Assessing the sensitivity of the sea floor

The sea floor is far from a homogenous landscape. Differing substrates, depths, and environmental conditions ensure diverse habitats that accommodate many species above, on, and below the seabed. Maintaining the integrity of the sea floor is an essential part of conserving marine biodiversity and living resources and is addressed by Descriptor 6 (D6)​ of the EU's Marine Strategy Framework Directive (MSFD).​

Different habitats in the benthic ecosystem have differing levels of sensitivity and the pressures from human activities impact to varying degrees. Having a variety of different indicators and methods is essential for assessing the state of benthic ecosystems if a robust picture of the seafloor and the changes that can be imposed by human activities is to be formed 

Highly sensitive areas of the seafloor including cold-water corals are known as vulnerable marine ecosystems (VMEs). These cannot endure any impact from pressures such as trawling, and once documented, are often protected from interference from human activities (for example through the establishment of marine protected areas (MPAs).

From pressure to impact maps

But what about the other areas of the sea floor that are not as sensitive? How do we assess the status of these areas after they have been impacted by trawling? If an area is trawled once every two years, will the majority of benthic communities present recover or do the species in that area require a longer recovery period? 

In 2014, ICES provided pressure maps showing the intensity of trawling on the sea floor throughout the Northeast Atlantic. These maps were a major step forward. Based on vessel monitoring system (VMS) data from ten ICES member countries, they indicated the hours fished by year and the different gear types: beam trawls, demersal seines, dredges, and otter trawls. These maps have been updated in 2015 and 2​016​. Earlier this year, the EUs Directorate for the Environment (DG ENV) requested advice from ICES on how to use these pressure maps of fishing intensity to move towards impact maps that could be used to better assess the state of seafloor habitats. With this advice, those implementing the MSFD would be better equipped to study the trade-offs that need to be made by management when attempting to keep the balance between a productive benthic community and fishing opportunities.

To develop this advice,​ the ICES Workshop on Fisheries Benthic Impact (WKFBI) evaluated material prepared by ICES working groups and compared similar approaches on assessing benthic impact of fishing developed within European-funded projects and regional seas conventions. The workshop sought to prepare a method of interpreting the pressure maps of fishing intensity so that it was possible to identify both the exposure of benthic habitats to bottom fishing pressures and the sensitivity of these habitats to fishing pressures in order to understand which habitats are likely to be further impacted.

Explaining the management context behind the work, Adriaan Rijnsdorp​, Chair of WKFBI pointed out, "We are dealing with habitats that are fished and that have to some degree been impacted by fishing. Within the ecosystem approach to fisheries management, we need to take account of the integrity of the seafloor therefore reducing wherever possible the pressure on the seabed to increase the status of the seabed."

A quantitative approach

Two approaches to develop indicators of habitat sensitivity in particular were analysed: a categorical approach, which employed expert judgement and a mechanistic quantitative approach, which is based on the mechanistic understanding of how trawling affects the sea-floor and the benthic community.​

While the categorical approach can be used to identify particularly valued and sensitive habitats and communities that require protection against bottom trawling, it is not suitable for assessing the condition of habitats where managers want to regulate existing fishing pressure and avoid further habitat degradation beyond a certain threshold. The reason for this unsuitability, as stated in today's advice is that, "class boundaries are arbitrarily defined and not quantitatively linked to trawling intensity." The categorical approach was considered inappropriate for this MSFD application as the categories are based on expert judgement with no quantitative links to clear fishing pressures. With this approach, an area can be classed as high impact if it's a highly sensitive one trawled only lightly, but another area can also be classed as high impact because it is trawled heavily even though ecologically these areas are completely different. This means that it is not possible to carry out any meaningful calculations with these scores.

Instead, ICES advises that a mechanical quantitative approach is adopted. More ecologically meaningful scores can be derived from using this type of approach. It is based on basic population dynamic theory, which allows us to estimate the impact of trawling as a function of the mortality imposed by trawling and the recovery rate of the benthic community.

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