Baltic Breakfast: No clear cause behind the collapse of the eastern Baltic cod

The fishery has been closed for three years, but the eastern Baltic cod stock has not recovered. A new study, presented at a recent Baltic Breakfast, shows that the most recent sharp decline in the stock can’t be explained by oxygen deficiency, and the causes of the poor condition of the cod remain unclear.

Researcher Henrik Svedäng and moderator Charles Berkow, Stockholm University Baltic Sea Centre.

The eastern Baltic cod has a dramatic history. It has been one of the major cod stocks in Europe, and in the 1980s it presumably reached an all-time high; photograph from this time show fishermen posing in piles of giant cods. Since then, the catches have declined, and three years ago, the eastern cod stock was in such a bad shape that the Commission decided on an emergency closure of the fishery.

- At the moment the stock is depleted, but also, the individual cod are small. So, there are two major causes of changes in cod productivity: decline in reproduction in the 1980s, and declines in individual growth and survival over the last three decades, explains Henrik Svedäng, researcher at Stockholm University Baltic Sea Centre.

Reproductions requires certain conditions

Biologically, the Baltic cod is unique – it may be regarded as a subspecies – as the reproduction has been adapted to the brackish environment. Still, the fertilising the cod’s eggs requires a salinity of 11 PSU and a sufficient oxygen level (more than 2 ml O2 per litre water). These circumstances are not found everywhere in the Baltic Sea, and since the mid 1980s, two of four spawning locations have been lost; the Gotland Deep (due to oxygen depletion) and the Gdansk Deep (due to a drop in salinity).

– This later loss is mostly related to a hydrographic shift in the Baltic Sea in the 1980s-early 1990s, says Henrik Svedäng.

Although the salinity in the Gdansk deep now is regarded sufficient, cod have not yet returned there to spawn.

– But one shouldn’t say that the cod’s case is lost in the Baltic Sea, because it can still reproduce quite well in the Bornholm and Arkona Basins, says Henrik Svedäng.

Most of today’s Baltic cod is also found in the southern part of the Baltic Sea, especially around Bornholm.

Not directly fishery related crises

Around 2010, the eastern Baltic cod recruitment was very good, and the prospects were bright.

– It was said that the cod was saved. However, a few years later it was clear that the biomass didn’t materialize as it should and the fisheries declined, says Henrik Svedäng.

The individual cod was smaller than before and matured earlier and the fishermen weren’t able to use up their fishing quotas. The decline in productivity, expressed as landing per recruit accelerated.

– The problem is not directly fishery-related, says Henrik Svedäng.

So, what happened? It has been suggested that this decline in productivity is related to oxygen depletion. In contrast, Henrik Svedäng argues that this is not the case, as the cod’s condition doesn’t seem to correlate with the extent of hypoxic areas in the Baltic Sea.

Researcher Anna Villnäs, Tvärminne Zoological Station, University of Helsinki.

Furthermore, oxygen depletion is primarily found below the halocline. Data on the temporal developments of oxygen content by depth in the Arkona Basin, the Bornholm Basin and the Gotland Deep have been relatively constant.

– Even if it is possible to find a trend, the seasonal variability is so high that any cod at any time would have encountered low oxygen content at some time, says Henrik Svedäng.

Benthic fauna still available for the cod to eat

Researcher Anna Villnäs, Tvärminne Zoological Station, has gone more deeply into one aspect related to oxygen depletion; the presence of benthic fauna – animals living on and in the seafloor. The diet of adult cod is mainly herring and sprat, but the younger ones (up to 35 centimetres long) also feed a lot on these animals.

– Specifically, the cod seem to prefer amphipods, mysids, polychaetes and the isopod Saduria entomon, says Anna Villnäs.

Lack of oxygen, hypoxia, affect all aspects of the benthic communities; the composition of the species, the abundance and the biomass.

– Some species can tolerate mild hypoxia during short terms, but most species die when we have severe hypoxia (oxygen deficiency) or anoxia (oxygen depletion).

Anna Villnäs and her colleagues have studied benthic monitoring data from the period 1990-2004 (when the cod was in good condition), and compared it with the period 2005-2018 (when the condition has been bad). In total, the study includes 1 900 sampling occasions at 364 monitoring stations.

– The benthic community biomass and composition is severely reduced in deeper areas, below the halocline. But there are large differences between the stations below and the ones above the halocline, Anna Villnäs summarises.

At shallow depths – below 40 metres – the study showed the benthic biomass to be high and long-lived species such as Macoma balthica (Baltic tellin) were still present. In the Gotland deep, the researchers could also see a change in the benthic community as the crustacean Monoporeia affinis has decreased and polychaetes such as the invasive Marenzelleria has increased since 2005. However, in the Bornholm basin, where the majority of the cod reside, such a change was not statistically clear.

– And the favourite food for cod – Saduria entomon – was still present in the shallow areas, adds Anna Villnäs.

Presentation of Anna Villnäs.

What then is the problem?

So, if there are still spawning areas, and still food available for the cod, why is the situation so bad for the eastern Baltic cod stock? The researchers at Baltic Breakfast could not present a clear answer, but Henrik Svedäng suggests that one explanation that could be further investigated is if the quality of the cod’s food has changed.

– Nearly no cod in the southern part of the Baltic Sea are in a not good shape; they are growing slowly and dying early. There must be an agent, or several agents, affecting on such large scale, he says.

Text: Lisa Bergqvist

Answers to the questions from the audience

Henrik, you showed that in a series of years the Cod landings not even touched the TACs – so from a management point of view: Management exposed the stock to a somewhat open access situation, because it did not use the TACs to restrict catches or limit fishing activities – for more than 10 years! Is that also what you concluded?

Henrik: Yes, I did and long before this study. TACs have not been limiting the fishery for a long time. It is similar to what was the case with the Kattegat cod in the 2000s.

How much do fishing practices matter...as in how we fish and with what gears?

Henrik: Quite a lot, the situation would have been better if we had fished with gillnets instead of bottom trawls, as trawls catch fish over a wider length range compared to many other fishing gears. It would have left a bulk of large cod, which could have been favourable for the subcomponent that once spawned in the Gdansk Deep.

Can we do anything to reduce natural mortality?

Henrik: Possibly, if we know the reason for the increased natural mortality (besides the fact that the cod are suffering from bad health. For instance, if it is related to food quality - some food stuffs are better than others - perhaps management actions could be taken.

To what extent do cods feed in shallow areas (e.g. 20 metres vs 40 metres vs 60 metres?

Henrik: Data are scarce, usually younger fish feed to a higher degree in shallower areas, but not always. It is very much dependent on season.

How has the predation on benthic fauna changed? Cod decreased, but sprat and stickleback increased?

Henrik: There are speculations that flounder might compete with cod, but I don’t think sprat and stickleback compete.

Processing industry tells that cod smaller than 15 cm are in good shape. The problems seem to start when cod is bigger than 20 cm. Do cod find herring today?

Henrik: That is a good piece of information, I agree the problems with health and so on are more acute in fish bigger than 20 cm. On the other hand, the median size at maturity is at 20 cm, so something has happened even before 20 cm which is related to growth/ physiological status.

Anna, did you also look at the biogeochemistry and CO2-sink and resuspension through e.g. trawling activities?

Anna: No, this monitoring data gave us information on benthic fauna biomass composition, and we did not have the opportunity to consider other questions.

Did the high densities of small cod (< 35cm) that Henrik mentioned maybe cause the lower benthic fauna biomass that Anna reported?

Henrik: These densities didn’t last for so many years, the recruitment has become lower in recent years.

Can we expect bottom trawling and the consequent destruction of benthic habitat to be the major reason for Cod population and size decline?

Henrik: No, that seems unlikely. Fishing activity has decreased over the last decade along with continuous low growth/survival of cod.

Regarding low oxygen and dead zones since the 1970s - what are the impacts of animal agriculture (e.g. feces) on the health of the Baltic

Henrik: Agriculture has contributed significantly to eutrophication, besides industry and municipalities. Eutrophication is the ultimate reason for oxygen depletion in the Baltic Sea. While decreasing, less nutrient leakage from various forms of agriculture will be needed to restore the Baltic environment.