On a trawled north Pacific seamount, reductions of benthic megafauna abundance, diversity, and ecosystem function are correlated with increased evidence of fishing.

Abstract

High-seas seamounts in the Hawaiian-Emperor Seamount Chain are exposed to bottom-contact fisheries, i.e., historical and contemporary trawl, coral tangle net, longline, and gillnet fisheries, that disturb vulnerable marine ecosystem megafauna, such as deep-sea corals and sponges. Koko Guyot is the largest of these features and has experienced destructive fishing practices for more than half a century. To better understand the state of vulnerable megafaunal communities on Koko, the submersibles Pisces IV and V obtained high-quality imagery from replicate transects of 500 m length at two sites along 400, 500, and 600 m depth contours. Visual evidence of fishing, trawl scars and anthropogenically sourced debris, were compared to the abundance, diversity, and ecosystem function of benthic megafauna. Multiple faunal assemblages were observed, ranging from depauperate communities dominated by sea urchins and cup corals to diverse coral gardens with octocorals and scleractinian reef-forming species. Overall, megafaunal abundance, diversity, and metrics of ecosystem function were significantly negatively correlated with increased visual evidence of fishing. Coralliid octocorals were previously targeted by fisheries on the seamount chain and were found in low abundance on Koko, supporting the evidence for disturbance. Yet the pockets of diverse octocoral gardens with small colonies of reef-forming scleractinian species support the presence of remnant or recovering populations. Therefore, Koko appears to host a mosaic of disturbed, recovering, and remnant communities requiring protection from future disturbance. This study highlights the importance of fine-scale analyses for assessing disturbed, remnant and recovering communities.