Ancient fish bones reveal how Arctic cod species survived the last ice age

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 from Stockholm University that examines small fish bones found in deep-sea sediments during an expedition aboard the Swedish Icebreaker Oden.

“These tiny bones, the otoliths, 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 research project.

Evidence of past vertebrate life

Otoliths are small calcium carbonate structures in the inner ear of bony fish. They function as part of the fish’s balance and hearing organs, detecting movement, gravity, and sound vibrations. The otoliths were found during a research expedition to the Arctic, coordinated by Stockholm University, aiming to map today’s fish stocks using echo sounding and fish catches in the water column. 

“But when the first bottom samples arrived on deck, we immediately noticed a fish otolith”, says lead author of the study, Professor Pauline Snoeijs-Leijonmalm from the Department of Ecology, Environment and Plant Sciences at Stockholm University.

The sediment otoliths provided direct evidence of past vertebrate life in the central Arctic Ocean, a region where such evidence is exceptionally scarce.

“The presence of fish indicates that a complex food web similar to the one today existed here during the last ice age as well. 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”, Pauline Snoeijs-Leijonmalm continues.

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.

Read more about The Swedish Research Councils and the ice breaker Oden

Access the study

Find the study in Nature Communications: “Survival and divergence of Arctic endemic cod species during 50,000 years of climate change”