Chub Mackerel in the Predator’s Crosshairs

Each winter, South Africa’s Wild Coast becomes the stage for one of the ocean’s great spectacles — the Sardine Run. It’s chaos at sea: baitfish boiling to the surface, predators striking from every angle, and the spotlight firmly on sardines. But this year, something else stole the show.

In June and July, while searching for gannet dives and bait slicks, we kept running into dense, fast-moving baitballs — not sardines, but chub mackerel. Bigger, stronger, and — this year — targeted with intense precision.

Change on the Menu

Chub mackerel (Scomber japonicus) aren’t usually the stars of the Sardine Run, but this season, they dominated. We saw multiple baitballs made up almost entirely of mackerel, over 30 cm long, hammered by common dolphins, dusky sharks, and Bryde’s whales.

The preference was clear: predators were choosing mackerel over sardines when given the chance. The payoff made sense — tighter baitballs, higher energy content, and bigger individual targets. For Bryde’s whales, especially, they were perfect: dense, fat-rich shoals ideal for high-effort lunges.

Too Much to Handle

Above the surface, it was a different story. Cape gannets dove eagerly into the action — but they struggled. The mackerel were simply too large and strong. Gannets surfaced gripping fish they couldn’t subdue, fighting to realign them for swallowing, often dropping them in defeat.

Other prey like saury, sardines, or redeye herring are far easier — smaller, softer, and swallowed quickly, reducing the risk of theft or escape. But mackerel? Too brawny. Time and again, gannets gave up mid-meal, leaving fish to scavengers — swift terns, subantarctic skuas, or giant petrels. One petrel spent nearly five minutes choking down a half-eaten mackerel.

Though gannets are known to feed chicks chub mackerel in sardine-poor years — with Bird Island diet studies showing up to 40% mackerel — this season exposed the limits of that adaptation. The fish were present, but not always usable.

No Problem for Whales

Bryde’s whales, on the other hand, showed no hesitation. We watched them bypass mixed shoals and hammer pure mackerel baitballs with repeated lunges. Stomach content studies from the Eastern Cape have shown chub mackerel making up 30% of whale diets in certain years — this behavior matched that data exactly.

These whales weren’t fooled by the Sardine Run hype. They knew what they wanted — and it wasn’t sardines.

A New Contender

Chub mackerel may not migrate like sardines, but this year they were central to the feeding frenzy. Big, abundant, and tightly schooled, they brought out the best in some predators — and exposed the limitations in others.

Is this a one-off season? Or a sign of changing dynamics along the Wild Coast?

Whatever the answer, one thing’s certain: chub mackerel aren’t the understudy anymore. This winter, they took the lead — and the ocean responded.