Next step – Åland
The study of the effects of reduced oxygen on cod in the experimental facility is just the first step in Magnus Husen's Ph.D. about the impacts of different pressure factors on cod:
"We know that increased levels of hypoxia affect fish, but if we can now demonstrate how just a little reduction in oxygen affects a commercially important species like the cod, then we can better understand some of the factors that have contributed to the miserable state of cod now," says Magnus Husen.
The next steps in the PhD will be to look at some other factors that may also affect the cod's growth and success, such as food quality, quantity and diversity. This spring, Magnus Husen will analyze the stomach contents of the 'super cod' found around Åland in Sweden:
"We will look at how their diet differs from the cod we know here at home," says Magnus Husen.
Up in the Åland Sea, just before the entrance to the northernmost part of the Baltic Sea, the Bothnian Sea and bay, the cod is distinguished by its large size - they grow much larger than their relatives in the more eastern, central and western Baltic Sea.
The Åland cod has good oxygen conditions, and over time there has also only been a small, local fishery for them. But otherwise, little is known about these cod far to the north.
Oxygen and cold water may not be the whole explanation
In connection with the MERTOR project, which Magnus Husen's Ph.D. is part of, both PhD student and supervisor Jane Behrens will go to Åland in northern Sweden in April to study the Åland cod.
"Overall, I am very excited to, together with Magnus, delve deeper into understanding why these cod on Åland are doing so well. I suspect that it is not only the better oxygen conditions and the colder water up there that is the whole explanation," says Jane Behrens and elaborates:
"Perhaps part of the explanation also lies in the food they have available, and here I have a feeling that a small circular animal - saduria, also called Baltic crayfish - might play a role."
In the Åland Sea, the water has lower salinity than the sea around Denmark, and also than in the Baltic Sea - is this also something the researchers want to look at?
"Yes, that is absolutely true - the salinity in the Åland Sea is significantly lower than both in the central Baltic Sea and in our internal Danish waters. However, it is not a parameter that we have to look at directly," says Jane Behrens and continues:
"But it is believed that this means that the cod in the Åland Sea cannot spawn locally, but must make spawning migrations to other places. However, it is still unclear where they swim to spawn.”
A broader insight
The MERTOR project and Magnus Husen's Ph.d. focuse on our national fish, the cod. But can the knowledge and experience gained from the experiments with cod be transferred to other species?
"Yes, you can," says Magnus Husen and explains:
"The set-up we have developed, and the learning from the experiments we are taking with us, can well be transferred to other species. However, this requires a good insight into the biology of the species and how they handle captivity. And then, of course, a facility that has the same capacity and modernity as here at DTU."
Back to the experiment in the experimental facility for fish biology at DTU Aqua in Lyngby, there are still many exciting samples waiting to be analyzed. Among other things, blood tests, which will be taken with Magnus to Aarhus University, where they have the right equipment to look at the blood's ability to bind oxygen.
In addition, the test fish's gills, pituitary gland, muscle tissue and otoliths must be examined to fully appreciate what a small reduction in oxygen means for our fish.
