Possible causes of a substantial decline in sightings in South Africa of an ecologically important apex predator, the white shark

Robin Fisher

doi:10.17159/sajs.2021/8101

Abstract

A decline in sightings of a top predator, the white shark (Carcharadon carcharias), in South Africa was quantified in order to identify possible causes for this decline. White shark sightings data across 8 years (2011–2018), recorded from a cage-diving vessel in Gansbaai are reported. A significant decline in mean total white shark sightings per boat trip (>6 in 2011 to <1 in 2018) and a 69% reduction in the probability of a sighting were found. Correlating with this decline in sightings is a rise in sightings of sevengill sharks (Notorynchus cepedianus) in False Bay and copper sharks (Carcharhinus brachyurus) in Gansbaai, as well as substantial ecosystem changes. The effects of lethal conservation measures such as the use of shark nets in KwaZulu-Natal; the direct and indirect effects of overfishing including a reduction in smoothhound (Mustelus mustelus) and soupfin (Galeorhinus galeus) sharks; and novel predation on white sharks are discussed as possible causative factors for this decline in white shark sightings. Significance:  The results of this paper highlight the need to reassess the impact of marine conservation initiatives and fishing practices. Failure to do so could seriously affect ecologically and economically important marine species. This paper reveals a potentially serious decline to the South African white shark population, characterised by a substantial decline in white shark sightings. This decline correlates with the overfishing of prey species, bycatch, the use of lethal gill nets and ecological changes such as the novel presence of orca. Better marine management is required if South Africa wishes to keep a healthy white shark population.

Highlights

Of the more than 1000 species of sharks and rays, approximately half are classified by the International Union for Conservation of Nature (IUCN) as vulnerable, endangered, critically endangered or near threatened.[1]
The main finding of this research project was that white shark sightings substantially decreased from >6 sharks per trip to
The aforementioned project estimated that the white shark population consisted of between 353 and 522 individuals, with a small contemporary effective population size of 333.6 if the sightings data in this paper are indicative of a decline in the white shark population, this population is at serious risk of extinction

Summary

Introduction

Of the more than 1000 species of sharks and rays, approximately half are classified by the International Union for Conservation of Nature (IUCN) as vulnerable, endangered, critically endangered or near threatened.[1] Between 63 million and 273 million sharks, belonging to 61 species, are killed by human activity each year – mainly from legal and illegal fishing operations.[2] The white shark, Carcharadon carcharias, is an example of one of these species.[3] White sharks perform important regulatory roles in coastal ecosystems by influencing the structure and function of communities via both direct and indirect predatory effects.[4] Their removal has been shown to cause reductions in marine biodiversity and cause negative trophic cascades.[5] For instance, white sharks control the population of meso-consumers, like Cape fur seals (Arctocephalus pusillus).[6] Without white shark predation, and a reduction in the threat of their predation, the fur seal foraging range has extended, resulting in the removal of refugia for seal prey species[7] which has reduced the populations of economically significant fishery species.[4,5,6]

Methods

Results

Discussion

Conclusion