Potential Impacts of Deep-Sea Trawling on the Benthic Ecosystem along the Northern European Continental Margin: A Review
John D. Gage, J. Murray Roberts, John P. Hartley, and John D. Humphery
Abstract. Little is known of the sensitivities of deep seabed communities to anthropogenic disturbance. We collate and summarize the sparse data related to effects on the seabed of deep-sea trawling and provide a new analysis of potential impacts on the benthic community in this still largely pristine environment. We concentrate particularly on the northern European continental margin. Here, the upper continental slope is now being impacted by new demersal fisheries using aggressive trawling techniques. Direct effects are caused largely by the physical impact of the trawl, where emergent sessile epifauna, such as corals and massive sponges, are particularly vulnerable to damage. Damage to coldwater coral reefs in the northeastern Atlantic is irrecoverable at ecological time scales. Large, semiburied glacial drop stones are uprooted in heavily trawled areas, impacting their rich covering of sessile epifauna. Seabed photographs suggest that trawl scour marks on soft sediment are widespread and persist longer than those in shallow sediments because of low rates of natural sedimentation. Loss of biogenic habitat complexity in the sediment and destruction or disruption of microhabitat provided by sessile epifaunal organisms caused by dragging trawls over infaunal communities disrupts sediment habitat structure. The scale of such physical impact, including possible smothering effects caused by sediment resuspended by the trawl, is unknown. Physical damage to sediment-rooted sessile epifauna, such as glass sponges, is inferred from seabed photographs, while removal of large motile epibenthic predators may alter community structure and ecosystem processes. Indirect effects on the deep seabed sediment community probably conform to disturbance effects seen in coastal benthos. High species diversity, rich in rare species, make it vulnerable to local-scale species loss. Effects of discards arriving as food falls to the bed are unknown and may cause a shift in the balance of ecological function through expansion of natural scavenger populations. We offer a unifying hypothesis from a deep-sea perspective. Coastal seas currently reflect stages in seabed community modification that predate rigorous scientific studies. Therefore, substantial habitat alteration and change in ecosystem function caused by loss of the most vulnerable, mainly sessile epibenthic species, probably occurred a long time ago. Deep-seabed areas off Europe are now starting to experience this early, probably most visible, stage from a previously pristine state.