The secret life of salmon during cryptic ice age lake isolation

Abstract Conventional thought holds that in formerly glaciated areas straying of anadromous fish from nearby unglaciated areas established contemporary salmon populations. An additional explanation for patterns of salmon life-history diversity and population structure derives from isolation of populations in proglacial lakes. We evaluate evidence for these potentially complementary hypotheses in chum salmon from two previously glaciated North American regions: the southern Alaska Peninsula/upper Cook Inlet and the Salish Sea of northwestern Washington and southern British Columbia. Some chum salmon populations in the southern Alaska Peninsula are genetic outliers compared with other nearby populations, while Salish Sea chum salmon populations have greater region-wide genetic divergence and lower gene diversity. Within-population genetic diversity and among-population divergence in both study areas support a hypothesis of salmon persistence relying on cryptic isolation and freshwater-resident (trout-like) life histories in proglacial lakes. We find that ice age adaptation of salmon to a trout life history helps explain aspects of contemporary population structure and life-history diversity.