The Great Sardine Migration: A Space-Visible Ocean Parade
Every summer, billions of Peruvian sardines surge off the coast of South Africa and swim north into the Indian Ocean. Sardines are small and defenseless, so they travel in a single, vast school: officially about 7 kilometers long and 1.5 kilometers wide, though some estimates double that. This mass is large enough to be seen from space and powerful enough to feed nations: in peak months, fishermen haul up to 300,000 tons of sardines in a single month. The sea around them erupts into a feeding frenzy as thousands of predators—dolphins, sharks, albatrosses, and baleen whales—converge to capitalize on the feast.
In This Article:
A Northbound Army: The 7 km by 1.5 km Sardine School off South Africa
Each summer, the sardines mass off the southern African coast and begin their northward crawl into the Indian Ocean. The official size of the school is about 7 kilometers long and 1.5 kilometers wide, though some estimates are even larger. Beneath the surface, predators close in: as the sardines pool, up to 18,000 dolphins herd them into tight balls and push them toward the surface. Albatrosses, sharks, and baleen whales crash the feast at the surface, hoping to swallow as many as they can. Divers and photographers from around the world descend to document the spectacle, turning the migration into a global visual event.
Why Do They Do It? The Uncertain Purpose of the Trek
What, if anything, do the sardines gain from this perilous trek? The species’ life history suggests many individuals stay near birthplace, and long-distance travel is not strictly necessary for reproduction. Nevertheless, a portion of the population undertakes thousands of kilometers of travel each year, and the reason remains unclear. Some scientists propose that the voyage helps exploit shifting feeding grounds, while others see it as part of a broader dispersal strategy to reduce predation pressure.
Two Theories, Two Mechanisms
Two leading theories try to explain the voyage. Upwelling hypothesis: Each summer, strong winds off East Africa push surface waters away from the coast, pulling up cold, nutrient-rich water from depth. The nutrient surge triggers plankton blooms, creating a feeding bonanza for sardines and their predators; many fish perish, but the survivors lay thousands of viable eggs. Genetic-programming hypothesis: Some sardine populations may be genetically programmed to migrate north whenever surface temperatures near Africa fall below 21°C. After spreading through the Indian Ocean to spawn, they may return southward in small groups via deep, cold currents to stay largely out of sight. There is also indirect evidence: sardine eggs have occasionally been found in the Indian Ocean, suggesting that reproduction occurs there, at least intermittently.
A Spectacle That Feeds People and Captivates the World
The migration is a lifeline for coastal communities. In a single month, catches can reach hundreds of thousands of tons, filling fleets and markets. Behind the scenes, divers, photographers, and researchers from around the globe document the event, sharing awe and data that bring the story to audiences far beyond the shore. Yet the deeper purpose of the migration remains partly a mystery, a reminder of nature’s immense scale and the limits of human understanding. This tale comes from The Animal Book. If you enjoyed it, you can join our Telegram community for updates.
