White sharks Carcharodon carcharias at Bird Island, Algoa Bay, South Africa

  • ML Dicken
  • MJ Smale
  • AJ Booth

Abstract

We present the first quantitative study of the occurrence, size and sex of white sharks Carcharodon carcharias at Bird Island, Algoa Bay. Twenty-two boat trips were made to Bird Island between November 2009 and October 2011 to chum for sharks. A total of 53 sharks was observed over the study period, ranging in size from 1.5 to 4.5 m total length (TL) and with the majority (60.3%) being <3 m TL. The sex ratio of sharks for which sex could be determined was not significantly different from unity. In both study years, sharks were present only in the winter between April and November with a maximum of 1.2 sharks sighted per hour. A zero-altered model, comprising a logistic regression to model presence/absence and a log-normal generalised linear model for abundance, showed that season explained the presence of sharks, peaking in mid-July, with abundance being significantly higher with higher barometric pressure. This study identified Bird Island as an important white shark aggregation site on the east coast of South Africa. These data are crucial not only to improve our understanding of white shark seasonal distribution and biology, but also for the long-term management and conservation of the species in South Africa.

Keywords: seasonality, zero-altered model

African Journal of Marine Science 2013, 35(2): 175–182

Author Biographies

ML Dicken
Department of Ichthyology and Fisheries Science, Rhodes University, PO Box 94, Grahamstown 6140, South Africa
MJ Smale
Port Elizabeth Museum at Bayworld, PO Box 13147, Humewood, Port Elizabeth 6013, South Africa; Department of Zoology, Nelson Mandela Metropolitan University, PO Box 77000, Port Elizabeth 6031, South Africa
AJ Booth
Department of Ichthyology and Fisheries Science, Rhodes University, PO Box 94, Grahamstown 6140, South Africa
Published
2013-08-06
Section
Articles

Journal Identifiers


eISSN: 1814-2338
print ISSN: 1814-232X