Megabenthic assemblage structure on three New Zealand seamounts: implications for seafloor massive sulfide mining

Seamounts are recognized for their biological importance and, more recently, mineral wealth. However, in most cases the biological information required to assess the risk to seamount assemblages from mining is lacking. This study uses towed video footage and environmental data to investigate the patterns of megafaunal distribution, assemblage structure and association with environmental variables, both within and amongst 3 seamounts along the Kermadec volcanic arc in the New Zealand Exclusive Economic Zone. These seamounts represent different levels of hydrothermal activity, with an overlapping depth range: Rumble II East has no history of hydrothermal activity, Brothers is hydrothermally active and Rumble II West is predominantly inactive. All 3 seamounts fall within an area previously licenced for the prospecting phase of seafloor massive sulfide (SMS) mining. In total, 186 putative taxa were identified from video samples and assigned to 20 assemblages. Both seamount and a priori defined habitat ( nested within seamount) contributed to explaining variation in assemblage structure, with a mixture of shared and unique assemblages found at each seamount. Magnetivity, as a proxy for hydrothermal activity, explained most of the variation in assemblage structure amongst seamounts, with depth, topography, substratum (and magnetivity for Brothers) explaining most within seamounts. Environmental management implications include the need to designate a network of 'set-aside' sites both within and amongst seamounts to adequately protect the range of faunal assemblages present. This study also suggests that inactive SMS areas may support faunal assemblages not found elsewhere within the region and would require suitable protection from mining activities.