French oceanographers say rising temperatures in the North Sea over the past 20 years disrupted supplies of plankton, the basic food for baby cod, according to their research, which is published on Thursday in the British scientific journal Nature.

During this time, plankton stocks have varied enormously in abundance and seasonal timing, and this has had a dramatic impact, leaving fewer young cod to survive.

Added to that is overexploitation of the North Sea, which means that many of those cod which survive to reproductive age end up in the net.

"Fluctuations in plankton have resulted in long-term changes" in the North Sea's cod population, say the authors, led by Gregory Beaugrand of the National Centre for Scientific Research (CNRS).

"(...) Rising temperature since the mid-1980s has modified the plankton ecosystem in a way that reduces the survival of young cod."

Conversely, a burst in plankton supplies triggered a surge in the cod population, fondly remembered by trawlermen, that ran for 20 years from 1963-83, Beaugrand's team suggests.

Data about the world's climate suggest that warmer temperatures began to kick in from about the middle of the 1970s as the result of emissions of "greenhouse" gases -- carbon pollution from fossil fuels that traps the Sun's heat.

Preliminary figures from some countries suggest 2003 will be the warmest year ever recorded.

SPACE.WIRE