Tradeoffs between fisheries and the seafloor

ICES explores five management scenarios that could be used to reduce pressure and impact on the seabed caused by bottom trawling.
Published: 24 June 2021

"As a fisheries manager, I want to know what are the financial consequences and environmental gains if bottom-fishing activity reduces its footprint by a fixed proportion." 

Can seafloor integrity be maintained while bottom trawling continues to take place? ​ICES has released advice to the European Union (EU) exploring different management measures that could reduce the impact of mobile bottom contacting gear. Within the advice, five management scenarios have been evaluated to show the possible trade-off analysis.

Seafloor pressure

While fishing with mobile bottom contacting gear is the main physical pressure exerted on the seafloor across the EU, management measures and fisheries actions can reduce the impacts of mobile bottom contacting gears on seafloor habitats. “Because bottom-contact fishing provides by far the major source of physical disturbance to the seabed around Europe, it affects many seabed habitats and is therefore a major pressure for which management options need to be developed if we want to improve the health of seafloor ecosystems" says David Connor, Policy Officer, EU Directorate-General for the Environment.


Over the past five years, ICES has been carrying out a stepwise process to deliver guidance on seafloor integrity for Descriptor 6 of the EU's Marine Strategy Framework Directive (MSFD) to ensure that with the sustainable management of human activities affecting our seas, it should be “at a level that ensures the structure and functions of ecosystems are safeguarded and benthic ecosystems, in particular, are not adversely affected".

In 2017, ICES developed indicators to assess pressure and impact on the seafloor from bottom-contacting fishing. The resulting advice presented measures of fishing pressure, its impact on the seafloor, and the value of fish catches and landings and laid the groundwork for the exploration of trade-offs, At that time, ICES also provided a number of scenarios of the advice, where a demonstration trade-off analysis was run for the Greater North Sea ecoregion over the period 2012–2015 using the indicators previously explained. The trade-off analysis indicated that a large fraction of landings and revenue from bottom fisheries are obtained from a relatively small part of the area fished in the North Sea. This finding underscored a potential management option that reduces fishing impacts on the seabed with a relatively small cost to the fisheries.

New data products

In 2021, ICES was also asked to provide  the EU with a set of management options to reduce the impact of mobile bottom contacting fishing gears on seafloor habitats. This recent advice breaks new ground in relation to the data products that are to be used in conjunction with the advice. A set of new data products have been provided which include interactive maps and regional assessments that allow users to explore the various options and assessments for specific (sub)regions using their web browser. The data layers are also made available as CSV files for further operational exploration.

In 2017, the Greater North Sea ecoregion was the only area to be operationalized for trade-off analysis. Three ecoregions have now been added - the Celtic Seas, Baltic Sea, Iberian Waters and Bay of Biscay ecoregions. In depth regional assessments for these four ecoregions are provided with descriptions of pressure, impact, core fishing grounds, fishing by métier, and implementation of management scenarios, including full assessments of 22 subregions.

Interactive maps showing the core fishing grounds, pressure and impact, MSFD protection - unfished, MSFD protection – PD impact, MSFD protection – L1 impact, as well as the availability of all the analysed data as data layers allows users, such as policy implementers, the opportunity to test trade-offs. While the advice gives an example of spatial thresholds, with these maps, it is possible to adjust the quality thresholds. Users can view unfished, lightly fished, or heavily fished areas and adjust the thresholds to find tipping points between a healthy, productive seafloor and an area that is no longer has good environmental status. The advice has been produced in ICES Transparent Assessment Framework tool (TAF), meaning that everything is reproducible and when new VMS data is received, it can simply be input.

What management measures were considered?

In their 2019 paper, Choosing best practices for managing impacts of trawl fishing on seabed habitats and biotaMc Connaughey et al. reviewed a number of management measures – gear design and operations, prohibitions by gear type, freeze trawling footprint, nearshore restrictions and zoning, prohibitions by habitat type, multipurpose habitat management, invertebrate-bycatch quotas, habitat-impact quotas, and removal of effort – all of which could be used to reduce the impacts of bottom on the seafloor. In preparation for WKTRADE3, a core group considered this set of management measures, operationally testing them with the available framework and data through different scenarios.


What are the benefits to the wider marine ecosystem if, for example, a percentage of the seafloor were to be closed off to trawling? What does the ecosystem gain? And what can the fisheries gain? One way to understand this is by looking at the indirect effects of a healthy productive seafloor. The two impact indicators (PD which indicates relative benthic biomass and L1 which focusses on the most sensitive parts of the ecosystem by quantifying the community biomass of benthos that is affected by trawling during its lifespan) highlight the underlying impacts and sensitivities of the seafloor. “Benefits include recovery in the relative abundance of sensitive and habitat-forming species and increased seabed complexity. This increased complexity can serve as refugia for species, help population connectivity, and enhance resilience to stressors and adaptation capacity to climate change."

Fisheries could also directly benefit from reduced pressure from bottom trawling. Reduced fishing effort in an area will increase fish abundance. Fish may spillover from closed areas to those that remain open, again increasing abundance and improving catch per unit effort.

All benefits that can be quantified can be included in the trade-off assessment (not all of the benefits mentioned in the Advice are currently understood well enough to be modelled at a regional scale for evaluating the management scenarios).

Read the full advice document ICES advice to the EU on how management scenarios to reduce mobile bottom fishing disturbance on seafloor habitats affect fisheries landing and value
and ​the accompanying Data products.

Find all ICES advice online.


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Tradeoffs between fisheries and the seafloor

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