whose diet is largely made up of sardines.

"When events like this are disrupted it can have a knock-on effect," warns Plön. Phenological shifts can disrupt entire ecosystems following relatively small climatic changes. But experts say there are few studies exploring shifts in the timing of migratory fish species like sardines – and more research is needed if we are to understand the interplay of warming atmospheric and oceanic temperatures, and what we can do to mitigate its impacts.

So, other than stopping climate change in its tracks, is there anything we can do to prevent the sardine run disappearing for good? Sardines are an important commercial fish species in South Africa. Following rapid declines in the landings of sardines during the mid-1960s, the industry changed its fishing strategy to use smaller-meshed nets to target juvenile anchovy. Anchovy dominated the catches for the next 25 years, allowing sardine populations to recover.

South Africa is also now home to 42 Marine Protected Areas (MPAs), including the Pondoland MPA – which stretches for 90km (56 miles) along South Africa's "Wild Coast", giving refuge for many overexploited fish species.

But Plön is not encouraged. "I have a lot of colleagues who have environmental anxiety," she says. "We have more and more endangered marine mammals around the globe, marine heat waves, more and more unusual mortality events. It's just increasing. And we can see it. We're reaching a point where it's happening very fast, with so many different factors – it's almost like we're just documenting extinction," she says. 

-bbc