The intense discussions in recent years about the situation of herring stocks in the Baltic Sea have their origins in coastal fishermen's alarms about a shortage of herring along the east coast, from the Bothnian Sea in the north to Hanö Bay in the south.

For several hundred years, coastal fishing has generally had good catches in spring and autumn when herring seek out shallower waters to spawn. But that is no longer the case. Catches have fallen drastically, as has the presence of older and larger herring.

In coastal municipalities, it is claimed that large-scale pelagic trawl fishing close to the coast is the main cause. However, due to a lack of environmental monitoring data and scientific evidence, it has been difficult to get the government and fisheries management to act.

Should have better control

– Herring has always been the dominant species for coastal fisheries along the east coast. Since it has been such an important species for Swedish fishing, it is quite strange that the authorities have not had better control of herring stocks in the coastal area, says Henrik Svedäng, fisheries researcher at Stockholm University Baltic Sea Centre.

If there had been widespread and systematic environmental monitoring of herring stocks in the coastal area over the last 20-30 years, today's problems would probably have been detected earlier.

– At best, the alarms about a herring shortage on the coast could have been checked fairly quickly, and then any measures could have been taken rather timely, says Henrik Svedäng.

Study confirms herring shortage

During a few summer weeks in August 2021, the Swedish University of Agricultural Sciences (SLU Aqua) conducted surveys in the southern Stockholm archipelago on behalf of the Stockholm County Administrative Board and Stockholm University. Using hydroacoustics and test trawling in Himmerfjärden Bay and around the islands of Askö and Nämdö, the researchers wanted to get a picture of the presence of herring juveniles, herring and other fish species.

The aim was to find out whether the local coastal fishermen's observations could be substantiated.

The results were recently published in the scientific journal Fisheries Research and showed that there seem to have been major changes in the fish community.

In similar test trawls carried out in Himmerfjärden 20 years ago, during the years 2002-2004, catches were dominated by juvenile herring (55%), followed by sprat (29%) and one-year-old or older herring (14%). At that time, sticklebacks were close to non-existent.

– We also got an indication that there were very few older herring compared to what it looked like almost 20 years ago. So, there is a risk that the herring stock have been severely decimated, says Henrik Svedäng.

The research vessel U/F Asterix full of trawl equipment and sonar devices. Photo: Gustaf Almqvist, Länsstyrelsen Stockholm.

Further studies are needed

For the areas around Askö and Nämdö there was no historical data to compare with. However, the test trawls in 2021 gave a similar picture to the one in Himmerfjärden:

• low incidence of herring juveniles (8% at Askö, <1% at Nämdö)
• large quantities of stickleback (36% at Askö, 9% at Nämdö)
• low prevalence of one-year-old or older herring (9% at Askö, 2% at Nämdö).

The biggest difference was that there was considerably more sprat at Askö and Nämdö (46% and 88% respectively) compared to Himmerfjärden.

Henrik Svedäng emphasizes that it is not possible to confirm for certain that the fish community in Stockholm's southern archipelago has experienced a regime shift at some point during the last 20 years.

– To do that, we would have had to do the same measurements for several years in a row. If there has actually been such a colossal change as our results suggest, it would probably be enough to repeat the surveys for 3-4 years to be able to make a more general statement, says Henrik Svedäng.

He and the other researchers have applied for funding to repeat the studies annually, but so far they have not received any funding.

– It's a disgrace, of course. Herring is the engine of the marine ecosystem and is incredibly important not only for the coastal waters and the fisheries, but for the entire Baltic Sea. Science and the authorities need to gain a better understanding of the situation, he says.

Stickleback abundance suggests a permanent shift

The alarms from the east coast are admittedly given additional weight by the single study from 2021. But it is not enough to establish that there is a shortage of spawning herring in the archipelago. The amount of juvenile herring can vary greatly from year to year.

But in a previous interview conducted in connection with the compilation of the results, another of the researchers, Thomas Axenrot at SLU, stated that the large amount of sticklebacks still indicates that the change was not only temporary.

– The spawning can fail from year to year, but it is not as likely that the stickleback stock could simultaneously increase so sharply from one year to the next. Such a large increase in stock size takes longer to build up, he said.

This stickleback was one of many that were caught in August, still carrying roe which suggests an extended spawning season. Photo: Gustaf Almqvist, Stockholm County Administrative Board.

Large-scale trawling along the coast

Herring, sprat and sticklebacks compete for the same food, zooplankton. In addition, stickleback like to eat herring juveniles. If the results from 2021 reflects an actual regime shift, the question is: what caused it?

Are less and less quantities of spawning herring coming in from the open sea, which gives the stickleback more and more space?

Or have the stickleback stocks increased for some other unclear reason, and has now become so numerous that they eat most of the herring juveniles?

At the moment, no one knows for sure. But there are some pieces of the puzzle point to the first option. One is that the large-scale pelagic trawl fishing for herring and sprat has increased sharply along the east coast in the last 5-10 years, according to analyses carried out by both the Baltic Sea Center and SLU. On the one hand, the actual catches have increased in terms of tonnes. On the one hand, the share of the trawl fishery's total annual catches taken on the coast, close to the trawl limit, has increased during the same period.

Herring eat sticklebacks

Another important piece of the puzzle is a review of knowledge on the distribution of sticklebacks , which was published in the ICES Journal of Marine Science in May 2022.

The so-called "stickleback wave" along the east coast began out in the open sea about 20 years ago, and has since spread inland. The researchers behind the compilation believe that the most important driving force is likely reduced predatory fish stocks. In many places along the coast, the amount of predatory fish decreased before the stickleback stocks began to grow. In addition, herring is an important predator of stickleback. The more herring there are in an area, the fewer sticklebacks.

Too many uncertainties

Despite this, there are still far too many question marks about what has happened and is happening to the Baltic Sea's most important fish species, says Henrik Svedäng.

– Our survey in 2021 was a test of how the composition of the fish community looked in large parts of the southern Stockholm Archipelago in that particular year. In the years 2002-2004 it looked one way, and in 2021 it looked completely different. There is no doubt about that. But what does that mean? Is herring recruitment always this low these days? We can't say anything for sure about that at the moment, says Henrik Svedäng.

According to him, this is precisely what fisheries research must find out.

– But that must not lead to inertia concerning the present fishing pressure. The herring stocks in the Baltic Sea are now so reduced, even in the open sea, that fishing must be severely limited or preferably paused completely until the stocks have recovered, he says.

Text: Henrik Hamrén