Detecting adverse effect on seabed integrity. Part 2: How much of seabed habitats are left in good environmental status by fisheries?

Highlights

• Propose a general modeling framework to detect thresholds of impact on an increasing anthropogenic disturbance gradient.

• Determine fishing related abrasion thresholds over several contrasted EUNIS level 4 habitats.

• Assess seabed environmental status in response to bottom trawling.

• The vast majority of the investigated sub-regions surfaces’ were found to be adversely impacted or lost.

Abstract

By relating observed changes to the pressures suffered, the Marine Strategy Framework Directive intends to better control the factors of environmental degradation and to manage their consequences in European waters. Several descriptors are defined within the framework of the MFSD and in particular descriptor 1 relating to the biological diversity of the seabed and descriptor 6 relating to the seabed integrity (i.e. the quality of their structures and functions). For each descriptor, indicators and threshold values must be defined and a novel conceptual approach to define and detect seabed integrity thresholds is proposed here. Bottom trawling being the main source of shelf continental disturbance, it is important to evaluate its impact on benthic habitat. The goal of this study is to propose a methodology to determine “Good Ecological Status” threshold values for each habitat type present in three contrasted MFSD sub-region (North Sea, English Channel and Mediterranean Sea). Trawling impacts are dependent of the spatial and temporal distribution of the fishing effort, fishing gears, intensity of natural disturbances and habitat types. Benthic community structures present in these areas were studied using by-catch non-commercial benthic invertebrates data collected during French scientific bottom trawl surveys. Swept area ratios derived from VMS data were used to quantify the intensity of fishery induced abrasion on the seabed. A modeling approach was used to determine abrasion threshold values on each EUNIS level 4 habitat. The values, beyond which trawling has an adverse effect on benthic communities, have been determined for each habitat. This made it possible to assess and map the ecological status of each of the habitats and to determine the percentage of each habitat impacted by trawling. The method proposed here to evaluate the impact of trawling on benthic communities highlighted that the vast majority of the investigated sub-regions were adversely impacted or lost as a result of seabed impacting trawling.

Introduction

In 2008, the European Union drew up the Marine Strategy Framework Directive (MSFD) to achieve or to maintain “Good Environmental Status” (GES) in the marine environment (EC, 2008). This directive sets out eleven descriptors of human uses of the marine ecosystem, each comprising a number of criteria and methodological standards for determining GES. Each member state must therefore develop quantitative indices and threshold values corresponding to each criteria to assess progress towards the GES (Rice et al., 2012). To measure the evolution of this environmental status, the evaluation of some criteria requires to develop appropriate indices able to detect changes in relation to anthropogenic disturbance (Leonardsson et al., 2009, OSPAR, 2012, Rice et al., 2012, van Loon et al., 2018). On the eleven descriptors defined in the MSFD, two of them specifically concern the benthic habitat: the descriptor 1 (biodiversity) and the descriptor 6 (seabed integrity). Criteria 1 and 2 of the descriptor 6 (D6C1, D6C2) are dedicated in evaluating the spatial extent of the physical loss or disturbance of seabed. The criteria D6C3 focuses on establishing pressure thresholds values for the adverse effects of physical disturbance. Finally, D6C4 and D6C5 must allow the assessment of the extent of benthic community “loss” or “alteration” and should set maximum admissible proportion of habitat loss and evaluate the status of each habitat in that respect (EC, 2008, EC, 2017).

The information of these criteria requires the development of transparent indices, allowing for a scientifically defensible assessment of the environmental status of the seabed. Since each type of pressure will result in either habitat disturbance or total physical destruction, it is expected that they will affect benthic communities in different ways. It seems therefore more appropriate to address each pressure effect separately and to develop specific indices and thresholds. In Europe, dredging and bottom trawling occur over large surfaces of the continental shelf and are the principal source of the anthropogenic disturbance to seabed habitats (Hiddink et al., 2007, Halpern et al., 2008, CNDCSMM, 2019). Based on an extensive assessment methodology, it was possible to identify four indices (Jac et al., submitted) that respond to trawling impact and may probably be used in all European waters. These were computed using benthic community data from scientific bottom trawl surveys which enable to work on a large spatial scale but also to focus on the epifauna, unlike other sampling methods such as grab or box-corer that perform small-scale sampling, mainly of the endofauna (Rumohr, 1999, Foveau et al., 2017). The set of indices retained were all based on species biological traits that are known to shape species sensitivity to physical abrasion such as that generated by bottom trawling.

The distribution and composition of benthic assemblages are known to be dependent of environmental conditions such as depth, hydrodynamism and granulometry (Gray and Elliott, 2009) or trawling pressure (Eigaard et al., 2017). Therefore, the evaluation of trawling impact on benthic community must be carried out by habitat type. As a great diversity of seabed habitats is present in the continental shelf of European waters, the development of an index that can be used in all European waters requires its evaluation in contrasted habitats, subjected to important gradient of trawling effort. Thus, a pan-european habitat map in a reasonably standardized typology is necessary to evaluate the relevance of each tested index at the scale of each MFSD sub-regions. A generic and hierarchical habitat classification of European Waters was developed by the European Nature Information System (EUNIS; http://www.emodnet.eu) and is currently available. This typology is based on a hierarchical classification of habitats allowing access, for the marine domain, to levels of precision ranging from the type of substrate to the precise identification of benthic stands, defined by the presence of characteristics species, while integrating the exposure level and depth (Galparsoro et al., 2012). Many studies on trawling impact have used EUNIS level 3 (Eigaard et al., 2017, van Loon et al., 2018) which takes into account depth, sediment grain size, light and hydrodynamism.

The characterization of GES, with regard to the impact of trawling, requires the definition of thresholds for each habitat type that may be trawled. Threshold values correspond to values below which no negative effect of the impact source (trawling in this study) can be observed on the community (here the benthic community). Thus, beyond this value, the observed effect results from the abrasion. Existence of these threshold values is linked to the community resistance to trawling. The more a community is resistant to the pressure; the more the pressure threshold value from which a negative effect may be observed will be high. Threshold can also be defined as the point at which small changes in a driver (fishing intensity for example) may produce large responses in the ecosystem (Groffman et al., 2006). It is therefore important to define the threshold at which GES is met as the use of trends-based targets gives no clear indication of the status achieved (EC, 2008).

The aims of this study were to propose a methodology based on four functional indices proposed earlier by Jac et al. (submitted) to determine GES threshold values for each habitat type present in three contrasted MFSD sub-regions: Western Mediterranean Sea, North Sea and English Channel. Maps representing the environmental status of these sub-regions were produced as a result of the application of this methodology.