Cod stocks in the North Sea have been hit by a double whammy of overfishing and warming waters, scientists say.
Cod are under pressure from two directions
A team of oceanographers report in the journal Nature that rising sea-surface temperatures in important fisheries have disrupted plankton supplies.
The microscopic animals are the primary food for larval cod and their decline has meant fewer fish are making it to adulthood to be caught by trawlermen.
Fisheries experts believe North Sea cod stocks have reached a historic low.
A recent report from the International Council for the Exploration of the Sea, a scientific advisory group to the European Commission, went so far as to recommend a total ban on cod fishing in some EU waters.
Drastic cuts in fishing quotas have been introduced in a bid to save the stocks from total collapse. And these quotas are expected to become even more restricted after a meeting of fisheries ministers next week.
Now, Gregory Beaugrand, of the French National Centre for Scientific Research, and colleagues have explained another pressure exerted on the cod population in addition to overexploitation.
The team says the surface temperature of North Sea waters has climbed an average one degree over the past 50 years and that this has had a substantial effect on the abundance and distribution of plankton.
This has been most pronounced from the mid-1980s onwards.
"There has been a northward movement of warm-water plankton and at the same time a decrease of colder water species," Dr Beaugrand told BBC News Online.
"Overall, there has been a major reorganisation of the plankton ecosystem and this has strongly impacted the quality and quantity of prey for cod."
He continued: "The increase in temperature also means larval cod have increased their metabolism - there is more energetic cost for larval cod.
"But, at the same time, there is less plankton available to have energetic gain. This is bad news for cod."
Previous studies had also tried to describe the relationship - with limited success, claimed Dr Beaugrand. But the strength of the link was evidenced in the so-called "gadoid outburst".
This was a burst in plankton supplies that triggered a surge in the cod population, fondly remembered by trawlermen, and which ran for 20 years from 1963-83.
"People had thought that overfishing was solely responsible for changes in cod recruitment but our study indicates clearly that plankton availability has an effect in addition to overfishing," Dr Beaugrand said.
He added that climatologists did not foresee an immediate return to the water temperatures that might boost plankton and cod success.