The substantial first impact of bottom fishing on rare biodiversity hotspots: a dilemma for evidence-based conservation

The present study set out to describe the impact of a single first passage of two types of bottom fishing gear on rare protected shellfish beds formed by the horse mussel Modiolus modiolus. From the available evidence it was clear that one of the study sites had been trawled and the other had been scallop-dredged. Divers collected high-resolution video imagery of epifauna from quadrats at the two study sites and directed infaunal samples at one.
The total number of epifaunal organisms was substantially less after a single pass of a trawl (90%) or scallop dredge (59%) as was the diversity of the community and the total number of Modiolus modiolus. A year later no recovery was evident at the site where the trawl had passed and substantially less infauna were recorded in the trawl track. The relative severity of the impact of the two types of fishing gear is unexpected and may reflect differences in community between the two sites or the undisturbed status of the habitats compared to previous studies. As a ‘priority habitat’ the nature of the impacts described on M. modiolus communities are important to the development of indicators of ‘Good Environmental Status’ under the European Union Marine Strategy Framework Directive.
Conservation managers are under pressure to support decisions with good quality evidence. Elsewhere indirect studies have shown declines of M. modiolus communities in fishing grounds but, given the protected status of the habitat, premeditated demonstration of direct impact is unethical and / or illegal in Marine Protected Areas. The present study therefore provides a unique opportunity to directly investigate the scale of the impact from bottom fishing gear whilst also to reflect on the paradox of evidence – based conservation management.