Natural and fishing disturbances combine to drive seafloor conservation outcomes from spatial closures on Georges Bank

Abstract

Marine reserves have emerged as a leading management response to address benthic habitat impacts from bottom-towed fishing gears; however, empirical evaluations of the efficacy of spatial closures for seafloor conservation are lacking. Here we assessed the effectiveness of spatial closures for benthic habitat management by comparing seafloor communities at sites closed to bottom-towed gears against adjacent areas open to harvest at two regions of Georges Bank, a productive fishing ground in the northwest Atlantic. We characterized fishing and natural seafloor disturbances at surveyed areas to assess the relative strength of these stressors. Fishing intensity in locations open to harvest was high, with annual area swept by bottom-towed gears equivalent to 1.0 and 4.2 times the total site area at the two surveyed regions. Vertical heights for some sessile taxa were lower in harvested areas indicating that bottom fishing can crop upright organisms. However, after 17 years of fishing closure, seafloor communities inside reserves were remarkably comparable in composition and structure to those in adjacent open areas, with low differentiation in the prevalence and density of mobile predators, sessile epifauna, or infauna. Estimated benthic shear stress forces across the study region were strong and sufficient to regularly mobilize seabed sediments. These results highlight that natural disturbances from currents and tides can be a powerful driver of benthic community dynamics in high energy shelf ecosystems such as Georges Bank. Thus, effective management of benthic habitats through spatial fishing closures warrants consideration of combined natural and anthropogenic disturbance regimes in siting reserve locations.