Schlacher, and 6 co-authors reported in 2014 that 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 on Hawaiian Seamount Chain; this scenario requires that environmental guidelines for mining operations on seamounts be developed. Here, the authors provide the biological information essential for effective conservation planning of deep-sea features targeted for such mining.
The authors concluded that 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.
Several implications for the design of spatial management and conservation tools with respect to mining emerge.The authors found clear spatial patterns in the composition
of the seamount fauna in the central Pacific that have conservation implications for seamounts targeted for future resource exploitation, we caution against extrapolation of all findings to other areas of commercial mining interests in the deep sea. Some of the general lessons and recommendations coming from this study will be important for other areas, but conservation and management plans need to be site-specific and based on extensive and robust scientific evidence obtained for a specific situation; these studies may also widen the taxonomic ambit beyond the benthic macrofauna investigated here.
Spatial planning measures for conservation have been proposed by the ISA in relation to the mining of cobalt-rich crusts, based on Hein et al. (2009) who developed a minesite model. This model is based on identifying multiple sub-blocks or ‘mine sites’ within a seamount, each mine site being 20 km2, and a total area of about 260 km2 (or 13 mine sites) being needed for a 20-year commercial operation.
These mine sites would be identified following exploration within 18 larger blocks (each 100 km2) distributed among a ‘confinement area’ (300,000 km2) within the region of cobalt-rich crusts. Hein et al. (2009) gave a hypothetical example with mine sites distributed over four seamounts (of six initially ‘explored’), two of which are adjacent. In their example, mine sites were on the summits of the seamounts, at approximately the same water depth, and clustered in contiguous groups of 2–5. Hein et al. (2009) estimated that only about 4% of the total surface area of a seamount < 2500 m would be mined. This type of mining site structure has been adopted in the ISA regulations (ISA, 2012).
The authors concluded that by noting that some of the results of our study (summarized below, 1–3) could be used by the ISA to refine the mine model of Hein et al. (2009) by including spatial planning strategies to conserve seabed biodiversity. For example, result 1 (species composition of the seamount fauna is strongly determined by depth) highlights concerns over concentrating mine sites at similar depths. Hein et al. (2009) estimated that only a small proportion of the seamount would be mined using their model. However, within the narrow depth stratum in which the mine sites are to be concentrated a much greater proportion of the seabed, and therefore the assemblages specific to this depth horizon, would be affected by mining. Thus, to protect assemblages unique to certain depth strata it may be better to consider distributing mine sites over as wide a depth range as possible within and among seamounts. Result 2 (geographic distance does not predict biological distinctiveness) cautions against the siting of mine sites on adjacent seamounts.
Because seamounts in proximity in the central Pacific do not necessarily have similar benthic assemblages (within a depth stratum), mined seamounts should be dispersed as widely as possible throughout the containment area. Result 3 (marked species turnover at distances > approximately 2 km) suggests that mine sites of 20 km2 (i.e. > 2 km in linear extent) will encompass multiple ecological communities and hence affect a greater range of ecological features than would smaller-sized mine sites. Although we recognize that smaller mine sites are likely to be less economically efficient (and thus unlikely to be adopted), our result suggests that for conservation purposes they are less likely to impact distinct ecological assemblages.
In summary, the authors found:
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; and
3) ideally, mining blocks on individual seamounts should not exceed 2 km 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.
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