Tracking life on the seabed after the trawl ban

For decades, bottom trawling has made life difficult for seabed-dwelling species such as starfish, crustaceans and worms—species that are essential for maintaining healthy biodiversity and fish stocks in Danish waters.

When heavy bottom-towed gear is dragged across the seabed to catch, for example, Norway lobster, cod, saithe, wolffish and redfish, it can scrape up large amounts of seabed fauna and sediments as unwanted and commercially worthless bycatch.

Reefs and the many different seabed structures can be flattened, causing habitats for bottom-dwelling organisms to disappear and fish to lose safe spawning and sheltering areas. In addition, the structure of the seabed is damaged, and the important processes taking place there are disrupted.

12 years is rare

“It is very rare to have such a long time—this project will run for 12 years—to assess whether trawling bans actually lead to changes and improvements in the marine ecosystem as a whole,” says Gary Banta, who, in addition to being a biologist and head of department at the Department of Biology, is also involved in the new research project.

He notes that environmental measures are often introduced without the necessary scientific documentation of their effects, and that this project is specifically designed to address that gap.

The project is called Passive Restoration of Marine Habitats (PReMaH) and is funded with just over DKK 24 million from the Danish Research Reserve. In addition to SDU, participants include DTU Aqua, GEUS and the University of Copenhagen.

Sediment processes and marine mammals

The research team will begin by collecting baseline (pre-intervention) data in the Kattegat, off Grenaa, and in the southern Belt Sea before the trawling ban takes effect.

Biologists from SDU will, among other things, contribute by studying sediment processes and whether there are changes in the distribution of large marine mammals, particularly harbor porpoises.

“A wide range of biological and chemical processes take place in the seabed that normally help keep the marine environment in balance. These can be disturbed by bottom trawling, which can, for example, weaken the seabed’s ability to ‘breathe’ — that is, to absorb oxygen and release CO₂ — as well as to process and store carbon and to transform nitrogen compounds,” Gary Banta explains regarding the importance of tracking sediment processes over the next 12 years.

This work will be carried out in collaboration with his colleague at the Department of Biology, Professor Bo Thamdrup.

Improvements in Sweden

Large marine mammals are the focus for Magnus Wahlberg, who is also a biologist at the Department of Biology and the daily manager of SDU’s marine biological research station in Kerteminde. He will contribute by deploying listening buoys and collecting environmental DNA to investigate the movements of large marine mammals and the development of populations in the new trawl-free areas.

“Harbour porpoises are a key species in Danish waters. We will examine whether the trawl ban leads to the desired effects even at the top of the ecosystem, where porpoises belong,” he says.

The researchers hope the project will document improvements in marine nature and biodiversity similar to those already observed in comparable initiatives in Sweden.

Project website is here.
Project press release is here.