Photo: Sirpa Ukura/Mostphotos

2025-05-15

Text: Henrik Hamrén

The negative development of commercial fish stocks in the Baltic Sea is not only affecting the marine ecosystem - but also society's relationship with seals and cormorants. This is particularly evident in the ongoing debate on herring stocks and fisheries, where the threats posed by seals and cormorants are becoming increasingly prominent. 

For example, when small-scale coastal fishermen raise the alarm about declining herring catches and accuse the large-scale pelagic trawl fishery of taking too large a catch, representatives of the large-scale fishery respond that the shortage of herring on the coast is likely to have completely different causes, such as climate change, warmer waters - and lots of seals and cormorants that eat a lot of herring.

Ministers: seals and cormorants ‘cause great harm’

The expanded threat picture seems to have taken hold in politics as well. When the government wants to explain its behaviour in EU negotiations on herring quotas or how to protect herring stocks in various debate articles and interviews, two measures in particular are mentioned: moving the trawl limit and increased hunting of seals and cormorants.

Recently, the government commissioned new studies on increased hunting of seals and cormorants to the Swedish Environmental Protection Agency (SwAM), SLU and Formas, mong others. The stated purpose was to ‘strengthen the development and recovery of fish stocks’.

In the press release, minister Kullgren noted that predation by seals and cormorants has ‘significant consequences for both ecosystems and fish stocks.’

And Climate and Environment Minister Romina Pourmokhtari (L) stated that the increased populations of seals and cormorants ‘create great harm to our marine environment and especially to our fish stocks’ - and that increased hunting of seals and cormorants will reverse the trend and ‘give fish stocks a chance to recover.’

The fishery takes up more fish

Seal and cormorant predation no longer seems to be seen as a problem related to depleted cod stocks and local coastal stocks of pike and perch, but is also associated with the more general problems of Baltic herring stocks.

However, according to Karl Lundström, a researcher at SLU, it is incorrect to claim that seals and cormorants have a significant negative impact on the large herring stocks in the Baltic Sea.

”No, you can't say that. It is not scientifically proven," he says. 

Earlier this year, he published a major review of the thate of scientific knowledge on the impact of seals on fish stocks. Some scientific publications show that seal predation can have negative effrects on different fish populations, others show that it has no negative impact or, in som ceses, even a positive impact.

"The general state of knowledge is very limited,” says Karl Lundström. 

But it is clear, he says, that seal and cormorant predation accounts for a much smaller proportion of the total catch of herring compared to fishing. 

”However, it may be the case that seal populations, in particular, are affected by the large harvest of herring in the Baltic Sea. Herring is the staple food of seals in the Baltic Sea and there are now signs that the seals' body condition is deteriorating and that they are starting to leave certain areas to seek out new foraging areas,” says Karl Lundström.

The population of grey seals in the Baltic Sea is increasing. Photo: Edwin Butters/Mostphotos

“Shaky results” from diet data

Research shows that in some areas, seals and cormorants can have a significant negative impact on local coastal stocks of, for example, perch and pike, which in turn can lead to an increase in the abundance of sticklebacks.

”Seals and cormorants are opportunists, eating whatever species are available wherever they happen to be. This makes it very difficult to draw generalised conclusions about the dietary habits of seals," he says. 

The dietary data available on seal predation is very limited in time and space. For example, if you find a lot of cod in the stomachs of hunted grey seals in a specific sea area at a certain time of year, it does not mean that all grey seals in all sea areas eat the same amount of cod all year round.

”Using data from a specific area and a specific time, and extrapolating up to a much larger area or a completely different area - or to a longer or completely different period of time - gives shaky results,” says Karl Lundström.

Cormorants eat a lot of stickleback

As for the link to the increasing amounts of sticklebacks along the east coast, he mentions a telling example from Kalmarsund, which harbours the most nesting cormorant pairs in the whole country.

Karl Lundström. Photo: SLU

"We examined shot cormorants there. They were in good condition - and they had almost exclusively stickleback in their bellies,” says Karl Lundström, and continues: 

"Can you then make the correlation that the amont of sticklebacks has increased because the cormorant has increased in the area? It could just as well be that the cormorant has increased in the area because there is a lot of stickleback there. I don't know which is true, but it's not as straight forward as you might think.” 

According to Karl Lundström, there is a general need for a better understanding of the factors that affect different fish stocks in different marine areas, and the dynamics between fish, seals and cormorants. 

”It is good to learn more about seals and cormorants, but we also need to focus on other important factors affecting fish stocks,” he says.

Different views on hunting

From near extinction in previous decades, the numbers of grey seals, harbour seals and ringed seals in the Baltic Sea (including Skagerrak and Kattegat) have recovered, particularly in the 2000s. There has also been an overall increase in the number of nesting cormorants.

Licenced hunting of grey seals started in 2020. How many seals that can be shot is decided by the Swedish Environmental Protection Agency (SwAM) and the quotas are governed by population trends and where and to what extent the seals are considered likely to cause damage to fisheries and sensitive fish stocks. 

In February, the Swedish Environmental Protection Agency decided to increase this year's hunting quota for grey seals from last year's 750 individuals to 1,000 individuals (licensed hunting) and at the same time increase the quotas for both harbour seals and ringed seals from zero to 200 individuals each (protected hunting). The new quotas were criticised from various quarters.

”Disappointing”, said Hans Olsson of the Swedish Hunters' Association, calling for higher and more long-term stable hunting quotas for seals.

”It's wrong to blame the seal for the collapse of the Baltic Sea,” said Ida Carlén of the Swedish Society for Nature Conservation, emphasising that it is reduced fishing and measures against eutrophication and environmental toxins - not increased seal hunting - that can help fish stocks recover.

Harbour seals are declining

Karin Hårding, professor and seal expert at the University of Gothenburg, is one of those who gave her opinion in the consultation round on this year's hunting quotas for the Baltic Sea's three seal species (grey seal, harbour seal and ringed seal). 

Karin Hårding. Photo: Johan Wingborg, GU.

She believes that there is some scope for hunting grey seals, as the population appears to be increasing by about five percent per year. 

”But the other two seal species I think we should be very careful with,” she says. 

The harbour seal population, which dominates on the west coast, is admittedly at a historically high level. But it has now started to decline. 

"We have counted the harbour seal pups in the northern Skagerrak and seen that they only get half as many pups as normal. We think they are struggling to make up their energy budget. If a female doesn't have a sufficient energy budget, they start having pups every other year instead of every year," says Karin Hårding. 

The population of ringed seals in the Gulf of Bothnia is around 25 000 individuals (compared to around 180 000 individuals at the beginning of the 20th century). As ringed seals only breed on ice, climate change and deteriorating ice conditions have a negative impact on the population. 

”Inventories of ringed seals have become more uncertain, but at the moment the population seems to be stable, neither increasing nor decreasing,” says Karin Hårding.

Difficult to draw conclusions

When it comes to the discussion on the impact of seals on Baltic fish populations, Karin Hårding emphasises that the dynamics and complexity of marine ecosystems make it difficult to draw any conclusions about the effects of seal predation.

For one thing, the three seal species have different ecologies, eat different amounts at different times of the year, and have different prey preferences. For example, harbour seals eat around 35 different species, including large quantities of sandeels and small blue whiting. 

On the other hand, seals can also have a strong regulatory effect in the ecosystem, meaning that seal predation can affect different fish populations in different ways, both positively and negatively, depending on how different fish species interact within the food web.

”Take the ringed seal up north, for example. It has small teeth and eats small prey, such as small herring and the crustacean Saduria entomon. And it can eat large amounts of stickleback,” says Karin Hårding.