Cod fish with mini-thermometers

Now for the first time and using modern advanced technology, it is possible to examine exactly at which temperatures cod are found in the Northeast Atlantic.

Researchers from several European universities, among them the National Institute of Aquatic Research in Denmark (DTU Aqua), equipped more than 2000 cod from eight different North Atlantic stocks, including the Baltic, the Skagerrak and the North Sea, with advanced temperature gauges. For over a year the gauges registered and stored water temperatures around the fish at fixed regular intervals. The results have just been published as a feature article in the "Marine Ecology Progress Series."

"It's absolutely unique to have data from such a large and comprehensive study," says Professor Ken Haste Andersen, one of four DTU Aqua researchers who participated in the international project.

The project has concentrated on cod because it is such an important fish commercially; at the same time, it is a large fish which can easily carry the electronic tag without being bothered by it.

Spawning is the cod's Achilles' heel The results indicate that adult cod can handle much warmer water than was originally thought.

"Some fish were found at temperatures as cold as -1.5 degrees, while others swam quite happily in water that was nearly 20 degrees above zero. This shows that cod are relatively adaptable fish that can tolerate higher temperatures than was previously thought. However, while this is true for adult cod, they appear to be somewhat more conservative in their choice of water temperature when they spawn. During this period, all the fish stocks studied consistently sought out water that had a temperature of between one and eight degrees. This indicates that the egg and larvae stages of a cod's life may constitute a particularly vulnerable time with regard to the effects of climate change," says Professor Andersen.

Subzero temperatures However, the fact that cod in the Northeast Atlantic can survive in water temperatures which fluctuate up to 20 degrees does not mean that all adult cod can tolerate all temperatures. If you took a cod from the North Sea and moved it to the -1.5 degree water north of Iceland, the coldest water measured in the survey, it would be seriously challenged.

"Each fish stock in each area is well-adapted to the local conditions. It is widely known that cod can live in water at subzero temperatures, because they can produce antifreeze proteins which protects them," says Professor Andersen from DTU Aqua.

About the survey

The research teams tagged a total of 3000 cod in different locations in the North Atlantic with electronic temperature gauges that measured and stored information about the temperature of the water around the fish about once an hour for a year. So far, 902 of the cod have been re-caught through fishing, and the tags holding the stored data sent back to the researchers.

To ensure that only information from fish whose tags worked long enough to give a real picture of the water temperatures the fish live in, short data series were discarded. Data from a total of 384 tagged cod from eight different populations are collated in the article's results, including cod from the Baltic Sea, the North Sea and the Skagerrak. The project Codyssey was funded by the EU and was coordinated by Professor David Righton from CEFAS (Centre for Environment, Fisheries and Aquacultural Science) in the UK.