The influence of kelp density on white shark presence within the Dyer Island nature reserve, South Africa

Large-scale and small-scale natural barriers have the ability to mediate the ecological dynamics within a region. In some instances, these barriers greatly influence the presence of predators and prey on spatio-temporal scales. For this study, we aimed to assess how varying densities of sea bamboo (Ecklonia maxima), a kelp species found within South African waters, could influence the presence of C. carcharias. Using baited remote underwater video systems (BRUVS), thirty-one different C. carcharias were identified and a total of 135 h of video were collected in high, moderate, and low kelp densities to create an organismal sighting index. Generalized linear models illustrated that the best fit model included the main effect of kelp density, with a clear inverse relationship between kelp density and C. carcharias sightings. However, although zero sharks were sighted within the high kelp density regions, ten different C. carcharias were sighted within the moderate kelp density regions, illustrating that the broad notion that C. carcharias do not navigate through kelp ecosystems is false and requires more specificity (e.g. high density kelp areas may result in decrease C. carcharias presence). In contrast, kelp density had no significant influence on the presence of five other elasmobranch species detected by the BRUVS. This study demonstrated that highly dense kelp forests (i.e. ≥1 stalk of kelp per 1 m2) serve as a local natural barrier for large C. carcharias; however, the need for a more inclusive analysis (e.g. routine inclusion of current speed and multi-location assessment) is warranted.