Small ear bones from fish (otoliths) found in deep-sea sediments during an expedition aboard the Swedish research icebreaker Oden demonstrate that the glacial Arctic Ocean was not a barren ice desert but a breeding ground for the evolution of cold adaptation in marine fish. This is the main conclusion of a new scientific publication in Nature Communications, stemming from an EU project coordinated by Stockholm University and carried out in collaboration with the Alfred Wegener Institute in Germany.
Direct evidence of past vertebrate life
The study began with an exciting, classic scientific discovery: an unexpected observation that suddenly strikes a scientist's mind. While the original EU project was designed to map today’s fish stocks using ultrasound and fish catches, a record from the past suddenly emerged and was incorporated into a new EU project. When the first bottom sample, originally collected to study the small organisms living in and on the sediments, arrived on deck, several expedition scientists immediately noticed a fish otolith. These sediment otoliths provide direct evidence of past vertebrate life in the central Arctic Ocean, a region where such evidence is exceptionally rare. Moreover, the presence of fish indicates that a living marine ecosystem existed during the last ice age. Fish are key organisms in the food web; their presence indicates the presence of plankton, which serves as fish food, and fish themselves provide food for seabirds and marine mammals. “These tiny bones provide a direct record of when and where fish occupied Arctic waters during major climate transitions,” says first author Julek Chawarski, who worked in the Department of Ecology, Environment and Plant Sciences at Stockholm University during the EU project.
Fish remains from the last 50,000 years
The researchers found more otoliths in the first sediment sample, and four additional samples from different locations across the Arctic Ocean also contained otoliths. After the expedition, the remains were identified and dated at the Alfred Wegener Institute’s radiocarbon-dating facility (MICADAS). “The measurements revealed the distribution patterns of two cold-adapted Arctic-endemic fish species, ice cod and polar cod, over the past 50,000 years”, says Pauline Snoeijs-Leijonmalm. “Otoliths were rare in the samples; we needed to carefully sieve through large volumes of sediment to find them, but it was well worth it. Otoliths provide an excellent way to study past Arctic ecosystems”, says co-author Flor Vermassen, a researcher in the Department of Geological Sciences at Stockholm University.
Two species, different survival patterns
The otoliths show that ice cod and polar cod were present in the central Arctic Ocean between 48,000 and 35,000 years ago, during the last ice age when ecosystem productivity was very low. Both species were found at latitudes of 87–89°N, close to the North Pole. During the Last Glacial Maximum, the coldest period of the last ice age, which lasted from around 27,000 to 19,000 years ago, both species disappeared from the North Pole area. However, ice cod survived the peak of this period around 20,000 years ago further south, at 84–85°N. The researchers suggest that it may have survived in polynyas — areas of open water surrounded by sea ice. As the climate warmed at the beginning of the Holocene, around 12,000 years ago, both species reappeared in the area and have persisted to the present. “That ice cod survived this far north during the Last Glacial Maximum whereas polar cod did not, suggests a mechanism for climate-driven evolutionary divergence between these two Arctic-endemic cod species, which are thought to have diverged between 5 and 10 million years ago”, says Pauline Snoeijs-Leijonmalm. The different survival patterns suggest that past climate changes may have helped shape the evolutionary differences between the two species.
What does this mean for Arctic fish today?
The same adaptations that allow ice cod and polar cod to survive in extremely cold conditions may also leave them vulnerable as the Arctic warms. Sea ice over the deep basins and submarine ridges of the central Arctic Ocean is becoming thinner, younger, and more dynamic, while midwater temperatures are rising. “Northward expansions of boreal species such as Atlantic cod and Alaska pollock will increase competition and predation in the Arctic Ocean. This puts the two extremely cold-adapted Arctic-endemic cod species at significant risk of population declines, as ongoing climate warming will soon exceed the limits of their evolutionary adaptations,” says Pauline Snoeijs-Leijonmalm.
Research cooperation and funding
The institutions participating in this study were Stockholm University (the Department of Ecology, Environment and Plant Sciences and the Department of Geological Sciences) and the Alfred Wegener Institute Helmholtz Institute for Polar and Marine Research in Bremerhaven, Germany. The work was funded by the European Commission (CINEA), the Swedish Research Councils VR and Formas, the Swedish Polar Research Secretariat, and the German Helmholtz Association’s Program-Oriented Research Period IV.
This news story was first published by Stockholm University.
Original publication
Julek Chawarski, Flor Vermassen, Kim Vane, Hauke Flores, Jutta Wollenburg, Hendrik Grotheer, Claudia Morys, Nicole Hildebrandt, Helen K. Coxall & Pauline Snoeijs-Leijonmalm: Survival and divergence of Arctic endemic cod species during 50,000 years of climate change; Nature Communications (2026). DOI: 10.1038/s41467-026-78810-4
