Stocks of cod in the North Sea are in serious danger of collapsing because of the twin threats of overfishing and rising sea temperatures, according to a new study published in today’s issue of the journal Nature. Over-exploitation of cod stocks over the past four decades means that most of the fish caught in the North Sea today are less than three years old, too young to have reproduced. The study’s authors, from Britain’s Centre for Environment, Fisheries and Aquaculture Science, also say that rising sea temperatures over the last decade have affected cod reproduction, which hit a low in 1997 and 1998. The study recommends that cod fishing quotas be reduced by at least 40 to 60 percent, a controversial cut also endorsed by the International Council for the Exploration of the Sea.