Herring stocks are increasing – Baltic Sea Centre researchers urge caution

Following last year’s gloomy forecasts, the latest stock assessments made by the International Council for the Exploration of the Sea (ICES) point to improvements for several of the Baltic Sea’s pelagic stocks. According to the latest model projections for next year’s fishing, recently presented by ICES, there is scope to increase TACs (total allowable catch) next year.

If EU fisheries ministers choose to ignore various uncertainties and shortcomings in ICES’s estimates when they decide on TACs later this autumn, this would lead to quota increases of up to 122% (274,332 tonnes) and 62% (90,391 tonnes) for the herring in the central Baltic and Gulf of Bothnia respectively, compared with this year’s quotas. For sprat, the upper end of the ICES TAC-range in the advice corresponds to an increase of up to 32%, to 324,277 tonnes.

"For several years now, there have been very clear signs that the Baltic Sea is going through a difficult period, with low and unproductive stocks. The individual size of the fish has also decreased. To open the door for such high quotas for herring and sprat – the species that form the core of the ecosystem – in this situation is indicative of an incredibly risk-prone management approach," says Henrik Svedäng, fisheries researcher at the Baltic Sea Centre.

Sustainable fishing pays off

The increased F-ranges in ICES headline advice for sprat and the herring stocks in central Baltic and the Gulf of Bothnia are primarily based on ICES assessments, which indicate that the spawning stock biomass (the quantity of fish ready to spawn) is increasing.

"It is encouraging to see that stocks appear to be increasing. This goes hand in hand with the lower fishing quotas set in recent years, which supports the view that restraint in fishing is good for stock recovery," says Sara Söderström, also a fisheries researcher at the Baltic Sea Centre, and continues:

"Given all the uncertainties involved in stock assessment, a prudent management should take this into account and not immediately raise quotas significantly. Instead, it should proceed with caution and ensure that the positive trend continues."

Herring stocks not meeting the MSY objective

In addition, the spawning stock biomass of both herring stocks remains below MSYBtrigger, i.e. the biomass reference value associated with biological sustainability. This means that neither of the two herring stocks currently fulfils the management plan’s objective of maintaining a spawning stock biomass sufficient to deliver maximum sustainable yield (MSY) in accordance with the MSY approach.

According to ICES' calculations, a 122 % increase in the quota for herring in the central Baltic Sea next year would mean there is a 40 % probability that the stock will not exceed the MSYBtrigger.

'Additional TAC' in Gulf of Bothnia herring

The situation regarding the herring stock in the Gulf of Bothnia is more complex.

This year’s TAC, which was decided by EU ministers last autumn, is 39,108 tonnes. However, following pressure from Finland, it may be increased to 55,869 tonnes during November–December this year, based on new data recently submitted to ICES by Finnish researchers. A decision on the ‘additional TAC’ may be taken later this year.

The ICES headline advice for next year is a TAC range of between 80,767 and 90,391 tonnes. At these levels, the probability of the stock falling below MSYBtrigger is 62–65 %. To achieve at least a 50 % chance that the spawning stock biomass will reach MSYBtrigger, the TAC must be set significantly lower: to a maximum of 45,544 tonnes.

"Given the unspoken but overarching aim of preventing stocks from collapsing, and in line with the EU’s objective of restoring stocks to productive levels, fisheries ministers should react swiftly when we see signs that stocks are declining – and gradually when we see that stocks might be recovering,” says Charles Berkow, policy analyst at the Baltic Sea Centre.

Rapid stock recovery – a matter of interpretation

Whilst the Baltic Sea Management plan refers to the objective of the Marine Strategy Framework Directive regarding ‘good environmental status’ for fish stocks – that is, spawning stock biomass above MSYBtrigger for commercially fished stocks – the plan does not specify an exact threshold for how high the probability of falling below MSYBtrigger may be. ICES normally applies a 50 % probability in its assessments.

However, the management plan sets out certain requirements when the spawning stock biomass falls below MSYBtrigger:

• the TAC must not be set at the upper end of the ICES quota range
• special measures must be taken (e.g. fishing bans or temporary closed areas) to ensure that the spawning biomass rapidly rises above levels capable of yielding MSY (i.e. above Btrigger).

However, what is meant by ‘rapidly’ remains to be determined. The herring stock in the central Baltic Sea has been below MSYBtrigger since the 1980s, and the Gulf of Bothnia herring has been below MSYBtrigger for at least ten years.

'Optimal’ fishing pressure may have been miscalculated

The relatively high level of risk associated with the advice for Gulf of Bothnia herring coincides with significant scientific uncertainties. The lack of clarity surrounding the possible ‘additional TAC’ means that no one yet knows how much herring will be caught this year. Furthermore, researchers have struggled to estimate the number of juvenile fish in the stock.

"This has generated a high degree of uncertainty, which is carried over into the stock assessment and forecasts,” says Sara Söderström.

The Gulf of Bothnia herring stock has declined by more than half over the last 30 years, falling from around 1.2 million tonnes to the current 500,000–600,000 tonnes. Over the last ten years, fishing mortality has fallen sharply and has for almost as long been well below the level considered to be the optimal sustainable fishing pressure (Fmsy). Despite the sharp reduction in fishing pressure, the spawning stock biomass continued to decline for several years.

Last year, the biomass began to increase slightly, but remains below MSYBtrigger. In order to bring the stock to MSYBtrigger, fishing pressure next year must be ‘at least 54 per cent below Fmsy’, according to ICES.

The trends for Gulf of Bothnia herring suggest that Fmsy for the stock may be overestimated, says Henrik Svedäng

"We can see that the age structure of the stock has changed dramatically over the last few decades; there are fewer and fewer older fish remaining. This suggests that the stock should be fished at a significantly lower level. Furthermore, individual growth rates have been lower since the early 1990s, which points to imbalances in the ecosystem," he says.

No measures to protect substocks

According to Henrik Svedäng, adding an ‘additional TAC’ at the end of this year, and then significantly increasing the TAC the following year, increases the risk of there being even fewer older fish in the stock.

"What’s more, we now know that the stock actually consists of a number of genetically distinct substocks or spawning components. It is likely that some of these are fished more than others, and some are naturally less productive than others," he says.

ICES addresses this issue in the latest advice and notes that it is "very likely that the Gulf of Bothnia herring stock is vulnerable to loss in genetic diversity. However, ICES is not in a position to advise on measures to protect substocks."

"But simply being aware of the problem should lead to greater caution and advice that sets TAC well below Fmsy, particularly if Fmsy is set too high," says Henrik Svedäng.

The ecosystem need plenty of forage fish

With regard to the Baltic sprat stock, ICES estimates that the spawning stock biomass has almost doubled (+80%) between 2025 and 2026. Prior to this, there was a corresponding decline during the period 2022 to 2024. The two most recent yearclasses are assessed to be the strongest in 10 years, whilst the three preceding year classes were the weakest in two decades.

"The problem remains that estimates of various upswings are uncertain; one cannot know exactly how large they are or how they will develop over time. This means that it is risky to immediately exploit these increases to the full, while the need to ensure a good supply of forage fish [herring and sprat] in the system has not been met, says Henrik Svedäng.

According to him, it comes down to how one chooses to interpret and respond to the changes observed in the sea.

"When fish stocks rise, the management authorities talk about ‘fishing opportunities’; when stocks fall, they often point to predators and environmental conditions. But the increases we are now seeing in herring and sprat stocks can, for example, also be seen as a sign that the grey seal does not have a population-regulating effect on the Baltic Sea as a whole, as has been suggested in the public debate recently," says Henrik Svedäng.

Decide which mistakes to avoid

Fisheries science is not an exact science. The uncertainties inherent in everything from data collection to analysis and modelling carry over into forecasts and advice, which in turn can lead to more or less unintended misjudgements. Politicians, who ultimately decide on fishing quotas, must therefore decide which types of errors they consider most important to avoid, argues Charles Berkow.

"If management errs on the optimistic side by increasing quotas too rapidly, the risk of stock collapse rises. By contrast, if it errs on the side of caution and reduces quotas somewhat more than strictly necessary, the outcome may still be beneficial, leading to stronger fish stocks in the future.," he says.

Text: Henrik Hamrén

ICES Headline Advice 2027 and current TAC