Demersal fish communities in KwaZulu-Natal, South Africa, indicate partial congruence with proposed conservation biozones

The KwaZulu-Natal shelf, on the east coast of South Africa, is inhabited by diverse communities of demersal fishes; however, previous studies deeper than 30 m have largely investigated these with extractive techniques. Using baited remote underwater videos (BRUVs), this study quantified how depth and substrate type correlated with the composition of demersal fish communities between 35 and 100 m, and assessed whether the communities displayed congruence with three benthic biozones, which were modelled using abiotic surrogates for purposes of conservation planning: Biozone 1 (warm; coarse sediment dominant; low seafloor oxygen, phosphate and organic carbon concentrations), Biozone 2 (flat; coarse sediment dominant; high seafloor oxygen and low phosphate and organic carbon concentrations) and Biozone 3 (shallow; warm; fine sediment dominant; low seafloor oxygen and higher organic carbon concentrations). A total of 118 fish taxa were recorded from 200 BRUV deployments conducted on different substrate types. All diversity indices differed significantly among substrate types, whereas depth correlated significantly with only the number of fish species per site and Margalef's diversity index. Multivariate analyses determined that substrate type and depth explained significant amounts of the variation in fish communities. A significant difference was also detected between the fish communities of Biozones 1 and 3, with Biozone 3 being the most distinct of the three biozones investigated. Overall congruence between fish communities and their associated biozones ranged between 41% and 45%. This suggests that, at a broad spatial scale, the demersal fish communities could be adequately represented by Biozone 3 and a second biozone comprising Biozones 1 and 2 combined.