Global Warming could put an end to North Sea Cod
Global warming will adversely affect the recovery of the Atlantic Cod in the North Sea, a new study shows.
Stocks of Atlantic Cod (Gadus morhua) have experienced overfishing and adverse environmental conditions for some years, but there has previously been no evidence to explain why some populations don't immediately recover even with protection from fishing.
Now a new mathematical model, using data collected continuously from merchant ships trawling the North Atlantic and North Sea (the Continuous Plankton Recorder) shows a link between numbers of zooplankton and the addition of young cod to populations, known as 'recruitment'. Although this link had been suggested before - though not explained - the new study shows that water temperature is an important factor governing zooplankton numbers and, consequently, cod stocks.
Newborn cod in the North Sea and the Irish Sea feed on certain species of surface-dwelling copepod (a small crustacean), which live in cold water. Warmer water pushes these copepod species further north, their place being taken by species that prefer more temperate conditions, but which don't appear until later in the year so they can't provide food for young cod. Consequently, there are fewer adult cod in southern areas of the North Sea, resulting in increased numbers of jellyfish and crabs. Another knock-on effect is a decline in flat fish such as plaice, whose young are eaten by the proliferating crabs.
Dr Richard Kirby of the University of Plymouth says these cold-water plankton are moving north by 30km per year, as the North Sea has warmed 1 degree Celcius over the past 40 years. The North Sea is already at the lowest limit of the Atlantic Cod's range.
The study predicts that with global warming and a continued rise in sea temperature, larval cod survival will be severely limited and stocks of cod in the North Sea will continue to decline, perhaps to extinction.
Helen Phillips, chief executive of Natural England, the UK government's conservation advisory body, says marine life around the UK could suffer a "permanent collapse" without drastic action.
"We can avoid the bleak future that England's fishing industry currently faces but we have to accept that far-reaching changes, from policy through to purchase, are now needed."
For more information, see 'Spawning stock and recruitment in North Sea cod shaped by food and climate' in the Proceedings of the Royal Society B 2011;278 504-510.