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RESEARCH ARTICLE
Contents Vol 60(6)

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

Nigel E. Hussey A E , Ian D. McCarthy A , Sheldon F. J. Dudley B C and Bruce Q. Mann D
+ Author Affiliations
- Author Affiliations

A School of Ocean Sciences, College of Natural Sciences, Bangor University, Menai Bridge, Anglesey, LL59 5AB, UK.

B KwaZulu-Natal Sharks Board, Private Bag 2, Umhlanga Rocks 4320, South Africa.

C Biomedical Resource Unit, University of KwaZulu-Natal, Durban 4056, South Africa.

D Oceanographic Research Institute, PO Box 107012, Marine Parade, Durban 4056, South Africa.

E Corresponding author. Email: nigel.hussey@bangor.ac.uk

Marine and Freshwater Research 60(6) 571-583 https://doi.org/10.1071/MF08280
Submitted: 3 October 2008  Accepted: 16 January 2009   Published: 19 June 2009

Abstract

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.

Additional keywords: displacement, GROTAG, migration, nursery concept, residency, tag–recapture.


Acknowledgements

We are indebted to the anglers and fishermen who participate in the ORI-WWFSA tagging program and who provided the data to enable this study to take place. The use of Colin Simpfendorfer’s GROTAG spreadsheet enabled aspects of the growth analysis. Comments provided by Sabine Wintner, Sean Fennessey, Simon Oliver and two anonymous referees greatly improved this manuscript. Gratitude is expressed to David Cocks for discussions regarding aspects of this work. This research, including the tag–recapture program, complies with the laws of South Africa. N. E. H. was supported by a NERC PhD studentship (NER/S/A/2005/13426).


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