IBTS - International Bottom Trawl Survey

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 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.

• Manual for IBTS: SISP 10 – Manual for the North Sea International Bottom Trawl Surveys
• Manual for MIK-trålning: SISP 2 Manual for the Midwater Ring Net sampling during IBTS Q1 

• Data from the bottom trawl survey can be downloaded from the ICES database DATRAS.
• Data from the survey suing a MIK-trawl are available in a ICES-database for fish larvae and fish eggs