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

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.

Tooth morphology elucidates shark evolution across the end-Cretaceous mass extinction

“…However, unlike these aquatic tetrapods, Cretaceous lamniform apex predators were supplanted by equally large-bodied hexanchiforms in some earliest Paleocene ecosystems [ 92 ]. This accords with our morphospace overlap of anacoracids and hexanchids (Fig P in S1 Text ) and is further compatible with some modern shark communities, in which the hexanchid, Notorynchus cepedianus , is known to invade apex predator niches once vacated by the lamnid, Carcharodon carcharias [ 93 ]. Unfortunately, shark body size is difficult to estimate accurately from fossil shark teeth [ 94 , 95 ], and the paleoecology of Cretaceous anacoracids and hexanchids is poorly understood.…”

The upper Miocene Deurne Member of the Diest Formation revisited: unexpected results from the study of a large temporary outcrop near Antwerp International Airport, Belgium

“…Modern great white sharks, closely related to the common Cosmopolitodus hastalis, often swim close inshore, attracted by pinniped colonies. Where great white sharks disappear, their niche is often taken by adult sevengill sharks (Notorynchus) (Hammerschlag et al, 2019). Recent tiger shark Galeocerdo also preys on turtles and large seabirds (see e.g.…”

Mercury isotopes as tracers of ecology and metabolism in two sympatric shark species

“…Within these complex ecosystems, long-lived organisms at the top of food webs usually display the highest Hg levels, due to bioaccumulation and biomagnification processes, which correspond to the increase of MeHg and other contaminant concentration over time and with the trophic position, respectively. Populations of marine predators are declining worldwide (Baum et al, 2003;Ferretti et al, 2008;Myers and Worm, 2003), including coastal sharks (Hammerschlag et al, 2019;Roff et al, 2018). The harmful effects of MeHg could thus represent an additional threat to the most depleted shark populations.…”