Population size of neonate and juvenile dusky sharks Carcharhinus obscurus in the Port of Ngqura, South Africa

Abstract

Identifying nearshore waters that serve as important habitats for neonate and juvenile sharks is crucial for effective fisheries management. This study examined the abundance  of neonate and juvenile dusky Carcharhinus obscurus within the Port of Ngqura, on the south-east coast of South Africa, between September 2006 and August 2007 using a combination of shore-angling catches and markrecapture data. During this period, the port was non-operational and still undergoing construction. A total of 480 dusky sharks, ranging in size from 50 to 123 cm (precaudal length), was captured. Catch per unit effort was greatest between October and February, peaking in November at 0.51 sharks angler^–1 h^–1. Of the 219 C. obscurus tagged and released during the study period, 37 (16.9%) were recaptured. Sharks were at liberty from 0 to 409 days with the majority (89.2%) recaptured within the port. Using a Jolly-Seber open population model, the abundance of sharks within the port was estimated at 552 (95% CI: 422–765). Annual apparent survival probability was 0.38 (95% CI: 0.30–0.46), with an average annual recapture probability of 0.28 (95% CI: 0.19–0.39). This study demonstrates that the Port of Ngqura is an important summer habitat and core activity zone for both neonate and juvenile dusky sharks. 

Keywords: abundance, mark-recapture

African Journal of Marine Science 2011, 33(2): 255–261