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Illustrasjon photo from the Barents Sea
Illustrasjon photo from the Barents Sea.
Photo: Kjartan Mæstad
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Climate change impact on fish stocks in the Barents Sea

Climate change is occurring more rapidly and produces many more changes in Arctic ecosystems than in any other part of the globe. Warmer water generally leads to a more productive ocean, but the effects on various parts of the food web might vary. 
Predictions suggest that the degree of change is expected to have particularly strong and possibly nonlinear effects in Arctic areas. In the Barents Sea, the long-term climate changes imply increased temperatures, less ice and a warmer ocean. 
 
Since 2006, however, the temperatures have decreased and the sea ice cover has increased, possibly as a result of complex interactions among several climate-related factors. Similar observations and predictions have been made in the Bering Sea.

Special issue

How the combined effects of a changing climate on oceanographic processes and food web dynamics are likely to influence marine resources and, thereby, fisheries at high latitudes
was the topic of the Annual Russian–Norwegian Symposium in 2011. From the contributions at the symposium nine articles were published in a special issue in Marine Biology Research (Volume 9, no 9). Although the journal is broader in scope, this article will focus on four articles dealing with effects on fish stocks.  
 
A study on Barents Sea capelin (Mallotus villosus), found that ocean temperature and ice cover set the large-scale terms for the capelin distribution, while the stock size determines how capelin uses the available area. Changes of 4 million tonnes in stock size or 1oC in temperature give comparable impacts on the distribution.
 
Another study compared Polar cod (Boreogadus saida) and capelin (Mallotus villosus) which are key species in Arctic and sub-Arctic marine food webs, respectively. Global warming, with reduction in sea ice and increase in temperature, will likely result in Polar cod loosing the ice-associated part of its life cycle and become more restricted in pelagic distribution during summer, whereas the capelin stock may expand to the north and east, although with considerable fluctuations from year to year.

Challenging uncertainty

Yet another study focused on changes in the relationship between sea temperature and recruitment of cod (Gadus morhua), haddock (Melanogrammus aeglefinus) and herring (Clupea harengus) in the Barents Sea. This study found a significant positive relationship between recruitment and temperature. Temperature during the first winter of life was found to positively correlate with haddock and cod recruitment residuals. Temperature during the first summer of life correlates positively with herring recruitment during some parts of the period, but also this correlation is weakened towards the end of the period.
 
Projecting stock response to climate change is a challenging task, which was the topic of the fourth and final study included in this article. Data suggest that there has been a positive relationship between the recruitment of cod in the Barents Sea and the sea temperature at the Kola section during the year of spawning. However, analysis of the most recent data indicates that this relationship no longer holds. Modelling populations based on the various recruitment hypotheses leads to vastly different results. This indicates the high difficulty of predicting the future development of a stock with any degree of certainty, or even with any quantifiable degree of uncertainty.