Polar and Atlantic cod share habitat, but not diet

Polar and Atlantic cod share habitat, but not diet

Top image: Small, but significant: The polar cod is important in the high Arctic ecosystem. Photo: Geir Johnsen/UNIS.

A warmer ocean climate has resulted in Atlantic cod and haddock extending further into Arctic waters, presenting a potential threat to the native polar cod which is an important part of the high Arctic ecosystem. However, a new study by scientists in Svalbard and Tromsø reveals there is little competition for food between the invaders and the polar cod.

30 April 2012
Text: Eva Therese Jenssen / UNIS

Increased sea temperatures, altered circulation patterns, and severe reductions in ice cover in high-latitude regions will have a profound effect on the High Arctic marine environment, and southern species have started migrating northward due to warmer water in the High Arctic.

During a recent period of increased influx of warm Atlantic water to the western coast of Svalbard, there are observations of northward expansion of boreal Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) into areas dominated by the native polar cod (Boreogadus saida).

Important key species in the high Arctic ecosystem
The polar cod is an important food source for seabirds, marine mammals and other fish, so disturbances to their diet and habitat would have a wide impact on Arctic Ocean ecosystems.

Polar cod

Polar cod. Photo: Øystein Varpe.

Scientists from Akvaplan-niva, the University Centre in Svalbard, Norwegian Polar Institute and University of Tromsø wanted to determine the potential impact of these species living and interacting in the same habitat, and studied the diet of co-occurring juvenile gadoids in fjords, open water, and sea ice around Svalbard. The study was conducted under the EU-funded Arctic Tipping Points (ATP) project.

Recent studies have found that young polar cod can coexist with others of closely related species. However, few studies have so far sought to understand how polar cod populations respond to competitors.

In this study, the scientists set out to investigate the extent to which the diets of polar cod overlap with those of Atlantic cod and haddock. The scientists collected gadoids from the three species during cruises in 2006, 2008 and 2009 from fjords around Svalbard, and from open water and beneath sea ice north of the archipelago.

Little overlap
The polar cod fed mainly on krill and crustaceans, such as Calanus and Themisto species. Atlantic cod and haddock fed on some of the same species, including Themisto species and krill, but where all three species inhabited the same waters, overlap in their diet was found to be less than 40%. Researchers identified only one of the regions – near a glacier in Billefjorden – where the diets of Atlantic cod and haddock were similar to each other, but overlap with polar cod was still low at just 37%.

Stomach content only reveal what the fish have eaten recently and the scientists also wanted to find out the long-term trends in feeding habits. They carried out stable isotope analysis, a method that can be used to investigate diet through chemical analysis of muscle tissue. Again, their results suggested distinct diets for all three species.

The low dietary overlap suggests little direct competition. Future increases in abundance and the high predation potential of the boreal species, however, may impact the persistence of polar cod on some Arctic shelves.

Renaud, P.E. Berge, J., Varpe, Ø. et al. (2012). Is the poleward expansion by Atlantic cod and haddock threatening native polar cod, Boreogadus saida? Polar Biology. 35: 401-412.


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