Outer Bristol Channel marine habitat study : 2003 investigations and results

This report describes the investigations undertaken by the British Geological Survey (BGS) and the National Museums and Galleries of Wales (NMGW) during 2003 for the Outer Bristol Channel Marine Habitat Study, and the results and interpretations completed by March 2004 for the elements of the study funded by the Sustainable Land Won and Marine Dredged Aggregate Minerals Programme of the Office of the Deputy Prime Minister. The complete study is a three year programme which is planned to end in March 2006. The other principal funder of the study is the Aggregate Levy Sustainability Fund for Wales, which is administered by the Welsh Assembly Government. The Crown Estate and the British Marine Aggregate Producers Association have also supported the study with some funding and contributions of data. Pressure on marine resources in the Bristol Channel continues to develop with a number of issues including aggregates, fisheries and wind farm developments. There are also legislative obligations with regard to marine conservation, in a European context with the Habitats Directive and in national initiatives such as the designation of Carmarthen Bay, the Pembrokesire Coast and Lundy as candidate Special Areas of Conservation (cSACs). To inform the policy and decision making process in terms of developments in the marine environment requires knowledge of its current physical state. This includes the morphology, geology, biology and sediments of the seabed. Baseline information on these is essential for strategic management and the conservation of biological diversity. Responsible stewardship requires an understanding of the way the marine environment functions and how the sea may respond to human activity. It also means involving stakeholders as an integral part of policy making. The Bristol Channel Marine Aggregates: Resources and Constraints Research Project (Posford Duvivier & ABP Research, 2000), indicated significant gaps in the biological and geological data available for the Outer Bristol Channel area. The only comprehensive study of the benthic invertebrates (Warwick & Clark 1977) was carried out in 1972-1973 and British Geological Survey maps of the area were based on surveys undertaken from 1971 to 1976

Benthic biodiversity in the southern Irish Sea

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Modification of marine habitats by trawling activities: prognosis and solutions

Fishing affects the seabed habitat worldwide on the continental shelf. These impacts are patchily distributed according to the spatial and temporal variation in fishing effort that results from fishers' behaviour. As a consequence, the frequency and intensity of fishing disturbance varies among different habitat types. Different fishing methodologies vary in the degree to which they affect the seabed. Structurally complex habitats (e.g. seagrass meadows, biogenic reefs) and those that are relatively undisturbed by natural perturbations (e.g. deep-water mud substrata) are more adversely affected by fishing than unconsolidated sediment habitats that occur in shallow coastal waters. These habitats also have the longest recovery trajectories in terms of the recolonization of the habitat by the associated fauna. Comparative studies of areas of the seabed that have experienced different levels of fishing activity demonstrate that chronic fishing disturbance leads to the removal of high-biomass species that are composed mostly of emergent seabed organisms. Contrary to the belief of fishers that fishing enhances seabed production and generates food for target fish species, productivity is actually lowered as fishing intensity increases and high-biomass species are removed from the benthic habitat. These organisms also increase the topographic complexity of the seabed which has been shown to provide shelter for juvenile fishes, reducing their vulnerability to predation. Conversely, scavengers and small-bodied organisms, such as polychaete worms, dominate heavily fished areas. Major changes in habitat can lead to changes in the composition of the resident fish fauna. Fishing has indirect effects on habitat through the removal of predators that control bio-engineering organisms such as algal-grazing urchins. Fishing gear resuspend the upper layers of sedimentary seabed habitats and hence remobilize contaminants and fine particulate matter into the water column. The ecological significance of these fishing effects has not yet been determined but could have implications for eutrophication and biogeochemical cycling. Simulation results suggest that the effects of low levels of trawling disturbance will be similar to those of natural bioturbators. In contrast, high levels of trawling disturbance cause sediment systems to become unstable due to large carbon fluxes between oxic and anoxic carbon compartments. In low energy habitats, intensive trawling disturbance may destabilize benthic system chemical fluxes, which has the potential to propagate more widely through the marine ecosystem. Management regimes that aim to incorporate both fisheries and habitat conservation objectives can be achieved through the appropriate use of a number of approaches, including total and partial exclusion of towed bottom fishing gears, and seasonal and rotational closure techniques. However, the inappropriate use of closed areas may displace fishing activities into habitats that are more vulnerable to disturbance than those currently trawled by fishers. In many cases, the behaviour of fishers constrains the extent of the impact of their fishing activities. Management actions that force them to redistribute their effort may be more damaging in the longer term