Effects of bottom trawling: IOW leads large Baltic Sea field experiment with four research vessels

From July 16, 2024, a three-week research expedition was conducted in the Baltic Sea near Kühlungsborn to experimentally investigate the acute effects of bottom-contact fishing (MGF) on marine sediments and communities. Led by the Leibniz Institute for Baltic Sea Research (IOW), a large-scale field experiment was carried out to study the immediate impacts of bottom trawling. Four research vessels participated: Elisabeth Mann Borgese (EMB, IOW), Alkor (GEOMAR Kiel), Clupea (Thünen Institute of Baltic Sea Fisheries), and Limanda (University of Rostock).

The study area was located at the outer edge of the 3-nautical-mile zone off Kühlungsborn (MV) and covered a total area of approximately 2000 m x 600 m. The Clupea created multiple trawl marks on the seafloor using a trawl net typical for the Baltic Sea to simulate different effects: intense fishing effort, sediment resuspension, and a single trawl track for small-scale detailed investigations. The trawl tracks were traced and investigated using hydroacoustic measurements and USBL transponders.

The primary goal of the experiment was to investigate the acute and short-term effects of MCF on the benthic community and biogeochemical processes, as well as to document the regeneration process. Various stations within the exposed and control areas were sampled weekly in a time series. The benthic community, from prokaryotes to protists, microphytobenthos, meiofauna, macrobenthos, and demersal fish, was analyzed. The biogeochemical investigations focused, among other aspects, on the fluxes of oxygen and nutrients, and the influence of bioturbation on these processes. Another goal of the experiment was to study the sediment plume, in which resuspended matter is transported into the water column for extended periods. Measurements included quantifying the release of sediment-bound carbon, nutrients, and methane.

Additionally, the experiment aimed to examine small-scale effects along the otter board and ground line tracks. Scientific divers from the Limanda were deployed to specifically sample the otter board mound and furrow, and to analyze the significance of this seafloor impact for organisms and biogeochemical processes.

For routine sampling, multicorers and Van Veen grab samplers were used aboard the EMB, complemented by non-invasive methods such as a video glider and a CTD rosette (eDNA). The Alkor focused on geochemical investigations of the seafloor, deploying landers with benthic chambers and an autonomous rover to analyze various fluxes directly at the seabed. These methods were supplemented by an OFOS (Ocean Floor Observation System).

The impacts of bottom-contact fishing on individual components of the community are already well-studied. Various factors determine whether and to what extent bottom trawling causes damage to the seabed, including sediment type, the existing community, the fishing gear used, and the frequency of its use. There is a lack of relevant studies in the Baltic Sea to assess the effects of these factors. The results of this experiment aim to provide deeper insights into the impact of bottom trawling on the entire food web, the short-term regenerative potential of communities, sediment functions, and the effects of resuspension on the pelagic system.

The trawling experiment has already attracted significant attention, including coverage on television (NDR), and reporters from the dpa accompanied the cruise; their report was published in newspapers such as the Süddeutsche Zeitung. Additional information can be found in the press release (german).

Due to the extend of the cruise, initial preliminary results are expected in several months.