The surveys conducted on board R/V Svea provide crucial knowledge about the state of fish stocks – a foundation for the long-term, sustainable management of marine resources. Foto: Sofia Bureborn
SLU - monitoring fish stocks
Page reviewed:
12/06/2025
How much fish is in the sea? To manage fisheries wisely, we need reliable data on stock sizes. On board R/V Svea, key surveys are carried out to support the EU’s fisheries policy.
SLU Aqua - the department of aquatic resources at the Swedish University of Agricultural Sciences – uses R/V Svea for seven annual research expeditions in the Baltic Sea and the Skagerrak/Kattegat. These surveys are international in scope and coordinated by the International Council for the Exploration of the Sea (ICES).
Unlike commercial fishers who actively seek out fish and optimise their fishing for the best possible catch, our aim during these expeditions is to fish as “poorly” and consistently as possible. We always sample in fixed areas using standardised methods. This creates long time series of data, allowing us to track changes in fish stocks over time.
The surveys cover both bottom-dwelling species (demersal fish such as cod and flatfish) and species that swim in the open water column (pelagic fish such as herring and sprat). Data are collected using both trawling and acoustic methods (echosounders). Eggs and larvae are also sampled and analysed, and all fish caught are weighed, measured, and assessed for age and sex.
Underwater video is also used to survey Norwegian lobster (Nephrops) populations.
SLU Aqua’s expeditions:
Internationally coordinated hydroacoustic expeditions in the framework of the Baltic International Acoustic Survey (BIAS), an International Council for the Exploration of the Sea (ICES) survey programme, are carried out annually. The main purpose of the survey is to show the changes in fish stocks (indices) and geographical distribution of fish (mainly herring and sprat).
The expedition is conducted in October. The survey is an international collaboration between several Baltic Sea countries. Since 2019, the Swedish expeditions have been carried out with SLU's research vessel R/V Svea. Sampling is done with echo sounder, and pelagic trawls, all fish species in the catch are measured in length and biological samples, including age, are taken on the target species, herring and sprat.
The Baltic Sea countries meet in the Baltic International Fish Survey Working Group (WGBIFS) and the results from each country are compiled in a common database. WGBIFS also develops common guidelines and manuals. The results form the basis for the WGBFAS estimate of the total stocks of herring and sprat in the Baltic Sea.
Trawl expeditions under the Baltic International Trawl Survey (BITS), an International Council for the Exploration of the Sea (ICES) trawl survey programme, have been conducted since the early 1990s. Data from BITS cover the entire Baltic Sea and include abundance and spatial distribution of cod and European flounder, as well as length, weight, age, sex and sexual maturity of caught fish.
The expeditions are carried out twice a year (in quarters 1 and 4). The survey is an international co-operation between several Baltic Sea countries. Sweden is responsible for parts of the southern and central Baltic Sea. Since autumn 2019, the Swedish expeditions are carried out with SLU's research vessel R/V Svea.
The main objectives of the survey are to monitor the abundance and spatial distribution of fish (mainly cod and flounder) and to provide data to calculate recruitment indices (different indices describing the growth of a population) and to monitor changes in spawning biomass of a number of fish species. All fish species in the catch, as well as some invertebrates, are measured in length. Biological samples (including sexual maturity and age) are taken from the target species. Hydrographic data (e.g. salinity, temperature and oxygen concentration) are also collected at each trawl.
Trawl expeditions within the framework of the International Council for the Exploration of the Sea (ICES) trawl survey programme, the International Bottom Trawl Survey (IBTS), have been ongoing in Skagerrak and Kattegat since 1990. The survey is conducted twice a year, in quarters 1 and 3. The Swedish part covers Skagerrak, Kattegat and parts of the eastern North Sea. Since 2020, the expeditions are carried out with SLU's research vessel R/V Svea.
The main objective of the survey is to monitor the abundance and spatial distribution of fish, to provide recruitment indices and to monitor changes in spawning biomass of a number of fish species. Fishing and sampling are conducted in accordance with a manual developed by the ICES IBTS working group. All fish species in the catch and some invertebrates are measured and biological samples (e.g. sexual maturity and age) are taken on the target species.
During the first quarter, a fine-mesh ring net (Midwater Ring Net, commonly referred to as MIK) is also used for fished at night for fish larvae, primarily to monitor the presence of herring, sprat and sardine larvae.
Hydrographic data (e.g. salinity, temperature and oxygen concentration) are also collected during each trawl.
Internationally coordinated hydroacoustic expeditions under the Sprat Acoustic Survey (SPRAS), an International Council for the Exploration of the Sea (ICES) survey programme, are carried out annually. The main objective of the survey is to show the changes in the fish stock (index) and geographical distribution of fish (target species is sprat).
The expeditions are carried out in May and are an international collaboration between several Baltic Sea countries. Since 2019, the Swedish expeditions have been carried out with SLU's research vessel R/V Svea. Data collected are the amount of biomass, as well as the weight, length, age, sex and sexual maturity of the sprat. Hydrographic data is also collected (temperature, salinity and oxygen).
The Baltic Sea countries meet in the Baltic International Fish Survey Working Group (WGBIFS) and the results from each country are compiled in a common database. WGBIFS also develops common guidelines and manuals. The results form the basis for the WGBFAS estimate of the total stocks of herring and sprat in the Baltic Sea.
UWTV Nephrops is a video-based survey of Norwegian lobster (Nephrops norvegicus) burrows in the Skagerrak and Kattegat. The survey uses a camera-equipped sled that is towed over a defined distance along the seafloor, filming a fixed-width transect. The method is known as UWTV – Underwater Television.
Because it is possible to assess whether burrows are inhabited, the footage allows for estimates of Nephrops density, which in turn enables calculation of the total stock size within the survey area.
The UWTV survey is coordinated by ICES and conducted under the EU Data Collection Framework (DCF). It provides a key basis for stock assessments of Norwegian lobster.
From data collection to fisheries quotas
The data collected during SLU Aqua’s expeditions is used, among other things, to estimate fish stock sizes. For internationally managed fish stocks, this work is carried out within the International Council for the Exploration of the Sea (ICES), which develops scientific advice on how much can be sustainably fished in different marine areas (TAC – Total Allowable Catch). Final decisions on annual fishing quotas are made by the EU’s fisheries ministers, who, in addition to the ICES advice, also consider economic and social factors.
The internationally coordinated fish surveys conducted by SLU Aqua are carried out under the Data Collection Framework (DCF) – the EU’s framework for the collection, management, and sharing of fisheries data and statistics. Data collection within the DCF is co-funded, with 60% of the cost covered by the European Maritime, Fisheries and Aquaculture Fund (EMFAF).
