ROV assessment of mesophotic fish and associated habitats across the continental shelf of the Amathole region

Abstract

Understanding how fish associate with habitats across marine landscapes is crucial to developing effective marine spatial planning (MSP) in an expanding and diversifying ocean economy. Globally, anthropogenic pressures impact the barely understood temperate mesophotic ecosystems and South Africa's remote Amathole shelf is no exception. The Kei and East London region encompass three coastal marine protected areas (MPAs), two of which were recently extended to the shelf-edge. The strong Agulhas current (exceeding 3 m/s), which runs along the narrow shelf exacerbates sampling challenges. For the first time, a remotely operated vehicle (ROV) surveyed fish and their associated habitats across the shelf. Results indicated fish assemblages differed between the two principle sampling areas, and across the shelf. The number of distinct fish assemblages was higher inshore and on the shelf-edge, relative to the mid-shelf. However, the mid-shelf had the highest species richness. Unique visuals of rare Rhinobatos ocellatus (Speckled guitarfish) and shoaling Polyprion americanus (wreckfish) were collected. Visual evidence of rhodolith beds, deep-water lace corals and critically endangered endemic seabreams were ecologically important observations. The ROV enabled in situ sampling without damaging sensitive habitats or extracting fish. This study provided information that supported the Amathole MPA expansions, which extended protection from the coast to beyond the shelf-edge and will guide their management. The data gathered provides baseline information for future benthopelagic fish and habitat monitoring in these new MPAs.

Figure 1

Study area, the Amathole continental shelf, on the south coast of South Africa where the East London (triangles) and Kei (squares) sampling sites were located. The 50 m bathymetric depth contours; existing MPAs, which were declared as official reserves in 1984 and later proclaimed as MPAs in 2011; and the new MPAs, proclaimed in 2019 are indicated. The MPAs are divided into zones with different restrictions namely (1) no-take; (2) controlled fishing, where extraction and harvesting of marine life is allowed with restrictions and limitations; and (3) controlled pelagic fishing, where only pelagic linefishing of specific species is permitted. Map created in QGIS ver. 3.8 (https://qgis.org/en/site/) using shapefiles provided by the South African National Biodiversity Institute (https://www.sanbi.org/) and sampling sites locations generated by this study.

Figure 2

Fish images extracted from the ROV transect videos. Fish species that were endemic to South Africa include (a) Chrysoblephus cristiceps (dageraad) (critically endangered), (b) Chrysoblephus gibbiceps (red stumpnose) (endangered), and (c) Petrus rupestris (red steenbras) (endangered). While (d) Polysteganus undulosus (seventy-four) was critically endangered and endemic to southern Africa. A living (e) Rhinobatos ocellatus (speckled guitarfish) and shoaling (f) Polyprion americanus (wreckfish) were recorded on video for the first time.

Figure 3

Images of dominant habitat for substrate clusters: (a) sand, (b) rubble, (c) rhodoliths and (d) rock. Biota clusters included: (e) no biota, (f) algae, (g) soft corals & sponges and (h) fan coral; and relief clusters (i) flat and (j) low, were extracted from the ROV transect videos.

Figure 4

Spatial distributions of the fish assemblages on the Amathole continental shelf, located on the south coast of South Africa. Fish assemblages are clusters of species determined using a Multivariate Regression Tree that includes depth, substrate clusters, distance from shore and the two broad areas of East London and Kei. These fish assemblages are labelled 1 to 10. The 50 m bathymetric contours as well as existing and new Marine Protected Areas (MPAs) are indicated. The existing MPAs were declared as official reserves in 1984 and later proclaimed as MPAs in 2011 and the new MPAs were proclaimed in 2019. The MPAs are divided into zones with different restrictions namely (1) no-take, (2) controlled fishing, where extraction and harvesting of marine life are allowed with restrictions and limitations, and (3) controlled pelagic fishing, where only pelagic linefishing of specific species may occur. Map created in QGIS ver. 3.8 using shapefiles provided by the South African National Biodiversity Institute and fish assemblages generated in R statistical software(CRAN ver. 4.0.2).

Figure 5

Generalized Additive Models (GAMs) for species richness in relation to (a) depth and (b) distance to shore and substrate cluster, which was the benthic explanatory variable with the lowest p-value. In the line graphs, the solid lines represent the predicted abundance or probability of presence per grid cell, and the dashed lines illustrate 95% confidence intervals. In the habitat category plots, dots represent the predicted probability of presence and whiskers illustrate 95% confidence intervals. Graphs created in R statistical software (CRAN ver. 4.0.2).

Figure 6

Generalized Additive Models (GAMs) for the five most abundant species (a) Cheilodactylus pixi, (b) Serranus knysnaensis, (c) Pterogymnus laniarius, (d) Chirodactylus brachydactylus, (e) Chelidonichthys capensis predicted abundance (for solitary species a,b,e,d) and presence (for shoaling species c) in relation to distance to shore and the benthic explanatory variable with the lowest p-value. In the line graphs, the solid lines represent the predicted abundance or probability of presence per grid cell, and the dashed lines illustrate 95% confidence intervals. In the habitat category plots, dots represent the predicted probability of presence and whiskers illustrate 95% confidence intervals. Graphs created in R statistical software (CRAN ver. 4.0.2). Illustrations were drawn and copyright permissions granted by Isabella Foulis.