Dredgers, bottom trawls, demersal seines… These fishing techniques all have one thing in common: the use of towed gear. Either nets – or equivalent – deposited on the seabed and connected by cables to one or more fishing boats which tow them over long distances, from the surface.
These techniques are extremely effective for catching shellfish, clams, langoustines, whiting… But what state do they leave the seabed in? Along with the overexploitation of resources and accidental catches, the degradation of the seabed is another major environmental issue in fishing.
Seabed little known but far from trivial
Several tens or even hundreds of meters below the surface of the sea, where the light no longer reaches, the ocean floors are nonetheless crucial habitats and larders for many species. “Nearly 3,000 invertebrates, fish and others have been identified in the deep coral areas that can be found off the French coast”, illustrates Pascal Laffargue, researcher in marine ecology at the French research institute for the exploitation of the sea (Ifremer).
“This issue of impacts on the seabed is at the heart of current fisheries policy”, confirms Clara Ulrich, Deputy Scientific Director at Ifremer. Even more so since COP15 biodiversity in Montreal, last December, which led to the objective of protecting 30% of the seas and oceans by 2030. “It is also a major concern of fishermen to ensure the sustainability of their practices, and therefore the sustainability of their activities,” says Clara Ulrich.
It is necessary to say then the importance of these towed gears in French fishing. Dredges, trawls and bottom seines land, between them, 36% of the volume of fish caught by French boats. “And 47% if we speak in value”, adds Clara Ulrtich.
68% of the English Channel swept each year
It remains to know the extent of the raked areas. This is one of the goals of a study initiated by fishing stakeholders in Normandy and Hauts-de-France, in which Ifremer is participating, the first results of which have been unveiled. Focused on the English Channel, it estimates that 68% of the area of this sea was swept each year by bottom towed gear, between 2013 and 2018. For 24%, the fishing pressure suffered is even considered very high, with an average of five passages per year, some areas having probably been swept ten times, explains Joël Veigneau, researcher at Ifremer and co-author of the study.
Determining the biological state of these most visited sea beds is difficult, due to the difficulty for scientists to access them. Ifremer identifies several pressures caused by these tows on the ocean floor. They vary according to the nature of this floor (rocky, sandy, muddy, etc.) and the size and intensity of the fishing gear. The most obvious of these pressures is abrasion, “ie the stripping of sediment”, explains Benoît Vincent, engineer in fisheries technology at Ifremer. An example with the bead, a heavy structure whose width can reach up to 35 meters and which keeps the net in contact with the bottom. It exerts an average pressure equivalent to that of a group of people dragging their feet on the bottom, compares Ifremer. The abrasion is certainly quite superficial on rocky soils, but deeper (5 cm) on sandy bottoms, and even more so on muddy bottoms. “This is enough to cause damage to benthic habitats and species (which live on the seabed), even more so if the passage of gear is frequent”, specifies Benoît Vincent.
Species that have learned to resist pressure?
Do these funds then become deserts? Not necessarily, we deduce at Ifremer. Example, again, in the Channel. “If these benthic species (sea urchins, brittle stars, small crabs and crustaceans, starfish, tube worms, etc.) had been greatly modified, we would have witnessed a change in the species of landed fish since most feed on them, begins Joël Veigneau . This is not the case. The system remains balanced and relatively productive compared to the volumes fished in the Channel thirty or fifty years ago. »
These species that inhabit the bottoms of the English Channel would therefore resist fishing relatively well. Joël Vigneau partly explains it by natural selection, in an area where “the fishing pressure does not date from yesterday but from the post-war period, and where the tides and currents are among the strongest in France, even of Europe”. In short: The communities present are those that have been able to adapt to these pressures.
Finding the balance between fishing and preserving ecosystems
This does not detract from the imperative to reduce the impacts of towed gear. If only because there are counter-examples. Pascal Laffargue cites plaice, a flatfish from the North Sea; scientific analyzes tend to show that it was indeed affected by the degradation of the seabed and the resources it found there.
An encouraging point for Ifremer, however: it would be possible to reduce the impacts on the seabed without reducing fishing efficiency, or even improving it. Benoît Vincent thus quotes the “Reverse” research project which aims to modify fishing gear to allow them to be less in contact with the ocean floor. “This also results in fuel savings for ships, up to 10% during experiments carried out between 2019 and 2021”, he assesses. Easier then to obtain the adhesion of the fishermen.