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Sardines by the billions live here year-round, swimming and feeding together in loose shoals of hundreds of thousands of individuals.
数十亿条沙丁鱼常年生活在这里，成群结队地游泳和觅食。
Then, in late April, ocean currents undergo a subtle shift.
然而，就在4月下旬，洋流发生了微妙的变化。
As the southern winter approaches, upwellings and eddies force a tendril of cold water toward the east.
随着南方冬季的临近，上升流和涡流迫使一股冷水向东移动。
The warm Agulhas current moves further offshore, allowing the cold stream to gain momentum.
温暖的阿古拉洋流进一步向近海移动，让这股寒冷的洋流更有动力。
The cold water sardines instinctively cluster in the chillier current.
生活在冷水中的沙丁鱼本能地往较冷的洋流中聚集。
150 miles out to sea on the Agulhas banks, schools begin to mass together until they form a mega-shoal 20 miles long.
在阿古拉斯河岸150英里外的地方，沙丁鱼们开始聚集在一起，直到形成一个20英里长的巨型鱼群。
Then the single super-organism pours into the cold water highway toward the coast and shallower water.
然后，这种单一的超级有机体沿着这股寒冷的洋流向海岸和浅水区倾泻而出。
It's the biggest biomass migration on the planet, exceeding the great herds of the Serengeti.
这是地球上最大的生物群迁徙，超过了塞伦盖蒂山脉的动物群。
Yet exactly why they do it remains a mystery.
然而，沙丁鱼为什么要这么做仍然是一个谜。
The journey serves no apparent benefit to the fish.
这段旅程对它们没有明显的好处。
It will inevitably dead end in a warm sea unsuitable for sardines.
这场迁徙将不可避免地在不适合沙丁鱼生长的温暖海洋中走向终点。
How far they travel depends entirely on the vagaries of this year's cold current.
沙丁鱼旅行的距离完全取决于这一年寒流的反复无常。
But one thing is certain, an entire food web is counting on the sardines to make the run.
但有一件事是肯定的，那就是整个食物链都指望着这些沙丁鱼群。
