Baltic cod. Photo: Robert Kautsky /Azote.

Cod in the Baltic Sea are not growing to the size they did in the past. At the same time, their size composition in recent years has become increasingly homogeneous.

– This is a completely unique situation. There is no historic data that shows a size structure as truncated as the one we see in the Baltic Sea today. This is a serious situation for the cod population, for the fisheries and for the ecosystem, says Henrik Svedäng, fisheries researcher at Stockholm Universitys’ Baltic Sea Centre.

The Baltic cod can have a life span of 25 years and grow up to one and a half metres in length. However, it is a long time since such magnificent specimens were found in the trawl nets of commercial fishing vessels. Today, most of the cod in the Baltic Sea are around 30 centimetres in length. Cod measuring 40–50 centimetres in length are rare specimens.

In their latest study, Henrik Svedäng and Sara Hornborg from the research institute RISE show that the trend towards an increasingly compressed size structure has been gradually on-going since the start of this century, and that the driving factors include extensive fishing and low growth rates.

– Since the low growth rates of recent years seem to be connected to a high density of truncated cod, this is probably what is known as density-dependent growth, says Henrik Svedäng.

Cod eat different kinds of food depending on how large they are. When a fish population consists almost exclusively of small cod of roughly the same size, the competition for food increases, which inhibits the individual’s growth.

– In practice this means that the number of cod in the population regulates individual growth, says Henrik Svedäng.

Affects both ecosystem and fisheries

While growth rates are in decline, and the proportion of small cod is increasing, trawling is being geared towards catching increasingly large cod.

–As of the mid 1990s there has been a deliberate increase in fish selectivity by the use of increasingly large mesh sizes and mesh that also opens during trawling. This means that an increasing number of small cod escape even if they swim inside the trawl net,” says Henrik Svedäng.

Each of the two trends – low growth rates and increased fish selectivity – probably reinforce the negative effects of the other, which has consequences for the fish stocks but also for the commercial fisheries. The smaller the size of cod in the sea, the greater the fishing effort that is needed to catch the same amount as before.

– We also have an ecosystem where large predatory fish have disappeared, and there is a risk that their vital structuring function in the ecosystem will be lost,” says Henrik Svedäng.

The cod adapts to being smaller

For the cod in the Baltic, the key to survival has always been about adaptation. Over the centuries the cod has adapted to low salinity and less favourable oxygen conditions. According to Henrik Svedäng, todays’ prevailing dominance of smaller cod could be a sign that cod are undergoing a genetic change, and that they are, quite simply, adapting to being smaller in size.

– During the 1990s the cod in the Eastern Baltic cod stock reached sexual maturity when they were just over 40 centimetres in length. Today the average length of a cod that has reached sexual maturity is around 20 centimetres. If this reflects a genetic change the current situation runs the risk of becoming a permanent one – and that risk needs to be highlighted, says Henrik Svedäng.

He recalls that the same phenomenon, of cod suddenly beginning to spawn when of a much smaller size than before, was also observed on the Grand Banks off Newfoundland shortly before that stock collapsed.

– Growth rates were low there too, and the fishing pressure was high, says Henrik Svedäng.

Öresund – a positive exception

Despite the generally poor growth of the Baltic cod, and despite the skewed size composition, there are still positive exceptions. In Öresund (The Sound), where a trawling ban has been in place since the 1930s, the stock has a normal size distribution, and above all there are more large individuals (around 50 centimetres).

– I believe that the trawling ban is a crucial reason for there being larger sized cod there. But it could also be that Öresund generally is a more productive ecosystem than the Baltic Sea as a whole, says Henrik Svedäng.

Bottom trawling is by far the most dominant method of fishing for cod in the Baltic. More than 90 per cent of all cod landed is caught in bottom trawling. Developments in trawling methods and trawl fishery management is therefore extremely important to the whole population dynamic of the cod.

– In order to bring back larger cod into the Baltic and prevent further instabilities in the ecosystem, we ought to review both the way we fish and how much we fish, says Henrik Svedäng.

Text: Henrik Hamrén