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Newsletter December 2017

Commercial fisheries and seabed disturbance: from black-and-white to complex and nuanced

What exactly are the consequences of demersal trawling for the seafloor? Thirty-three partners, including ILVO, spread across all the coasts of the EU, have conducted 4 years of intensive research on this topic. The results indicate that the consequences of seafloor disturbance are highly dependent on the types of trawl, on fishing intensity and on the type of habitat. The issue of seafloor disturbance is therefore not as black-and-white as it is sometimes suggested: it is in fact full of complexity and nuance.

BenthisFishing with bottom trawls such as otter trawl, beam trawls and dredges accounts for almost a quarter of the global catch, but these trawls are also the main source of physical disturbance of the seafloor by humans worldwide. The discussion the consequences of the seafloor disturbance has been ongoing since the beginning of the 1990s. The discussion in and around the North Sea focused on one type of gear - the beam trawl - where other types of fishing gear have received hardly any attention. This has generated a rather controversial discussion which is now coming to an end thanks to the BENTHIS project. The objective was to collate all knowledge of seafloor disturbance, to develop instruments to assess the effects of bottom trawling on the structure and functioning of benthic ecosystems, and to provide solutions.

Four years of intensive research resulted in clear insights into the consequences of demersal trawling. Scientists agree that bottom trawls have a number of well-documented effects. They tend to smooth out the sediment, reduce the complexity of the seafloor and stir part of sediment into the water column. They reduce the biomass, the numbers and the diversity of marine organisms and benefit small animals that reproduce quickly. In this way, the bottom trawl fishery changes the composition of the marine ecosystem and the mutual relationships in the food chain. However, sea fishing is not distributed homogeneously: a significant surface area in our North Sea (37%) experiences little or no fishing, so the consequences are not uniform. It is now clear that certain habitats are very sensitive to seabed disturbance but others are not at all. The problem of seabed disturbance is therefore not as black-and-white as is sometimes suggested.

Based on the results of the project, a trawl impact indicator can be calculated. It reflects the status of the benthic community as the surface of a particular habitat in which the animals can recover. This indicator can be used directly for the European Marine Strategy Framework Directive. The usefulness of this indicator is reflected in the absence of the need to measure the situation before disturbance (a T0-measurement). This is quite relevant because usually there is a need for an area in the sea that has not been affected by human activities, which is now practically no longer exists. The proposed method is based on physical properties of fishing gear and habitat, on the biological characteristics of the animals, and the interaction between these factors.

The results emphasize the complexity of the problem of seabed disturbance. They indicate that good knowledge of the marine ecosystem and interaction with fishing is necessary to assess the impact of fishing and to manage the fisheries properly. The evaluation framework drawn up in the Benthis project now offers a tool to include in an objective and quantitative way seabed disturbance in the management of the sea. In the meantime, this tool has been approved and accepted within ICES (the International Council for the Exploration of the Sea), the principal advisory body of the European Commission. The big advantage of this method is that it can be applied for every type of gear and every type of habitat.

The different steps of the Benthis methodology are explained in 5 short animated videos

  1. Fisheries’ footprints
  2. Characteristics of benthic life
  3. Vulnerability of habitats
  4. Impact reduction?
  5. Impact determination

Project: BENTHIS - “Benthic Ecosystem Fisheries Impact Studies”
Term: 2012 - 2017
Partners: 33 partners uit 12 landen
Funding: Europese Unie in het 7de Kaderprogramma
Links: www.benthis.eu
Contact: Hans.Polet@ilvo.vlaanderen.be