Climate Change could make a sea in southern Scandinavia too warm for Atlantic cod and rising water temperatures may be stunting the growth of young fish, a study showed on Monday.
The report, drawing on records since 1919 of more than 100,000 juvenile cod caught and measured in the Skagerrak area off south Norway, gives some of the most detailed evidence yet of how global warming may affect commercial fish stocks.
Juvenile cod grew less in summers when waters were warmer than usual in Skagerrak, which is between Sweden, Denmark and Norway, it said. The summer slowdown more than offset a small boost to growth when spring temperatures were above normal.
“The coastal Skagerrak will become ill-suited for Atlantic cod,” if a projected rise in summer temperatures continues to have most influence on growth, according to scientists in the United States and Norway.
“We found that spring and summer temperatures have opposite effects on the growth of cod,” said Leif Stige, of the University of Oslo and among authors of the study in the U.S. journal Proceedings of the National Academy of Sciences.
“This shows some of the complexity in predicting the effects of a warmer climate,” he told Reuters of the study, which is rare in being able to draw on a long historical record. Other fish species may also react to warming in similar ways, it said.
The study estimated that a sea surface temperature of 19 degrees Celsius (66.20F) in summer, against an average 15.9 C over the past century, lopped 0.1 cm (0.04 inch) off a typical cod that reaches almost 10 cms long by autumn, it said.
Cod may already be suffering since temperatures have often been above average in recent years. “It is likely that coastal Skagerrak cod are experiencing decreased growth rates because of high metabolic costs in warm summers,” it said.
Warmer temperatures may mean less food for cod in summer. In the nearby North Sea “warming temperatures have decreased the quantity and quality of plankton prey for cod,” it said.
Climate models predict a rise of 2-3 degrees Celsius in summer surface temperatures in Skagerrak over the next century.
Apart from climate change, many commercial fish stocks also face problems including over-exploitation and pollution.
Total landings of cod from Skagerrak and the linked Norwegian sector of the North Sea fell to about 24,000 tons in 2008 from 67,000 tons in 2000, according to Norwegian statistics. Most was by European Union vessels.
The U.N. panel of climate experts has said global warming is tending to push many stocks of fish toward polar areas.