在末次冰期期间，北美西北部的太平洋大马哈鱼（白鲑）面临河流冰封的严峻挑战，阻断了它们在海洋与内陆产卵地之间的回游通道。传统科学界长期认为，被冰川阻隔在内陆、无法完成海洋生命周期的鲑鱼种群均已灭绝；然而，业余科学家帕特里克·马丁在《第四纪研究》发表的最新基因研究表明，部分受困种群不仅成功存活，而且在封闭的淡水环境中繁衍演化。

大马哈鱼广泛分布于从东亚的日本、朝鲜半岛到北美俄勒冈州沿海的环太平洋区域，并在阿拉斯加的科迪亚克岛、库克湾以及华盛顿州与不列颠哥伦比亚省之间的萨利希海呈现出极端的遗传与行为变异。例如，萨利希海种群选择在夏季而非传统的秋冬季回游产卵，库克湾的部分种群更演化出在湖泊而非河流中产卵的习性。地质证据显示，在距今2.3万至1.5万年前，冰川在科迪亚克岛截流形成了一个面积达1000平方公里、深达300米的巨型冰川湖，华盛顿州奥林匹克山脉周边亦形成了类似的冰川堰塞湖。

为验证这些变异是否源于冰川湖的长期地理隔离，马丁联合华盛顿大学与美国国家海洋和大气管理局（NOAA）的研究团队，对环太平洋310个种群的32,817个样本进行了大规模基因测序。结果排除了亚洲种群基因渗透的可能，证实被困大马哈鱼将封闭湖泊当作“微缩海洋”并独立演化出特有的淡水生存策略，且在冰坝消融重归大海后依然保留了这些生理与行为特征。这种独特的遗传多样性不仅为物种抵御灭绝风险提供了天然屏障，也为开发无海虱侵害、病原体更少的高纬度淡水鲑鱼养殖提供了关键依据。

During the last ice age, while large megafauna flourished, Pacific chum salmon in north-western North America faced severe disruptions as glacial ice locked up migration rivers between marine habitats and inland spawning grounds. Scientists long assumed that salmon trapped inland perished without access to the sea; however, a genetics study published in Quaternary Research by Patrick Martin reveals that landlocked populations not only survived but thrived in isolated glacial refugia.

Chum salmon inhabit coastal rims spanning from Japan and the Korean peninsula to Oregon, displaying profound genetic and behavioral anomalies in three distinct hotspots: Alaska's Kodiak Island, Cook Inlet, and the Salish Sea between Washington state and British Columbia. Unlike typical populations that migrate during autumn or winter, Salish Sea salmon spawn in summer, while a Cook Inlet subpopulation spawns in lakes rather than running rivers. Geological records indicate that between 23,000 and 15,000 years ago, glacial dams formed extensive freshwater bodies, including a 1,000-square-kilometre, 300-metre-deep lake on Kodiak Island and glacial meltwater lakes around Washington’s Olympic Mountains.

To test whether geographic entrapment drove this divergent evolution, Mr Martin collaborated with researchers from the University of Washington and the National Oceanic and Atmospheric Administration (NOAA) to sequence 32,817 specimens across 310 Pacific-rim populations. Genetic analysis ruled out cross-Pacific admixture from Asian lineages, demonstrating that ancestral salmon adapted by treating trapped glacial lakes as miniature oceans, retaining their freshwater modifications even after glacial melting reconnected them to the Pacific. This extreme genetic divergence offers vital resilience against ecological extinction while providing valuable biological templates for freshwater, cold-water aquaculture free from marine parasites such as sea lice.

Source: How salmon evolved to cope with the ice age

Subtitle: Their odd behaviour seems to be a relic of the time of the glaciers

Dateline: Aug 27th 2026