Two people sorting fish
Sorting of herring and sprat during an expedition with the research vessel R/V Svea. Photo: Sofia Bureborn

Moving out the trawl boarder in the Baltic Sea - scientific monitoring

Page reviewed:  25/10/2025

The Swedish Agency for Marine and Water Management (SwAM), on behalf of the Swedish Government, is carrying out a scientific project involving a relocated trawling boundary in the Baltic Sea. The aim is to examine the effects of the shifted trawl border on herring (Clupea harengus) stocks.

SLU Aqua, the Department of Aquatic Resources at the Swedish University of Agricultural Sciences (SLU), is responsible for the scientific monitoring, in which we study stock development and ecosystem responses within six work packages.

Herring (Clupea harengus) is a key species in the Baltic Sea ecosystem. They serve as important prey for predatory fish, birds, and seals, while also providing an important resource for both coastal and large-scale pelagic fisheries. In recent years, a worrying trend has become evident: catches in coastal areas have declined sharply, and large individuals have become scare. For coastal fisheries, which predominantely target large herring, this development has resulted in a severe downturn.

To investigate whether stronger protection in coastal zones could reverse this trend, the government tasked SwAM in 2022 with conducting a scientific trial. The central question is whether a moved trawling boundary, the distance from the coast within which trawling is prohibited, can contribute to stronger herring stocks and re-establish the occurrence of large herring in coastal waters.

What does the government assignment involve?

The government’s assignment entails that the trawling boundary has been temporarily moved outward in parts of the Baltic Sea. In the central Baltic Sea, a time-limited trawling ban for pelagic species within 12 nautical miles from the coast has been in effect since February 2025. The trial will continue until spring 2027.

Map of swedish coast
Area closed to trawling

SwAM has the overall responsibility: the agency decides on regulations, consults internationally, and coordinates the work. SwAM has also appointed a scientific committee with Swedish and international experts to ensure the quality of the monotoring program implemented by SLU Aqua.

SLU Aqua’s scientific monitoring

SLU Aqua is responsible for collecting and analyzing the data required to evaluate the experiment. The monitoring program builds on both the long-term environmental monitoring and recurring fish surveys that SLU Aqua already conducts in the Baltic Sea, as well as new targeted studies adapted to the experimental area. The program has a broad design intended to capture any potential direct effects on herring and longer-term changes in the ecosystem.

  1. Environmental factors – Collection and analysis of abiotic data such as temperature, salinity, and nutrients, as well as monitoring of zooplankton, an important food source for herring.
  2. Pelagic fish – Monitoring of pelagic stocks, primarily herring and Baltic herring, but also sprat and stickleback. Through scientific trawl surveys and acoustic studies at sea and along the coast, stock development, biomass, and size and age distributions are monitored.
  3. Genetics and otolith chemistry – DNA analyses and chemical tracers in otoliths (ear stones) are used to distinguish spawning stocks and map migration patterns.
  4. Pelagic fisheries – Analyses of fishing pressure, i.e., how fishing patterns change when areas are closed. The analyses are based on logbooks, AIS data, and landing statistics.
  5. Seals and cormorants – Analyses of predation pressure from these species through studies of population development, distribution, behaviour, and diet.
  6. Ecosystem effects – Modelling of how changes in trawling fishery affect interactions between species and habitats in the Baltic Sea.

What has been done so far?

The monitoring program began in 2023 and was expanded in 2024 and 2025. The surveys have been adapted to the designated trial area in the central Baltic Sea and are now underway in all parts of the program.

On board the research vessel Svea, four acoustic fishing expeditions are conducted annually. Additional coastal fish surveys are carried out at Forsmark and Simpevarp.

Map of coastline
Left: Map showing the 2024 acoustic surveys with R/V Svea. Right: Map showing haul locations (black circles) in the extended logbook reporting for 2024 and SLU’s coastal monitoring stations.

Further acoustic data are also collected using a solar-powered sailing drone and a stationary acoustic buoy.

Samples have been collected for genetic analyses and otolith chemistry from coastal fish surveys, scientific expeditions, and commercial fisheries. This work has already contributed to a scientific publication on the population structure of herring in the Baltic Sea.

In the monitoring of seals and cormorants, tagging, diet analyses, and population surveys are being conducted in focus areas in the Bothnian Sea and off Bråviken, with reference areas on Gotland and in Blekinge. Work on ecosystem models has begun to analyze interactions between fishing, stocks, and the environment.

What happens next?

In 2026, the monitoring program will continue with the same structure as in 2024–2025. SwAM will report the final results of the assignment to the government in spring 2027.

Contact

Environmental factors: Carolyn Faithfull and Lovisa Wennerström

Pelagic fish: Joakim Hjelm and Diana Hammar Perry

Genetics and otolith chemistry: Lovisa Wennerström

Pelagic fisheries : Katja Ringdahl

Seals and cormorants: Monica Mion

Ecosystem effects: Joakim Hjelm

Contact