Nursery grounds, movement patterns and growth rates of dusky sharks, Carcharhinus obscurus: a long-term tag and release study in South African waters

Knowledge of movement patterns of sharks in coastal waters is critical for the structuring of regional management plans. Through a long-term tag-recapture program, 9716 dusky sharks (Carcharhinus obscurus) were tagged and released along the east coast of South Africa. A total of 648 C. obscurus, principally small sharks (< 100 cm PCL), were recaptured. Most recaptures were within 100 km of the tagging location in the nursery habitat in KwaZulu-Natal (KZN) with small scale movements occurring throughout the year. Forty-eight C. obscurus undertook movements >200 km that indicated a southerly migration between KZN and Eastern/Southern Cape (E/SC) between June and November. Seasonal northerly migrations were less well defined. The largest southerly and northerly movements were 1323 km and 1374 km, respectively. For sharks moving 1-100 km south from their tagging locality in KZN, an increase in displacement occurred between June and September identifying animals beginning their migration to the E/SC. With increasing displacement, there was also an increase in minimum swimming speed. Calculated growth rates of small sharks of 10.3-11.5 cm year(-1) were in agreement with current literature values. Established tag-recapture programs provide an important tool in understanding the ecology of early life-stages of coastal shark species.