The future of cod in the North Sea
For years, there has been worries about the future for cod in the North Sea. Until recently it was thought that this was down to overfishing; but some scientists, including Dr Richard Kirby and Dr Grégory Beaugrand believe that that even if cod fishing was outlawed – that cod could disappear completely from the North Sea. This is down to rising temperatures in the North Sea due to global warming.
In the past 40 years, a 1oc average temperature increase has been recorded in the North Sea, leading to the current average temperature of 6oc in winter and 17oc in summer increase . This increase is double the increase in the Earth’s surface temperature of approximately 0.5oc in less than half the time it took .
The change in temperature is dramatically affecting the numbers of cod who live in the North Sea and especially the survival of cod through the first year of their live. There are a number of reasons why they are affected like this, including:
1) Young cod feed mainly on copepods, along with many other species of small crustaceans. The copepods that these cod eat prefer colder waters, and have been moving north as the sea warms by 1200km, with their numbers declining by 60% in the past four decades. With the main source of food for young cod becoming much rarer in the sea, it is only natural that cod are less likely to survive to the stage where they are large enough to eat what adult cod eat .
2) Adult cod will eat crabs and shrimp on the seabed. Both cod and shrimp eat plankton, competing with cod larvae for food. With less cod surviving to reach adulthood, there has been an increase in the numbers of crab and shrimp, therefore increasing the competition that cod larvae has to face in order to feed. As a bonus, crabs and shrimp also spawn more larvae in warmer waters. So with the number of crustaceans blooming in the North Sea, there is a lot more competition for food with young cod.
In addition to these two reasons: cod, like many other fish, have their favoured temperature range for spawning and may not spawn at all if the temperature isn’t right; cod eggs and young cod are very susceptible to changes in water temperature, which can even kill them .
Work undertaken by scientists at the Cefas Lowestoft Laboratory have shown a relationship between the average sea-surface temperature of the North Sea during spring, and the numbers of cod surviving their first year 
As these two graphs show, the overall pattern is that the higher the temperature of the water, the less cod survive to be 1-year old.
So if cod will disappear from the North Sea, where will they go? The most popular opinion is that the Barents Sea, north of Russia and Norway, will be the most likely choice.
1) http://www.mumm.ac.be/EN/NorthSea/facts.php, undated.
4) http://www.stemnet.nf.ca/cod/history4.htm, undated