Movements and growth rates of the broadnose sevengill shark Notorynchus cepedianus in southern Africa: results from a long-term cooperative tagging programme

Abstract

Top predators are important components of healthy ecosystems but are at risk of overexploitation due to insufficient data on life-history characteristics and population dynamics to guide management. We investigated the movements and growth rates of the broadnose sevengill shark Notorynchus cepedianus in southern Africa, using data from the Oceanographic Research Institute’s Cooperative Fish Tagging Project (ORI-CFTP). A total of 3 513 N. cepedianus were tagged between 1984 and 2017, with 195 (5.6%) recaptured. Distribution ranged from Cape Fria, northern Namibia, to the Great Kei River, Eastern Cape, South Africa, representing a range extension of ~200 km in comparison with that previously reported. Most captures occurred along the west coast of South Africa, and recaptures showed connectivity between the west and south coasts, but not between South Africa and Namibia. Site fidelity was evident, with the majority of recaptures (66.6%, n = 130) occurring <50 km from the release site, and many recaptures (32.8%, n = 64) occurring <1 km from the release site. Few sharks (22.6%, n = 44) were recaptured >100 km from the release site, while only three sharks (1.5%) were recaptured >500 km from the release site. Growth rates for N. cepedianus at reference precaudal lengths of 100 cm and 160 cm were estimated to be 4.7 cm y^–1 and 4.0 cm y^–1, respectively, which are lower than previous growth estimates. The slow growth rate and site fidelity evident in N. cepedianus, coupled with being captured in several commercial fisheries, necessitate the need for improved management of this species.

Keywords: apex predator, displacement, population connectivity, range extension, site fidelity, tag–recapture data