Impacts of demersal fishing on sedimentary organic matter: a global meta-analysis.

Marine sediments represent a hot spot of ecosystem services, but their integrity is increasingly put at risk by anthropogenic disturbance, most notably by demersal fisheries. The need for global action to minimize the impacts of destructive fishing techniques on the marine environment is urgent. The urgency to act, however, needs to be met with caution, as scientists are pushed for action, global predictions of trawling impacts are tempting, yet poor validation and oversimplified assumptions can lead to large uncertainties. We visit the scientific literature on trawl studies to map out current evidence from the literature and report on a global meta-analysis to quantify the effects of demersal fishing on sedimentary and biogeochemical properties.  Studies examining the direct impacts of bottom fishing revealed significant reductions in total organic carbon (TOC; -10%), chlorophyll-a (Chl-a, -10%), phaeopigments (-21%) and proteins (-24%), and largest impact was detected on surficial sediment (0-2 cm). Implications of methodological biases as a result of inappropriate sampling in trawl studies and the importance of context-dependency for effect size is flagged up. Environmental parameters such as bottom current velocity and surface primary productivity significantly influenced both the direction and magnitude of fishing effects. We highlight where the lack of evidence lies that might create bias in regional and global models that require empirical data for validation. The objective is to summarize current knowledge and to direct future studies towards more robust analysis of the impacts of bottom trawling, which will provide a basis of sound advice to fisheries managers and policy makers.