”Nobody knows what has caused the current situation”


New international project to solve the cod mystery

The eastern Baltic cod stock is in crisis, the fishery is on its knees – and nobody knows why. An expert evaluation, initiated by the Stockholm University Baltic Sea Center, will try to find answers to what has happened, and what to do to reverse the trend.

Text: Henrik Hamrén

– One important first step is to make sure that we have all hypotheses and possible causes on the table, says Keith Brander, professor at Danish DTU Aqua and scientific leader for the two-year project.

With the help of scientists at the Baltic Sea Centre, and two external international experts, he will conduct a scientific evaluation and synthesis of all relevant knowledge and try to determine what factors actually caused the cod's decline - and what factors may be less important.

– There is a great deal of uncertainty as to why the eastern stock is in such bad shape. Eutrophication and fishing are often mentioned as possible explanations. But, in fact, nobody knows what has caused the current situation, and what to do about it. It is not obvious at all, Keith Brander says.

His hope is to contribute to improved scientific understanding by evaluating everything from oceanographic information and habitats to benthos distributions and the impact of fishery.

– Our goal is to help to deliver a stronger scientific basis for future management measures that can improve the cod stock.

The eastern cod stock is the largest and commercially most important cod stock in the Baltic Sea. In recent years it has shown increasingly worrying signs, for instance in terms of declining size at maturation. In the 1980s, cod spawned for the first time when they were around 45 cm in length. Today, they reach maturity when they are just over 20 cm. If this reflects a genetic change, the current situation – with a lot of small cod and few or no larger individuals – runs the risk of becoming permanent.

According to Keith Brander it is highly unlikely that the poor state of the stock can be linked to a single and isolated factor.

– As in all biological systems, most things are connected. For the Baltic Sea, I think there are several different factors in play here. The question is which these factors are and how they interact, he says.

Besides analysing historical samples and information about the cod and its physical, chemical and biological environment, Keith Brander and his team will also gather experts from the Baltic Sea Region and other places to evaluate and apply various models to the Baltic Sea specific case.

The uniqueness of the Baltic Sea environment becomes a special challenge in this work.

– Some of the biological processes that are so vital for cod stock management in the Baltic are not considered at all in other waters, says Keith Brander.

He mentions the low salinity, which is a decisive factor for cod survival in the brackish Baltic Sea.

– But in almost all other marine environments in the world, salinity is not an issue and therefore is not included in the stock models. Models from other areas are therefore incomplete with respect to the Baltic, says Brander.

Keith Brander has been involved in marine research since the mid-1960s and is a well-known name in the international fisheries research community. He has studied fish, plankton and underwater fauna in many parts of the world, from the Indian Ocean to the Irish Sea. But the Baltic Sea is so far a new environment for him.

– At the moment, I'm reading a lot trying to catch up, which takes quite a bit of my time. The Baltic Sea is a very well-studied marine environment, he says.

Paradoxically, lack of knowledge is one of the biggest problems for today's cod stock management in the Baltic Sea. For example, it has become increasingly difficult to tell the age of cod by otolith reading. And if you don’t know the age of the fish it is very hard to say anything about how the stock develops.

– The problem with age determination is fundamental. I think there are possibilities to tackle this problem better than has been done in the past, for example by studying older historical information. We will have access to a lot of such material, says Keith Brander.

To shed light on why the eastern Baltic cod stock is declining – and deliver a better basis for future management actions – is a difficult challenge. Keith Brander has high hopes for the outcome, but is also humble in the face of the task.

– I have no doubt that what we end up with may be better, but it will not be definitive. There will still be uncertainties and the management decisions will need to balance the risks and uncertainties, he says, and continues:

– I accepted to lead this project out of curiosity. That is what drives me, he says. So I’m optimistic, but also mindful of a motto I heard from a math professor many years ago: They said it couldn’t be done, so he tackled it with a smile on his face – and couldn’t do it.