Running scared: when predators become prey

Abstract

Apex predators play an important role in structuring food webs and are thus key components of healthy, stable ecosystems. While the loss of apex predators has been shown to disrupt ecosystems and trigger trophic cascades, the introduction of novel apex predators to functionally intact systems is less well understood. False Bay, South Africa, is an aggregation site for both white (Carcharodon carcharias) and broadnose sevengill sharks (Notorynchus cepedianus) which together fulfill the role of coastal apex predators. However, since 2009, their position at the top of the food chain has been subverted by the increased presence of killer whales (Orcinus orca) in False Bay. These super predators are known to specialize on certain prey species, and up until 2015 were only documented preying on marine mammals within False Bay. However, in 2015 and 2016 we documented two events in which killer whales preyed upon broadnose sevengill sharks, using a specialized feeding method in which only the liver of each shark was consumed. Although selective feeding on shark liver by killer whales is established, this is the first record of killer whale predations on sevengill sharks in False Bay, and the first documentation of a novel feeding technique, in which killer whales used force applied to the pectoral fins of each shark to rupture the pectoral girdle and thereby access the liver. These predation events resulted in the prolonged absence of sevengill sharks from what is the largest known aggregation site for this species globally, which remained abandoned for up to a month. We briefly review the literature on killer whale behavior, dietary specialization, and population delineation globally and locally, and hypothesize that the novel predations on broadnose sevengill sharks in False Bay are possibly indicative of the arrival of a different sub-group or ecotype of killer whale in the bay, which habitually feeds on sharks. Due to the unique predatory niche occupied by sevengill sharks in False Bay, the increased presence of these particular killer whales in False Bay could have profound impacts throughout the ecosystem.

Direct observation of killer whales preying on white sharks and evidence of a flight response

“…In one case, a killer whale attempted to roll and invert a white shark into a position that would result in tonic immobility (Henningsen, 1994 ; Pyle et al, 1999 ) before biting into its abdomen just behind the pectoral fins. This confirms the strategy hypothesized from the examination of previous shark carcasses based on missing livers and bite marks on pectoral fins and torn pectoral girdles (Engelbrecht et al, 2019 ; Towner, Watson, et al, 2022 ). The consumption of what appears to be a large piece of drifting liver confirms that the liver is positively buoyant and may escape the torn open body cavity of the shark and float to the surface.…”

A synthesis of three decades of socio-ecological change in False Bay, South Africa: setting the scene for multidisciplinary research and management

“…The most abundant dolphin species in False Bay is the long-beaked common dolphin, which has a mostly pelagic habitat over the continental shelf (Best and Folkens, 2007) and is known to be hunted in the bay by the killer whale . There is also increasing evidence of killer whales preying upon broadnose sevengill cowsharks Notorynchus cepedianus in the bay (Engelbrecht et al, 2019).…”

Backing up from negative stimuli: A back‐thrust mechanism during escape‐like response in wild sevengill sharks (Notorynchus cepedianus)

“…Fast-speed predation events are highly expected in sevengill sharks because of their predatory nature and foraging strategies (Ebert, 1991a), and the highly mobile prey species found in their diet (Barnett, Abrantes, Stevens, Yick, et al, 2010b;Braccini, 2008;Ebert, 1991b;Lucifora et al, 2005). Similarly, fast-start events can be expected during escape responses to elude its major known natural predator, the killer whale (Engelbrecht et al, 2019;Reyes & García-Borboroglu, 2004).…”