Study gives clues to cod's response to climate change

Populations of cod on both sides of the Atlantic have survived past climate extremes, albeit with a reduced population, a new study from an international team of researchers shows. The
scientists hope that their results will improve our understanding of cod’s vulnerability to future global warming.

Fish stocks are already under threat from over-fishing, and there are concerns regarding their vulnerability to climate change. One way of gauging a species’ response to changing temperatures
is to find out how it responded to such events in the past.

In this latest study, which is published in the Proceedings of the Royal Society B, scientists studied how populations of Atlantic cod (Gadus morhua) changed during the last ice age, when both
temperatures and sea level were lower than they are today.

Studies of land animals have revealed that at this time, most terrestrial species moved further south. Carrying out similar studies of marine species is complicated by the three-dimensional
nature of the seas. ‘Conditions can vary greatly over a very few metres in water depth, and different life-cycle stages may occupy different habitats,’ the scientists explain.

Furthermore, the fall in sea levels meant that many of the productive, shallow water habitats around Europe and North America simply ceased to exist. In addition to this, changes in oceanic
circulation patterns meant that the temperature change between tropical and polar conditions occurred over a much shorter distance, meaning there was less room for temperate species such as
cod.

The scientists used computer models to estimate ice-age habitats which would have been suitable for cod, and combined these with genetic studies of current cod populations and ecological
information.

These analyses revealed that cod’s range during the ice age reached down as far as northern Spain, but the total area of suitable habitat was just a fifth of what it was today. Nevertheless,
cod populations continued to exist on both sides of the Atlantic throughout this period.

‘This research shows that cod populations have been able to survive in periods of extreme climatic change,’ said Professor Grant Bigg of the University of Sheffield. ‘However, this does not
necessarily mean that cod will show the same resilience to the effects of future climatic changes due to global warming.’

‘The effects of future climatic conditions need to be considered in relation to demographic structures, as well as the major population changes due to human exploitation,’ the scientists
conclude.

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