BITS – Baltic trawl survey

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.

• Fishing and sampling are conducted in accordance with a manual developed by a BITS working group within ICES: SISP 7 - Manual for the Baltic International Trawl Surveys (BITS).
• Map - trawl stations within the Swedish parts of BITS (jpg)

Biological data are made available through the ICES database DATRAS.