The large-scale Swedish trawling for herring in the Baltic Sea has flourished in recent years. The herring catches in the Central Baltic and the Bothnian Sea have increased by 70 percent over the past five years. At the same time, a larger proportion of the fishery has moved closer to the east coast.

There are now worrying reports about a decreased abundance of herring in several coastal areas. These concerns include depletion of local herring subpopulations, with grave consequences for coastal ecosystems and lower productivity of the whole herring stock.

These signals from the coast, and the overall negative trends for some of the major Baltic Sea herring stocks, call for caution. Fisheries management and stock assessment frameworks should urgently ensure a better and more detailed understanding of 1) the stock- and subpopulation structure of the Baltic Sea herring, and 2) how the current herring fishery affects the coastal ecosystems.

In order to halt the negative development, it is also necessary to restrict the large-scale trawling for herring close to the Swedish east coast, in line with the precautionary approach.

Photo: Tobias Dahlin/Azote.

Less herring in archipelagos

Most pelagic trawl fishery for herring along the east coast takes place from January to April, and, to some extent, also during October–December. It is focused on herring that gather off the coast before spawning in the coastal zone or in shallower water.

There have been numerous warnings from local fishermen and authorities in recent years, saying that there is significantly less herring than usual in many coastal areas and archipelagos. In some areas, for instance, in the southern parts of the Bothnian Sea, the amounts of large herring have decreased over the past decade.

Swedish trawl catches in the Baltic Sea have increased over the past five years – and more and more of the catch is taken near the coast. Of all herring caught by Swedish trawling vessels in the Central Baltic Sea and the Bothnian Sea, ICES subdivisions (SDs) 25-32, during the years 2010 – 2014, an average of 29 % was taken in the coastal areas Hanö Bay, Svealand Coast or Gävle Bay. In 2015 – 2019, the proportion was on average 40 percent.

Trawl catches increase ...

Baltic Sea Centre’s analysis of national official catch data from the past ten years show that herring catches by Swedish trawl fisheries have increased in three areas near the Swedish east coast:

• Gävle Bay: Along the southwest coast of the Bothnian Sea, the catches started increasing in 2013/2014. During 2015–2019 the catches tripled compared with the previous five-year period – and the average annual catch went from around 4 600 tonnes to about 12 000 tonnes per year.
• Svealand coast: Outside the Stockholm archipelago and in the Åland Sea, the catches more than tripled in 2015–2019 compared with the previous five-year period, with a significant increase in 2015 – and the average annual catch went from around 2 800 tonnes to about 9 000 tonnes per year.
• In Hanö Bay, outside the Blekinge archipelago, the catches increased by 60 percent during 2015–2019 compared with the previous five-year period – and the average annual catch went from around 3 600 tonnes per year to around 5 800 tonnes.

These three areas attract large amounts of spawning herring each year. The local small-scale fishery in the two northernmost areas has had decreasing catches during the past ten years.

The Swedish share of the total catches of herring (all countries) in the Central Baltic Sea (SD 25-29 and 32) has been relatively constant, around 29%, during the last ten years, while the Swedish share of the total catches in the Gulf of Bothnia (SD 30-31) has doubled in the last five years, from an average of 7% in 2010 – 2014, to 15% in 2015 – 2019.

... and more is caught near the coast

The relative development of Swedish herring catches in three coastal “hot spots” along the Swedish east coast, and their adjoining subdivisions (SDs), between 2010 and 2018. Note that each time series is normalised at 100 % in 2010.

At the same time as both Swedish and foreign trawl vessels catch more herring in the Central Baltic Sea and the Bothnian Sea, a larger proportion of the Swedish yield is caught near the coast:

• Central Baltic Sea: While total Swedish trawl catches increased by an average of 70 percent during 2015-2019 compared to 2010-2014, the coastal-near catches off the Svealand coast increased by an average of 218 percent. The proportion caught in the area Svealand coast almost doubled, from 9 to 17 percent, over the compared five-year periods. Catches in Hanö Bay also increased during 2015-2019, but in proportion to the increase in total yield.
• Bothnian Sea: The total catches increased by an average of 131 percent during 2015-2019, while the catches off the Gävle Bay increased by 161 percent. The proportion caught off the Gävle Bay increased from 72 to 81 percent over the past five years compared with the previous five-year period.

Of all herring caught by Swedish trawling vessels in both the Central Baltic Sea and the Bothnian Sea (SD 25-32) during 2010-2014, an average of 29 percent were caught in areas close to the east coast (Hanö Bay, Svealand coast and Gävle Bay). For 2015-2019, the proportion of coastal-near catches went up to an average of 40 percent.

Managing the big picture

The question is how increased large-scale trawl fisheries close to the east coast might affect the distribution of herring in archipe- lagos and other coastal areas – and what consequences it might have for coastal ecosystems and the Baltic Sea herring population as a whole. One obvious risk is that overly intensive fishing of offshore aggregations of herring near the coast can deplete or even wipe out local subpopulations, leading to, in general, lower productivity in coastal habitats.

Current fisheries management fails to take into account the complex subpopulation structure of herring. Instead, it focuses on the big picture, based on four management units, also called stocks: Central Baltic herring, Western spring spawning herring, Gulf of Riga herring, and Gulf of Bothnia herring.

Each management unit annually gets a politically agreed catch quota (TAC) based on scientific stock assessments. Where and when, within the management unit, the fish is later caught, is up to the fishermen themselves to decide.

Subpopulations – a scientific controversy

Many of today’s concerns about the risks associated with large-scale herring fisheries along the Swedish east coast boils down to an old scientific controversy; that of herring subpopulations.

Should the current management units for Baltic herring be regarded as a relatively homogenous stock, with similar migration and spawning behaviours? Is it sufficient to manage all herring trawl fishery in the Central Baltic Sea as fishing on a single stock, regardless of where and when the catches take place?

Or does each management unit, in fact, consist of several genetically and behaviourally different subpopulations and local components, with more or less specific learned and/or genetically driven migration and spawning patterns?

Today’s fisheries management is organised according to the previous model. In practice, that presupposes that all herring fished close to specific spawning areas will automatically be replaced by herring from the same or neighbouring management units the following year.

However, both historical and new research shows that the latter model is closer to how fish populations are structured.

Foto: © OCEANA.

It is known that different herring spawning subpopulations are to some extent separated from each other based on when and where they reproduce and that they possibly adapt to the environment during the egg and larval phases.

For instance, tagging and genetic studies suggest that Western spring spawning herring has a complex mixture of several different subpopulations (e.g. Rügen herring, Fehmarn herring, herring from the Kattegat and Inner Danish waters). They predominantly spawn during spring, but there are also local spring-, autumn- and winter spawning stock components – each of which has different contributions to the fishery and ecosystem.

During summer, these subpopulations migrate to feeding grounds as far away as Skagerrak and the North Sea before returning to their different spawning grounds to reproduce (i.e. similar to the homing behaviour observed in salmon).

DNA reveal genetic “toolbox”

In a recent study lead by a research team at Uppsala University, new DNA-techniques were used to map the entire genome of At- lantic and Baltic Sea herring. The results revealed a large number of genetic variations between herrings sampled at different locations in the Baltic Sea. Genetic differences were also discovered between herrings caught in the same area but spawning at other times of the year.
The most significant genetic differences between different populations are related to season of spawning and the local environmental conditions (salinity, temperature, light conditions, etc.) during spawning and early larval development.

Since herring also have a well-documented homing behaviour (i.e. spawns in the same place, or at least area, as it was once hatched), it is highly probable that the herring’s choice of spawning grounds is not made randomly but is largely genetically controlled or learned.

The scientists behind the study conclude that the Baltic herring population has a ”toolbox” of different genetic variations linked to different reproduction strategies, and that the extensive genetic characterization of herring populations has established highly informative genetic markers that can be used for cost-effective monitoring of herring populations.

A more finely calibrated fisheries management

Foto: © OCEANA.

It is not yet apparent how and where the genetic variations occur and exactly how they divide the Baltic herring into different supopulations.

The depleted state of several herring stocks, combined with increased catches and reports of decreasing herring quantities along the east coast, are clear warning signs. At the same time, existing knowledge about the herring population structure shows that the management goal of maximum sustainable yield (MSY) can be assumed to be very different between different subpopulations.

Reducing fishing pressure on stocks that are currently below sustainable levels is a given. The same applies to measures to reduce coastal trawling in areas where severe herring shortages are re- ported in coastal ecosystems.

But it is also time to further safeguard the herrings’ genetic varia- tion by taking greater account of subpopulation structure within each current management unit.

The work of providing fisheries management with detailed knowledge about the herring’s population dynamics in relation to coastal ecosystems and the impact of fisheries should begin immediately.

Knowing which herring are caught where and when may help remedy the mismatch between management and biological reality – and reduce the risk of further depleting the Baltic Sea herring.