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Cod in a sweat: some like it hot!

Concerns about climate change have led Cefas researchers to investigate whether sea temperature plays an important role in fish distribution.

Recent tagging studies suggest that the movements and distribution of individual animals is much less restricted by environmental temperature than laboratory studies showed.

Laboratory research indicated that the preferred temperature range of Atlantic cod (Gadus morhua) was 11–15ºC. However scientists saw a different pattern when they followed the migration patterns of wild cod equipped with electronic tags. The tags, which record depth and temperature, showed that whilst some fish prefer deeper cooler waters, others tagged at the same time prefer to swim in shallower habitats in the southern North Sea where summer temperatures are consistently above 17ºC.

Although individual cod can adapt to changes in temperatures, they are thought to behave in a way that reduces thermal stress and to be unable to tolerate large, abrupt changes in temperature. Consequently, it was believed that temperature changes of only a few degrees could constitute fairly impenetrable boundaries and that this might contribute to the segregation of cod populations in nature.

“We have found that cod in the northeast Atlantic repeatedly experience abrupt temperature changes of up to 8ºC, suggesting that temperature may not be so crucial in constraining the movements and distribution of adult cod,” explains Cefas’ Dr Julian Metcalfe.

“However this doesn’t mean that climate change won’t impact the numbers or distribution of cod populations since there may be other environmental factors, such as prey distribution or suitable flow refuges, that could be affected by a rise in sea temperatures.”

This research is part of the EU-funded CODYSSEY programme, which aims to identify key environmental forcers of horizontal movements of cod. The programme ends in early 2007, but the Department for Environment, Food and Rural Affairs (Defra) continues to fund research on the distribution and movements of key fish species in relation to their environment.

Centre for Environment, Fisheries & Aquaculture Science (Cefas). April 2006.

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