Warming water temperatures provide cod as they were in the 1980s

In recent decades, North Sea cod and other threatened stocks have been maturing earlier and earlier, and are therefore getting smaller. The likely cause of this is overfishing. However, new research results from the Department of Bioscience, Aarhus University, show that the warmer climate is putting this right.
“It’s the increase in the sea temperature that has saved the cod from becoming the small fry we’d expected. If the temperature had been constant for the last thirty years, things would probably have appeared very different,” says fish ecologist Peter Grønkjær.
Opposite trends
“In the 1980s, an average North Sea cod reached sexual maturity at 4 years and measured 70 cm. By the turn of the millennium, however, an adult cod in the same location was only 2.5 years and 50 cm long. The fish can nevertheless reach the same size today as they did thirty years ago,” says Associate Professor Grønkjær. The impact of climate change and the warming sea water on cod growth is the crucial question behind the results that Associate Professor Grønkjær has just published along with Anna Neuheimer from DTU Aqua (National Institute of Aquatic Resources) in the international journal Global Change Biology.
“Our results show two opposite trends. To start with, the cod reach sexual maturity at an earlier age. This would normally mean that we get smaller fish, because fish of a smaller size start using the energy that should really be used for growth to produce offspring. However, we’re also seeing increased growth in the mature fish, and this is due to the rising temperatures. For this reason, we haven’t seen the changes in the North Sea that would be expected as a result of the drastically earlier age of maturity,” says Postdoctoral Scholar Anna Neuheimer. However, continued warming is not a solution to the problems of the North Sea cod.
“Still higher temperatures can make the cod smaller – at least if the fish remain in the North Sea – because warming could set so many life processes going that the cod wouldn’t be able to maintain them,” explains Associate Professor Grønkjær.

Basis for preservation
The two researchers drew on data collected for administrative work at the ICES Data Centre, an international database with marine data from all over the world. They developed a method that can help provide an understanding of the impact of global warming on marine life, and their discoveries about the growth dynamics of fish provide important input to the work that is necessary to develop preservation strategies.

Contact
Associate Professor Peter Grønkjær, Department of Bioscience, Aarhus University, +45 8715 6114/2338 2177,
peter.groenkjaer@biology.au.dk Postdoctoral Scholar Anna B. Neuheimer, Centre for Macroecology, Evolution and Climate, DTU Aqua/University of Copenhagen, +45 2343 6037,
anna.neuheimer@bio.ku.dk Link to article: http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2486.2012.02673.x/abst...
Read more (in Danish only): Global warming benefits the cod population in Jyllands-Posten, 22.04.12