Benthic habitat characterisation of soft-bottom continental shelves: Integration of acoustic surveys, benthic samples and trawling disturbance intensity

Abstract

Eleven sites were located on Mediterranean continental shelves to explore the link between the physical characteristics and epibenthic fauna from soft-sediment habitats. These sites, at 32–82 m in depth, were associated with fishing grounds and the trawling intensity was estimated at the site scale to assess the effects of trawling on benthic communities. Each site was surveyed with Multi-Beam (bathymetry and backscatter), side-scan sonar, benthic grabs and a surface dredge. The sites were clustered in three habitat types. Habitat 1, with moderate trawling disturbance, was characterised by homogeneous mud and associated epifauna that was also highly homogeneous across sites. Habitat 2, with sandy mud and scattered gravel and rocks, had a high abundance of sessile suspension feeders that probably attach to the coarser substratum and benefit from the low fishing disturbance in these sites. Habitat 3 included sites with heterogeneous sediments with maërl as the prevailing biocenosis and having the highest species richness, despite being subjected to variable trawling intensity. Statistical models were used to relate environmental parameters and the species abundance. More than 3 physical variables were necessary to explain the epifaunal patterns across sites, including the percentage of mud, sediment heterogeneity and fishing effort. These analyses are an essential step for extrapolating information from benthic samples to the larger scale of habitats, mapped through acoustic surveys. Despite this, a good integration is required between the mapping of physical habitat distribution and the ecological knowledge of communities.

Introduction

Continental shelves worldwide are highly disturbed by human activities with dredging and fishing with towed gears accounting within the most damaging activities (Thrush and Dayton, 2002). In this context the widely-advocated ecosystem-based management demands the development of adequate tools to evaluate the status of ecosystems at large scales (Frid, 2003). Broad spatial knowledge of the habitats and associated biological communities overlapped with human uses is essential (Kenny et al., 2003; Pickrill and Todd, 2003; Holmes et al., 2008). If the distribution of marine habitats and human pressures is known, then the spatial distribution of the potential risk of impact can be estimated, thereby helping to identify the most efficient management solution (Stelzenmüller et al., 2010). This information is a high priority in the European context, and the implementation of several EU strategies such as the Marine Strategy Framework Directive (directive_20008/56/EC) requires a comprehensive classification of marine habitats and their correspondent biodiversity (Barberá et al., 2012).

Recent advances in our ability to survey large areas of the seafloor using acoustic and visual techniques offer a great potential (Kenny et al., 2003; Lo Iacono et al., 2008; Birchenough et al., 2010; Freitas et al., 2011). Remote sampling techniques allow large scale acquisition of data (Hewitt et al., 2004) and different acoustic systems have been used to characterize the physical parameters of benthic habitats (e.g. Morrison et al., 2001; Freitas et al., 2003; Foster-Smith et al., 2004; Coiras et al., 2011). However, there is still a mismatch between acoustic techniques that principally focus on the abiotic characters of habitats and ground-truthing sampling that focuses on a small portion of the seabed to obtain the corresponding biotic information (Diaz et al., 2004). An increasing number of studies have used remote techniques to produce habitat suitability models for sensitive benthic fauna, such as cold water corals (Guinan et al., 2009; Davies and Guinotte, 2011), and fish habitats (Eastwood et al., 2001; Ordines et al., 2011). This work should now be extended to benthic communities from the highly impacted continental shelves. A large proportion of continental shelves correspond to muddy and sandy habitats and harbour benthic communities that provide many ecosystem services (Thrush and Dayton, 2002). Even though with few exceptions, e.g. reef forming organisms such as corals or densely aggregated species such as shellfish, most biological attributes in these areas would not be easily measurable using remote sensing techniques, due to lack of dense aggregation of organisms or dominance of burrowing invertebrates (Hewitt et al., 2004; de Juan et al., 2007; Davies and Guinotte, 2011). Therefore, an important advance now is to establish strong links between biotic and abiotic components of the seafloor in homogeneous soft-sediment areas (Freeman and Rogers, 2003).

Mediterranean continental shelves have been historically exploited and yet in this region there is a general lack of information on the distribution of benthic species and habitats beyond coastal areas and their sensitivity to human pressures (Sardá et al., 2012). The existing knowledge is dispersed and not systematically compiled, while most of the monitoring studies target commercial species or the more accessible coastal areas (de Juan et al., 2012). The principal objective of this work was to investigate the relationship between abiotic and biotic components in soft-sediment seabed linked to commercial trawling grounds. We surveyed 11 sites across Mediterranean fine and coarse sediment dominated continental shelves, off the coasts of Spain (6 sites), Italy (2 sites) and Greece (3 sites). Aiming to relate physical habitats and biological communities, each site was surveyed with Multi-Beam bathymetry and backscatter, side-scan sonar, van Veen grabs and epibenthic dredges. Additionally, the trawling intensity was estimated at the site scale. Relationships between epibenthic communities and physical variables, including fishing effort, were assessed aiming to identify community variability across sites and the environmental factors linked with this variability. We highlight the importance of incorporating trawling disturbance in the study as knowledge on the distribution of ecosystem sensitivity over continental shelves is essential to advance in the ecosystem-based fisheries management.