The KwaZulu-Natal sardine run, popularly known as the “greatest shoal on Earth,” is a mass migration of South African sardines from their temperate core range into the subtropical Indian Ocean. It has been suggested that this represents the spawning migration of a distinct subtropical stock. Using genomic and transcriptomic data from sardines collected around the South African coast, we identified two stocks, one cool temperate (Atlantic) and the other warm temperate (Indian Ocean). Unexpectedly, we found that sardines participating in the sardine run are primarily of Atlantic origin and thus prefer colder water. These sardines separate from the warm-temperate stock and move into temporarily favorable Indian Ocean habitat during brief cold-water upwelling periods. Once the upwelling ends, they find themselves trapped in physiologically challenging subtropical habitat and subject to intense predation pressure. This makes the sardine run a rare example of a mass migration that has no apparent fitness benefits.

中文翻译：

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非洲东南部的沙丁鱼大迁徙是大规模迁徙进入生态陷阱

夸祖鲁-纳塔尔沙丁鱼流，俗称“地球上最大的浅滩”，是南非沙丁鱼从温带核心区域向亚热带印度洋的大规模迁移。有人提出，这代表了独特的亚热带种群的产卵迁移。使用在南非海岸附近收集的沙丁鱼的基因组和转录组数据，我们确定了两种种群，一种是冷温带（大西洋），另一种是暖温带（印度洋）。出乎意料的是，我们发现参与沙丁鱼奔跑的沙丁鱼主要来自大西洋，因此更喜欢较冷的水。这些沙丁鱼从暖温带鱼群中分离出来，在短暂的冷水上升流期间迁移到暂时有利的印度洋栖息地。一旦上升流结束，他们发现自己被困在具有生理挑战性的亚热带栖息地，并受到强烈的捕食压力。这使得沙丁鱼成为一个罕见的大规模迁徙例子，没有明显的健康益处。