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9 of the world's largest animal swarms

school of herring

Photo: Leonardo Gonzalez/Shutterstock

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Herring

Many kinds of fish congregate into shoals, but few rival the mega-shoals of herring, which have been called the world's biggest animal groupings. The fish shoal to avoid predation or when it's time to breed. As a shoal grows in size, more and more fish tend to gravitate toward it until it reaches a mammoth size. Some shoals have been recorded to contain tens of millions of herring, covering dozens of square kilometers.