- The deepest waters of the Baltic Sea are becoming uninhabitable.
- Cod and flatfish are at particular risk as these deep waters lack oxygen.
- One solution is to pump near-surface oxygen-rich water to these deep-sea basins.
- Benefits may be felt by all marine life throughout the Baltic Sea.
The picturesque Danish island of Bornholm may be famous for its medieval round churches, but the surrounding Baltic Sea is home to an altogether tastier treasure. Anglers are drawn to the area, attracted by the deep waters of the Bornholm Basin, which are one of the few areas in the world with relatively healthy cod populations.
It is no secret though, that cod are in trouble. Populations around the world are in decline due to extensive targeted fishing, habitat loss, and the harmful effects of eutrophication. The deep-sea basin off the Bornholm coast, whilst having relatively healthy cod populations, is unfortunately no exception.
Here, oxygen levels have decreased over the last 20 years, resulting in a lack of fish and fish food, which impacts on all marine life in the basin.
Now, marine scientists from Sweden and Switzerland have developed a computer model to simulate the movement of water in and around the Bornholm Basin. Their model predicts that artificial pumping of shallow oxygen-rich water to this deep basin returns oxygen, salinity, and other key nutrients, back to habitable levels. In real life, this would mean a return to conditions suitable for cod reproduction and an increase in cod numbers.
Cod are perhaps the Goldilocks of the sea. They need salty waters for their eggs to float, and their eggs need oxygen-rich waters, at around 1.5oC or warmer, to successfully fertilise and hatch.
The results are encouraging news for cod, flat fish, and all other fishy inhabitants and users of the Baltic Sea. Including fishermen and tourists. As oxygen levels increase, other marine life would likely return to the deep-sea basin. This not only provides a source of food for cod, but also provides a natural mechanism to help maintain healthy levels of nutrients and prevent the development of harmful algal blooms in summer.
This could be a practical and simple technique to increase cod populations in the Baltic Sea whilst at the same time improving overall marine health and satisfying all who depend on it.
Photo Credit: Flickr Maciek
Time for some extra info? Read on:
This research paper is available free to view in Ocean Science. In addition to the paper, you can also view and download the pre-review discussion paper (this is original version of the paper that was submitted to Ocean Science for peer-review), and the reviewer comments and author responses.
This paper develops an interesting solution to issues of marine health with important implications for cod reproduction in the Bornholm Basin and fisheries in the Baltic Sea at large.
Why not judge for yourself by reading both of the original articles? Try reading the peer reviews to see how fellow scientists judged the paper and suggested improvements or alternative approaches. See this months extra info for definitions of the terminology used throughout this paper.
As you read, consider the following:
Does the model accurately capture the natural environmental processes in the Bornholm Basin? One of the reviewers suggests that readers may not be totally convinced by the models fit with the measured oxygen, salinity, and temperature data. However, considering the type of model and how it is used here, they believed it to be accurate enough and therefore suitable.
In particular, the model was fine-tuned to fit the observational data. In this case, the purpose of the model was to see how pumping near-surface water to the deep basin would change oxygen levels (and salinity and temperature) at the same location and over the same period for which they have observational data I.e. 1990 to 2010). So fine tuning the model to a particular set of conditions was justified. However, if the model was to be used to predict future behaviour of the Baltic Sea, then the authors may have chosen to not fine tune the model so much, or perhaps to use a different model entirely. Can you suggest why?
What may the authors need to do to use their model to describe other marine environments? What other cod producing regions could benefit from this type of modelling exercise?
Reviewers also criticised the original paper for having a limited discussion of the potential impacts of their pumping experiment over longer periods of time, and on all marine life throughout the Baltic Sea in general. The authors have made a number of changes to address these concerns in their final paper. Can you think of any other issues not raised here? Finally, what future work could you see leading on from this paper?
Any questions? Post them here or get in touch, and we will try our best to answer them.