Distribution and diversity of octocorals from longline by-catch around South Georgia, UK

Abstract

In recent decades man’s forays into the deep sea have been increasingly frequent and wide-reaching; the most immediately intense and widespread activity being fisheries. United Nations (UN) General Assembly Resolutions on Sustainable Fishing on the High Seas call for assessment of fishing impacts with the aim of helping fisheries to avoid damaging vulnerable marine ecosystems. Octocorals form the majority of by-catch in the bottom-longline fishery for Patagonian toothfish, Dissostichus eleginoides Smitt, 1898, around South Georgia. I employed several methods to investigate octocoral extent, location and diversity using by-catch from this fishery. Preliminary video analysis from camera arrays attached to bottom-longlines found whip corals interacted with longlines less than bushy, fanshaped corals or Stylasteridae. Stylasteridae were however very fragile with 42% of interactions likely resulting in mortality through dislodgement. Deep-sea octocorals are relatively understudied with many families and genera requiring revision and, as such, morphologically identifying octocorals is a difficult task; I thus investigated by-catch diversity by embedding octocorals caught from this South Georgia fishery into a wider Octocorallia phylogenetic tree. Three polyphyletic groups were resolved. Putative species numbers were estimated from morphological and phylogenetic analysis. Morphological identifications of one family of octocorals, Primnoidae, were compared to phylogenetic trees and the probability species were monophyletic tested using the statistical programme, Species Delimitation for Geneious. Six families, at least 21 genera (including three new discoveries) composing of a minimum of 34 species were caught as by-catch. Thouarella Gray, 1870, was the most common genus caught and a revision of this genus is presented. Using recently created fine scale (150 m by 150 m) bathymetry of South Georgia, a depth-derived megahabitat map was created and octocoral occurrence data used to predict octocoral habitat suitability. Shelf troughs, crest and slope areas were highlighted as highly suitable octocoral habitat which substantially overlaps fishing areas. Current management protection regulations were found to protect highly suitable octocoral habitat.