Analysis of sightings of white sharks in Gansbaai (South Africa)

P Micarelli,D Bonsignori,L J V Compagno,A Pacifico,C Romano,F R Reinero

doi:10.1080/24750263.2021.1892216

Abstract

In Gansbaai (South Africa), at Dyer Island Nature Reserve, a large White shark population is present and can be observed due to the support of local ecotourism operators authorised to reach the field observation sites. Between 2009 and 2019, it was possible to create a database including information about each individual observed. In total, 423 white sharks were sighted during 462 direct observation hours from the boat, that included 220 hours from the diving “cage”. The mean sighting rate was 0.91 (range 0.18–1.53) sharks per hour and sighting rates dramatically declined in the last three years of the study period. Ninety-nine unique Photo-Ids of the dorsal fin were collected and only five re-sightings occurred, which indicate a transient behaviour for the Gansbaai White shark population. The sex ratio showed that females were always prevalent over males throughout the duration of the observations: the ratios were 1:2.2:0.8 for males, females, and unsexed sharks, respectively, and showed the prevalence of immature female individuals (immature: 51 males, 201 females, and 40 unsexed; adults: 49 males, 14 females, and 1 unsexed; undefined maturity: 5 males, 19 females, and 43 unsexed sharks). The predominance of immatures only applies to the females; there were as many immature males (51) as mature (49). The total length for all the individuals was between 150 cm and 500 cm (mean 308 cm, n = 423) with few young-of-the-year and adults recorded, indicating that Gansbaai Area is not a nursery area nor an adult aggregation site, but a seasonal feeding ground. The interannual sighting trend showed a consistent long-term increasing peak (ca. 4–5 years) and this could confirm that, in Gansbaai, the White shark frequency is not affected by ecotourism but, since 2017, a consistent loss of sightings was also due to recorded transient killer whales’ unusual fatal attacks.

Highlights

The White shark Carcharodon carcharias (Linnaeus 1758) is a protected species in many countries such as United States, South Africa, New Zealand, Australia, and Mexico (Fergusson et al 2009) and this species has been categorised as Vulnerable since 1996 (Fergusson et al 2009)
In South Africa, overall estimates by Cliff et al (1996) were 1279 sharks (95% confidence limits, 839–1843) from Richards Bay region in Kwazulu-Natal to Struis Bay in Western Cape; Ferreira and Ferreira (1996) recorded 255 sharks in False Bay, Gansbaai, Struis Bay, Mossel Bay, and Algoa Bay; Towner et al (2013a) in Gansbaai identified 532 unique individuals and estimated the super-population size of 908; Andreotti et al (2016) estimated the South African super population at 438 sharks, but one year later Irion et al (2017) stated that, due to the complex stock structure of White sharks and the model assumptions made by Andreotti et al (2016), the conclusions drawn could not be supported by their methods and data
Sightings varied between 2009 and 2016, with two peaks in 2011 (63 sightings) and 2015 (65 sightings), were found: (1) the sighted female white sharks were always higher than the sighted males; (2) female sighting rates were higher than the mean (0.50) over time (6 years overall); (3) when female sighting rates were lower than the mean, for example, in 2012, 2015, and 2019, unsexed sighting rates were larger and they had probably been females; (4) male sighting was recorded during the 11 years of observations and rates were generally constant over time, contrary to female ones

Summary

Introduction

The White shark Carcharodon carcharias (Linnaeus 1758) is a protected species in many countries such as United States, South Africa, New Zealand, Australia, and Mexico (Fergusson et al 2009) and this species has been categorised as Vulnerable since 1996 (Fergusson et al 2009). This shark is relatively abundant in only the following eight “Great White hot spots” (Bass 1975; Ferreira & Ferreira 1996; Martin 2003; BecerrilGarcìa et al 2020; Leone et al 2020): central California (especially the Farallon Islands and Año Nuevo Island), central Chile (from Punta Angamos to Punta Lavapie), New England, the Mediterranean Sea, South Africa (especially the Indian Ocean side of the Western Cape and the Southern Cape regions), southern Australia, New Zealand, and Japan Even if the estimates of the Gansbaai and South African

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