Even a minor drop in oxygen levels can have serious negative effects on cod. Hypoxia in Danish waters has been widespread in late summer and autumn months and has become increasingly severe in recent years. Surprising new results from DTU Aqua indicate that even a small reduction in oxygen is having significant consequences.
Biologist Magnus Husen’s PhD project focuses on cod which are under severe pressure. Together with his supervisor, senior researcher Jane Behrens, Magnus has just carried out his first experiments, and the preliminary results are striking.
‘We have investigated the effect of an oxygen level, which is at a level that cannot even be defined as oxygen depletion, but is simply called ‘low oxygen content’ in the annual reports from Aarhus University. This corresponds to 4-6mg oxygen/L. The effect on the cod was significantly more negative than we had actually expected,’ Magnus Husen said.
‘The fish that had been at this oxygen level for five weeks had grown significantly less, were thinner and had a smaller liver in relation to their body size than normal. The liver is the cod’s ‘food package’, and with a small liver the fish have less energy available. Their swimming ability was also affected’.’
Cod attempted to compensate for the poorer oxygen conditions by up-regulating the amount of red blood cells and the concentration of hemoglobin, which transports oxygen in the blood around the body. According to the researchers, they turned into different cod. Which the researchers could also see in the behaviour of the fish.
‘Our observations also show that the cod here are less active and react less to external influences. In short, they seem duller, and one can fear that they are therefore also more vulnerable to external threats, for example being hunted and caught by predators,’ Magnus Husen explained.
All in all, the preliminary results from Magnus Husen’s experiments show that moderate or severe oxygen depletion does not need to occur before fish such as cod become measurably and chronically affected – the alarm bells should ring as soon as a minor reduction in oxygen in the sea is noticed, and this is often not even defined as oxygen depletion.
The growth experiments with reduced oxygen and cod have been carried out by Magnus Husen and Jane Behrens in DTU Aqua’s experimental facility for fish biology in Lyngby over six weeks. Cod from Øresund were divided into two groups, in which one group of fish was in fully oxygenated water, while the other group was in water with 50% oxygen saturation.
The actual set-up with wild cod, in which it has to be ensured that they thrive and eat and do not get sick, alongside the fact that a stable oxygen level has to be maintained in many tanks at a time, requires very special facilities and solid knowledge and experience.
‘DTU Aqua’s experimental facility for fish biology in Lyngby has a large capacity, which means that we can have many fish swimming in several tanks and over longer periods. We can also control the temperature, salinity and oxygen content of the water and monitor these parameters online, which is why we can do many types of experiments and with different species of fish,’ Jane Behrens said.
‘And then our sister facility, which works with fish diseases, has several vets in their group, so we can always quickly get advice and help if some fish show signs of disease.’
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,’ he said, adding that the next steps in the PhD will be to look at other factors that may also affect the cod’s growth and success, such as food quality, quantity and diversity.
This spring, Magnus Husen will analyse the stomach contents of the ‘super cod’ found around Åland in Sweden. 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.
‘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
‘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.”
