Trends in sightings and population structure of white sharks, Carcharodon carcharias, at Seal Island, False Bay, South Africa, and the emigration of subadult female sharks approaching maturity

Abstract

A long-term sightings and photographic identification (photo-ID) database documented the inter-annual and monthly trends in white shark (Carcharodon carcharias) sightings, sex ratios and life-history stages at Seal Island, False Bay, South Africa, over the period 2004–2012. A total of 1105 sightings were recorded during 171 scientific surveys incorporating 577 h of observation (annual mean 64.10 h, range 23.71–178.69 h). The mean annual sighting rate was 1.87 (range 0.90–3.19) sharks per hour and sighting rates declined over the nine-year study period. It is unknown whether the decline resulted from an actual population decline, or was due to changes in shark distribution or environmental conditions, but it highlights the need for ongoing monitoring at this site. The overall sex ratio was 1.0: 1.0: 2.3 for male: female: unsexed sharks, respectively. Of the 1105 sightings, 39% (n = 433) were photo-ID’d, representing 303 individual sharks (112: 111: 80, male: female: unsexed) and 130 resightings. Of the 303 photo-ID’d sharks, 71% were sighted in a single year only, indicative of transient behaviour. Of the 29% of sharks that were resighted in more than one year, 65% were resighted in the following year, indicating site fidelity to Seal Island, whereas 35% skipped one or more years between encounters, indicative of a temporary absence. The majority (60%) of the photo-ID’d sharks were immature, 32% were subadult and only 8% were mature. No young-of-the-year sharks and few adults were recorded, indicating that Seal Island is not an adult aggregation site, nor a pupping or nursery area, but rather is best described as a seasonal feeding ground. Large females were rarely resighted again after they approached length-at-maturity (≥450 cm TL), whereas some adult males were recorded consistently across years. The emigration of subadult female sharks approaching maturity from Seal Island, combined with the small number of mature sharks of both sexes reported from any South African location, indicate that adult aggregation sites, and thus areas of reproductive importance, still remain unknown.

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Acknowledgements

This research was made possible with the financial support of the Save Our Seas Foundation through a grant provided to AAK. Bursaries for AMH’s MSc research were provided through the National Research Foundation (NRF- http://www.nrf.ac.za), the University of Cape Town and Prof. C. Moloney (Marine Biology Research Centre, UCT). Bursary funding for AAK was provided for her MSc research from the NRF and for her PhD research through a grant to CG from the NRF-SEA Change Programme. We are grateful to A. and J. Hewitt for their indirect financial support of this research and to K. Underhill, M. Hardenberg, T. Seckler, A. Casagrande and all other field assistants for their assistance with data collection. We acknowledge R.Ryklief for guidance in the development of the photographic catalogue, and we are grateful to the anonymous reviewers who provided useful feedback. All research protocols were approved by the University of Cape Town and data were collected adhering to the legal requirements of South Africa’s, Department of Environmental Affairs: Oceans and Coasts, permitting authority (permit # V1/1/5/1, V1/8/5/1).

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Hewitt, A.M., Kock, A.A., Booth, A.J. et al. Trends in sightings and population structure of white sharks, Carcharodon carcharias, at Seal Island, False Bay, South Africa, and the emigration of subadult female sharks approaching maturity. Environ Biol Fish 101, 39–54 (2018). https://doi.org/10.1007/s10641-017-0679-x

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Keywords

  • White shark
  • Sighting rate
  • Sex ratio
  • Photo-identification
  • Population
  • Life-history