Elasmobranchs and the Inshore Reefs of Southern Mozambique

Abstract

The coastal waters off southern Mozambique host a diverse community of elasmobranchs. The complexity of studying sharks and rays in remote locations means that limited research attention focusses on its marine environment. As the region is famed for its coastal reefs and the diversity of its elasmobranch species, a novel opportunity was presented to study elasmobranch species that have become rare in most other regions. This thesis aims to investigate the relationship between elasmobranch species and coastal reef habitat in southern Mozambique. This aim is addressed by investigating the frequency with which elasmobranch species visit reef systems and by comparing this with the environmental and biological conditions on the reefs. A reef camera trap was deployed 60 times on four reefs to record elasmobranch visits and to estimate the relative abundance of each reef’s fish and coral communities. SCUBA-based transects (n = 738) were conducted on 18 reefs in the same region to count elasmobranchs and measure environmental parameters that influence the presence of elasmobranchs on the reefs. Two large Carcharhinus sharks were observed in low numbers during video and SCUBA observations. To monitor their interactions with regional reefs, 18 acoustic tags were fitted to Carcharhinus limbatus and Carcharhinus leucas individuals. The sharks were tracked on 13 omnidirectional acoustic receivers positioned on reefs throughout the Inhambane province of southern Mozambique. The community composition of reef fishes appeared to influence the species of elasmobranch that frequented each survey area. Sharks visited reefs with the most abundant piscivorous fish life, which we determined to be linked to foraging opportunities. Small-bodied stingrays were only observed visiting shallow reefs near intertidal sand flats. Pelagic rays (e.g. Mobula spp.) were most often observed while cleaning on reefs that were proximate to their foraging grounds.Elasmobranch abundance on reefs was found to vary across several spatiotemporal and environmental parameters, resulting in patterns of reef use across various temporal scales. Most elasmobranch species became abundant on reefs during the summer, and tide, water temperature, and moon illumination were found to be strong predictors of their presence and abundance. Closely related species were similar in their responses to the measured environmental and spatiotemporal parameters and these species visited the same reefs with specific biological characteristics. Reducing pressures on the reefs that elasmobranch species rely on, and eliminating the extraction of elasmobranchs around these reefs, could benefit the populations of vulnerable elasmobranchs off the coast of southern Mozambique.