Abyssal demersal fish fauna composition in two contrasting productivity regions of the Crozet Plateau, Southern Indian Ocean

Superimposed on the topographically uniform Crozet Plateau is an annually occurring pronounced gradient of surface productivity during the austral summer. Primary productivity is enhanced due to leached dissolved iron from the Crozet Islands resulting in high nutrient input to the seafloor east of the islands (+Fe; M5), and low nutrient input in the south (HNLC; M6). The demersal fish community at the +Fe site, 4160m, depth, and the HNLC site, 4190m depth, were sampled by trawl. Demersal fish species richness was greater at the +Fe site with 14 vs. 10 demersal fish species. The same major taxonomic groups were present at both sites, with the macrourids dominating both abundance and biomass. At both sites the macrourids and Bathypterois oddi dominate numerically, and the macrourids dominate biomass. Biomass was 374kgkm−2 at +Fe and 199kgkm−2 at HNLC and abundances were 1336 fishkm−2, and 881 fishkm−2, respectively. Despite elevated demersal fish biomass, biodiversity and abundance at +Fe compared with HNLC, an imbalance in sampling regime resulted in no statistical difference between the two sites, calling in to question current deep demersal fish sampling on multidisciplinary cruises. ► The demersal fish of the Crozet Plateau were sampled by trawl. ► Two sites with contrasting productivity were compared (M5, +Fe and M6, HNLC). ► Macrourids dominate both abundance and biomass at both sites. ► There is no statistical difference between sites due to inadequate sampling. ► Dedicated trawling cruises are required to enable appropriate numbers of replicates.