Everyone agrees that the Baltic Sea's iconic cod stock, the eastern stock, is in severe crisis. But on the causes of the situation, there is still disagreement. Over the past ten years, several studies have pointed to the lack of oxygen and the spread of dead bottoms as the major culprit in the drama, leading to lower growth and survival. It is believed, for instance, that the lack of oxygen at the seabed has knocked out the small bottom-dwelling small animals, which are an essential food source for cod.
These claims are contradicted by a new study, written by researchers at Stockholm University's Baltic Sea Centre and the University of Helsinki, Tvärminne Zoological Station.
– Our results show that it is not possible to prove a link between the eastern cod stock's reduced productivity and oxygen deficiency per se. It is probably sufficiently with benthic feed in the form of benthic animals above the halocline in the Bornholm Basin, says Henrik Svedäng, researcher at the Baltic Sea Centre and the study's lead author.
Reduced productivity of the eastern stock
Productivity is a measure of the relative increase in biomass of fish stocks. Since the beginning of the 2000s, the productivity of the eastern cod stock has fallen sharply. The cod grow extremely poorly, are small and bony, and seem to die off early.
The new study does not determine what has caused the poor growth. However, it notes that it cannot be due to lack of oxygen.
– In and around the Bornholm Basin, which is the spawning area of eastern cod, we see that oxygen conditions are good and have remained relatively constant for the past 60 years, says Henrik Svedäng.
In large parts of the deep bottoms of the Central Baltic Sea there is a lack of oxygen, but still, 50 percent of the seafloors are oxygenated. At the Bornholm Basin, which is the spawning area for the eastern cod stock, the oxygen and salinity of the halocline in the deepest part is still favourable for reproduction. Above the deeplayer, there are large areas with enough oxygen for bottom-dwelling animals to thrive. The researchers thus conclude that oxygen deficiency may not have caused the decline in the stock over the past three decades. Illustration: Elsa Wikander/Azote
Oxygen conditions differ
At the same time, there are frequent reports that the oxygen-free bottoms are spreading in the southern Baltic Sea. How can the oxygen conditions for cod be good at the same time?
The answer is that the oxygen content of the water can vary depending on the depth. The bottom of the Baltic Sea is not a flat surface but is hilly, with peaks and valleys.
At a depth of about 50-60 meters, the water mass is divided by a leap layer – the so-called halocline – which prevents the sweeter and oxygen-rich surface water from mixing with the saltier and more oxygen-poor bottom water.
The deepest point at Bornholm is at a depth of just over 100 metres. Right there, about half of the water mass is under the halocline. But large bottom areas at and around the Bornholm deep are above the halocline, where there is enough oxygen for both the reproduction of cod and the benthic animals.
An illustration from the new study shows how the water masses with sufficient salinity and oxygen content to be suitable for cod spawning have varied over time and space in the Baltic Sea. The spawning area southeast of Gotland has permanently disappeared, while in the Bornholm Basin it has existed throughout the period studied.
No drastic change in bottom fauna
The researchers have also compiled data on how the benthic fauna has developed in the entire Eastern Gotland Basin and in the Bornholm Basin.
– We compared two time periods, 1990-2004 and 2005-2018, and looked both at the total biomass of benthicanimals and specifically at polychaetes and crustaceans, such as amphipods (Monoporeia affinis and Prontoporeia femorata) and the glacial relict isopod, Saduria entomen, which are the benthic animals that small cod like the most, says Anna Villnäs, researcher at the Finnish Tvärminne Zoological Station.
The results showed that there had been no drastic change in the bottom fauna of the bottoms above the halocline since 2005, neither in terms of biomass nor composition.
– It looks very similar to what it did in the 1990s in areas above the halocline, both in the southern parts of the Eastern Gotland Basin and at the Bornholm Basin, says Anna Villnäs. In the northeastern Gotland Basin, however, the incidence of amphipods has decreased since the 1990s, while the invasive polychaete (Maenzelleria spp.) has increased in abundance.
Deteriorated food quality?
The dramatic decline in growth, chances of survival and productivity for the eastern cod stock in recent decades does not seem to coincide with any noticeable change or reduction in benthic fauna in the areas where the cod is located.
In the main area of the stock (SD25), which extends from Bornholm up to southern Öland, half of all bottoms are above the halocline - and have both oxygen and living benthic communities.
– Quantitatively, there is food for the cod, but we don’t know much about the quality of the food. Something may have happened to the nutritional value of the benthic animals. It remains to be investigated, says Anna Villnäs.
Two of three spawning areas are gone
Although the Bornholm deep still seems to have enough oxygen, oxygen deficiency has generally had a significant negative impact on cod in the Baltic Sea as a whole, says Henrik Svedäng.
– It has decreased the natural distribution area of cod over the past 50 years. Two of the three spawning areas are gone: the Gotland Deep and the Gdansk Deep. Now, only the Bornholm Basin remains, where the salt and oxygen levels are sufficient for the cod's eggs to be able to develop, he says.
With only one spawning area left, it is extra important to find out why the eastern cod stock is also collapsing and how it can be avoided, says Henrik Svedäng.
– The deteriorating, individual condition of the Baltic cod, which led to a fishing closure in July 2019, also shows severe imbalances in the Baltic Sea environment. The problems with the cod’s health could be related to the quality and composition of the food. Finnish researchers, for example, have said that salmon find too little herring relative to sprat in the southern Baltic Sea, but it is difficult to show clear connections. It is therefore important to try different explanatory models.