Disappearance of white sharks leads to the novel emergence of an allopatric apex predator, the sevengill shark

Neil Hammerschlag,Monique Fallows,Chris Fallows,Lacey Williams

doi:10.1038/s41598-018-37576-6

Abstract

Despite global declines of apex predatory sharks, evidence for ecosystem consequences remains limited and debated. This is likely a result of both the logistical difficulties of measuring such processes in marine systems and also due to shifting baselines, whereby the ecosystem changes have occurred prior to monitoring. Between 2000–2018, we conducted standardized monitoring of white shark (Carcharodon carcharias) abundance patterns (N = 6,333 shark sightings) and predatory activity (N = 8,076 attacks on seals) at Seal Island, a Cape fur seal (Arctocephalus pusillus pusillus) colony in False Bay, South Africa. Over the 18-year study, declines in white shark abundance and attack rates were documented between 2015–2018, with anomalous lows occurring in 2017 and 2018. This included prolonged periods of complete white shark absence from Seal Island. The disappearance of white sharks from Seal Island coincided with the unprecedented appearance of sevengill sharks (Notorynchus cepedianus; N = 120 sightings), an otherwise allopatric kelp-associated apex predator in False Bay. We also recorded a sevengill shark attacking a live seal in the absence of white sharks. These data provide empirical evidence for behavioral shifts in an allopatric marine predator following the decline and disappearance of white sharks from a foraging site. This study demonstrates the importance of historical data and long-term monitoring for disentangling ecological consequences of apex predator declines.

Highlights

It has been suggested that the loss of apex predators is humankind’s most pervasive influence on the natural world, with far reaching ecological effects via trophic downgrading[1]
Since 2000, we have been monitoring the relative abundance and predatory behavior of white sharks (Carcharodon carcharias) at Seal Island, an offshore rookery inhabited by ± 60,000 Cape fur seals (Arctocephalus pusillus pusillus), in False Bay, South Africa (Fig. 1)
White sharks concentrate their movements at Seal Island during colder months of the year (May-September) to actively predate on Cape fur seals[8] (Fig. 2A)

Summary

Seal Island Millers Point

Predation rates by white sharks on Cape fur seals occurring in 2014 (Fig. 3), after which predation rates declined. In 2017 and 2018, we recorded for the first time in our surveys, prolonged periods (10+ consecutive sampling days) of complete white shark absence, even during the colder months when white shark abundance and hunting activity had historically peaked (Fig. 5) This included no observations of white sharks over 20 consecutive sampling days that occurred between 27 March and 21 May 2017 and again over 60 consecutive www.nature.com/scientificreports/. On 28 March 2018, at 0945 h, during a period when no white sharks were recorded from our surveys at Seal Island, an estimated 2-m in length sevengill shark rose to the surface and attacked a young-of-the year Cape fur seal Taken together, these data suggest that the emergence of sevengill sharks at Seal Island was associated with the disappearance of white sharks. Without historical data and time-series monitoring[28], documenting and disentangling ecological consequences of marine predator declines and recoveries will likely continue to prove challenging

Methods

Additional Information