Environmental Influences on the Abundance and Sexual Composition of White Sharks Carcharodon carcharias in Gansbaai, South Africa

Alison V Towner,Malcolm J Smale,Oliver J D Jewell,Les G Underhill,Athanassios C Tsikliras

doi:10.1371/journal.pone.0071197

Alison V Towner, Malcolm J Smale + Show 3 more

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https://doi.org/10.1371/journal.pone.0071197

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Abstract

The seasonal occurrence of white sharks visiting Gansbaai, South Africa was investigated from 2007 to 2011 using sightings from white shark cage diving boats. Generalized linear models were used to investigate the number of great white sharks sighted per trip in relation to sex, month, sea surface temperature and Multivariate El Niño/Southern Oscillation (ENSO) Indices (MEI). Water conditions are more variable in summer than winter due to wind-driven cold water upwelling and thermocline displacement, culminating in colder water temperatures, and shark sightings of both sexes were higher during the autumn and winter months (March–August). MEI, an index to quantify the strength of Southern Oscillation, differed in its effect on the recorded numbers of male and female white sharks, with highly significant interannual trends. This data suggests that water temperature and climatic phenomena influence the abundance of white sharks at this coastal site. In this study, more females were seen in Gansbaai overall in warmer water/positive MEI years. Conversely, the opposite trend was observed for males. In cool water years (2010 to 2011) sightings of male sharks were significantly higher than in previous years. The influence of environmental factors on the physiology of sharks in terms of their size and sex is discussed. The findings of this study could contribute to bather safety programmes because the incorporation of environmental parameters into predictive models may help identify times and localities of higher risk to bathers and help mitigate human-white shark interactions.

Highlights

White sharks Carcharodon carcharias (Linnaeus 1758) are large apex predators that occur circumglobally in cool temperate marine systems [1]
Trips were made to offshore sites mainly from March to August and offshore trips were made in all months except December
The average number of white shark sightings was higher in autumn and winter months (March to mid-September), at Dyer Island and at Geyser Rock than in summer months (December to February) (Table 1 and 2, Figure 2)

Summary

Introduction

White sharks Carcharodon carcharias (Linnaeus 1758) are large apex predators that occur circumglobally in cool temperate marine systems [1]. Through advances in tagging technologies, that these predators undertake extensive pelagic migrations, crossing ocean basins and inhabiting tropical waters before returning and exhibiting fidelity to temperate coastal aggregation sites [8,9,10,11,12,13,14,15,16]. Hunting, mating and breeding have been suggested as possible causes of this migratory behaviour [8,10,13,20,21,22] Environmental cues such as changes in water temperature and upwelling influence the time spent in the pelagic and coastal phases of these migrations [8,11,14,15,16,18,20,21,23]

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