Sympatric anadromous-resident pairs of threespine stickleback species in young lakes and streams at Bering Glacier, Alaska

1. Resident freshwater and anadromous pairs of threespine stickleback species (Gasterosteus aculeatus species complex) have been reported from only a few lakes. 2. We discovered such pairs in four lakes and four streams, including recently deglaciated terrain, near the Bering Glacier, Alaska. 3. Bering Glacier surges are accompanied and ended by massive glacial outburst floods, which create unmatched dynamics in the surrounding lakes and streams. 4. This is the first report of a stickleback species pair in a proglacial lake (Vitus), which did not exist 100 years ago. 5. These species pairs may have arisen through sympatric speciation from an anadromous ancestor, double invasion of anadromous stickleback, or independent colonization by resident and anadromous forms.