Torsk på rev. Foto: Karsten Dahl

Cod save energy on stone reefs

Thursday 20 Feb 20


Jon Christian Svendsen
Senior Researcher
DTU Aqua
+45 93 51 16 63

Opportunities for students

DTU Aqua is looking for students for projects on stone and mussel reefs:


Description of stone reef project

Description of mussel reef project


Reed more

Read more about stone reefs and cod following the restoration of the stone reef at Læsø:

Behavioural changes of Atlantic cod after marine boulder reef restoration: Implications for coastal habitat management and Natura 2000 areas  (Fisheries Management and Ecology)

Cod sheltering in stone reefs can conserve energy, allowing them to spend more energy on growing or on reproduction

Researchers from DTU Aqua have found that cod save about 20 per cent of their energy when sheltering in a stone reef compared to fish resting on a sand bottom habitat. The study was conducted in the lab and included measurements of the energy consumption of cod either placed in a tank with a simulated stone reef or in a tank with a simulated sand bottom habitat.  It enabled the researchers to compare the energy consumption of the fish in a stone reef and a sand bottom habitat and showed that cod spend less energy when sheltering in the stone reef than on the sand bottom.  The studies suggest that cod that have the opportunity to shelter in a stone reef have more energy at their disposal. The findings have just been published in the scientific journal, Marine Ecology Progress Series.

“It’s a bit like saving energy in your house. The money you save can be spent on other things. Similarly, cod can use the saved energy for other purposes—e.g. growing larger or increased fish production,” says Adina Schwartzbach, who has recently completed her MSc thesis at DTU Aqua. 

Cod thriving in stone reefs

Researchers at DTU Aqua have been working on several projects to restore stone reefs. Following restoration of a stone reef at the Danish island of Læsø, it has been shown that the new reefs act as a good habitat for cod. The fish on the reef have rich opportunities to shelter and find food. Large numbers of cod have been observed on the restored stone reef. Studies of individual cod showed that the fish spend more time in the area following restoration, suggesting that the cod respond favourably to stone reef restauration. The same development has also been observed following restoration of stone reefs at Sønderborg.

“Many have discussed whether stone reefs merely attract fish or if such complex habitats also facilitate increased cod growth. The new findings indicate that stone reefs are not merely attracting more cod. Stone reefs can also help ensure greater production of cod because cod save energy when sheltering in a stone reef, thus enabling the fish to grow or have more offspring,” says Jon C. Svendsen, Senior Researcher at DTU Aqua.

New stone reefs good for cod

Stone reefs have been removed for over 100 years in Denmark. The boulders were used for piers and other coastal construction. This means that there are now very few stone reefs in Danish waters. While it is difficult to determine what the past removal of stone reefs has meant to fish stocks, studies suggest that the conservation and restoration of stone reefs can benefit many fish species.  It is still uncertain to what extent the surrounding fishery benefits from the positive effect that reefs have on cod. There are plans for future studies to investigate this issue. Researchers are also planning to examine whether mussel reefs also have a positive affect on a number of fish species.

Read the scientific article in Marine Ecology Progress Series (MEPS):
Atlantic cod Gadus morhua save energy on stone reefs: implications for the attraction versus production debate in relation to reefs.

By Adina Schwartzbach, Jane W. Behrens and Jon C. Svendsen, DTU Aqua. National Institute of Aquatic Resources

This video shows more about the study and the results: