Why do young Tanganyika sardines swim upward soon after hatching?

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clupeiform A young common herring (*Clupea harengus*).

Young Tanganyika sardines swim upward immediately after hatching because this vertical movement is essential for their survival. If they were to sink below the level of oxygenated water—which typically ranges from 80 to 200 metres (roughly 260 to 650 feet)—they would not survive.1

Because their tails are the only flexible part of their bodies, these larvae must actively swim to maintain their position; they sink as soon as they stop moving. As they grow, they gradually transition toward surface waters before eventually moving toward the shore.1