Seamount benthos in a cobalt-rich crust region of the central Pacific: conservation challenges for future seabed mining

Aim The benthic fauna of seamounts typically includes organisms that are slow-growing, long-lived and sensitive to mechanical disturbance, making seamounts susceptible to anthropogenic impacts. Such impacts may arise from mining cobalt-rich crusts, envisaged for seamounts in the central North Pacific; this scenario requires that environmental guidelines for mining operations on seamounts be developed. Here, we provide the biological information essential for effective conservation planning of deep-sea features targeted for such mining. Location Central North Pacific, Hawaiian Seamount Chain. Methods Spatial analysis of seamount benthos using a large biological dataset (>600 taxa) obtained from 144 submersible dives (depth range: 113-1985m) on 27 seamounts covering a distance of over 2200km of ocean. Results Benthic assemblages of invertebrates are structurally different between seamounts located inside and outside a region with cobalt-rich crusts. This spatial contrast results from variations in species composition and relative abundance of species, rather than differences in species richness, challenging historical notions of an impoverished cobalt-rich crust fauna in the region. Seamount assemblages also have high species turnover with depth and distance at the scale of individual seamounts, but geographic separation was a poor predictor of ecological separation for the region at large. Main conclusion Several implications for the design of spatial management and conservation tools with respect to mining emerge: (1) conservation of seamounts outside the cobalt-rich crust region is unlikely to capture the full range of ecological features found inside the region; (2) conservation areas need to encompass a broad bathymetric gradient; (3) ideally, mining blocks on individual seamounts should not exceed 2km in length. Overall, the life history characteristics and morphological traits of the deep-water invertebrate fauna typical of seamounts in the region imply that any recovery from mechanical impacts is likely to be very slow.