Polar cod have become larger in the Barents Sea over the last 30 years

Polar cod is a key fish species, transferring energy from zooplankton to larger animals. Polar cod depend on sea ice for spawning and during the early parts of its life. Reduced ice cover may therefore influence the survival and growth of young polar cod directly through e.g. loss of predation refuge, and indirectly by e.g. affecting the abundance and availability of prey.

(Picture: Peter Leopold; Picture collection Norwegian Polar Institute)

In a statistical analysis of a 30-year data series on the mean body length of polar cod cohorts, Nansen Legacy post doctor Nicolas Dupont and coworkers show that polar cod growth in the Barents Sea has been increasing as the winter sea ice cover has decreased. The increased growth is evidenced by increased body sizes at a given age. The reason: reduced winter sea ice cover may have led to an earlier onset of the spring bloom, and an increased zooplankton production, prolonging the overall length of the feeding season for the fish. While an increase in growth should have positive effects on the polar cod population, the stock biomass of polar cod in the Barents Sea did not increase with decreased sea ice cover. Increased overall predation rates from increased dominance of boreal species in the Arctic may counteract an increase in stock biomass, as may a reduction in suitable spawning area. 

The positive, linear effect of decreasing ice cover on polar cod mean body size seen for the past 30 year may be transient, as nonlinear effects may set in when the Arctic warms beyond the past range. A further decrease in sea ice concentrations could for example lead to a mismatch between primary and secondary production, or a shift in zooplankton species composition; both adversely affecting the feeding conditions of polar cod, and hence leading to a reduced growth in the future.


(Illustration: Nicolas Dupont, UiO)

As the sea ice cover in the Barents Sea has diminished, the feeding season for polar cod has become longer over the past 30 years. That has led to overall better feeding conditions for these arctic fish, resulting in increased body growth and sizes of polar cod in the Barents Sea.