Duration: 06 to 26 October 2025

Area: Baltic Sea

Purpose: Hydroacoustic recording of pelagic fish stocks (GERAS/BIAS)

The voyage (GERAS - German Acoustic Autumn Survey), which is carried out every year in cooperation with Denmark, is part of an international hydroacoustic survey of pelagic fish stocks in the Baltic Sea (BIAS - Baltic International Acoustic Survey), which is coordinated by ICES (International Council for the Exploration of the Sea). Vessels from Estonia, Latvia, Poland and Sweden are also taking part in this survey.

Tasks of our research trip:

1. Hydroacoustic surveys: Recording the stocks of herring (Clupea harengus) and sprat (Sprattus sprattus) in the Kattegat, western Baltic Sea with Belt Sea, Öresund and Arkona Sea. The recordings are made with echo sounders. Additional transects are planned to compare day and night distribution. Broadband data will be recorded during the fishery (if necessary, additional recordings will be made in the Öresund, see below).
2. Calibration of the echo sounder: this is ideally carried out at a water depth of at least 20 metres and in an area protected from the wind. The exact location depends on the weather and is determined together with the ship's command.
3. Biological sampling: Fishing hauls are required to identify the echoes and determine lengths, ages and species. Samples of the target species herring and sprat are taken with pelagic trawls. Species and length distributions of all catches are recorded. In addition, herring, sprat, anchovy (Engraulis encrasicolus), sardine (Sardina pilchardus), cod (Gadus morhua), three-spined stickleback (Gasterosteus aculeatus) and possibly dogfish (Squalus acanthias) are sampled for biological analyses.
4. Hydrographic investigations: Temperature, salinity and oxygen profiles are recorded throughout the area using a CTD multi-probe (CTD stands for conductivity, temperature and depth) with a crane water sampler after each haul and at other stations. Water samples are used for calibration.

The Öresund has seen significant declines in herring stocks since 2016. If time permits, comparative studies should be carried out to check whether the design of the standard survey is still representative.