Insights into the Mobbing and Hunting of Cape Fur Seals by Great white sharks at Robberg Pensula, Plettenberg Bay, South Africa

Insights into the mobbing and hunting of Cape fur seals (Arctupulis artc) by Great white sharks (Carcharodon carcharias) at Robberg Pensula, Plettenberg Bay, South Africa. 

Special Report 001-2019 BWRU

Ryan L. Johnson, Barry Skinstad, Fiona L. Ayerst

 Abstract. We report on a series of directly observed and previously reported observations of mobbing and hunting behaviour by great white sharks (Carcharodon carcharias) on a newly established colony of Cape fur seals (Acr) occuping the Robberg Pensula at Plettenberg Bay, South Africa. First observations of interspecific interactions were made by C. Stewederson in 1992, however in recent years, the increase in Cape fur seal colony has coincideded with an increase in sightings of great white sharks patrolling the colony. This report describes both hunting and defensive stratigies adopted by predator and prey respectifully in a unique environment characteriesed by shallow (2 - 4m) water, sandy bottoms and high cliff. Patrolling by sharks occurs primary during the winter months (May to August) by a number of great white sharks that vary in size and gender. Cape fur seals utilise mobbing and harrassment techniques as a predator avoidance strategy to disuade white sharks. Concurrently white sharks patrol with the aim of hunting Cape fur seals. Successful hunting appears to be infrequent and occurs when a white sharks gains an element of surprise over a cape fur seals or encounters a weakended and vulnerable cape fur seal. 

 Keywords: Mobbing, Great white sharks, Cape fur seals, Robberg, Predator prey games

 

INTRODUCTION

 

 

 

The state encompasses nearly the entire volcanic Hawaiian archipelago, which comprises hundreds of islands spread over 1,500 miles. At the southeastern end of the archipelago, the eight main islands are—in order from northwest to southeast: Niihau, Kauai, Oahu, Molokai, Lanai, Kahoolawe, Maui, and the Island of Hawaii. The last is the largest island in the group; it is often called the "Big Island" or "Hawaii Island" to avoid confusion with the state or archipelago.

The archipelago is physiographically and ethnologically part of the Polynesian subregion of Oceania.

The Hawaiian islands were formed by volcanic activity initiated at an undersea magma source called the Hawaii hotspot. The process is continuing to build islands; the tectonic plate beneath much of the Pacific Ocean continually moves northwest and the hot spot remains stationary, slowly creating new volcanoes. Because of the hotspot's location, all currently active land volcanoes are located on the southern half of Hawaii Island. The newest volcano, Loihi Seamount, is located south of the coast of Hawaii Island.

MOBBING BEHAVIOUR

Mobbing behaviour is a common strategy utilised by potential prey species when confronted by a predator. The prevelance of mobbing is dependent on a number of factors, including (a) the

Specific behaviours observe, and proposed functions of cape fur seals mobbing great white sharks include

Grouping behaviour

Cape fur seals routinely formed rafts (groups) when in the water adjacent to the colony. Sometimes these were as few as three of four seals, but often numbers upto 100 seals. The advantage of rafting in gorups is that it increase the overall probability to detect the pressence of a predator, and thus increase the individual safety of a seal. For this strategy to be successful, interspecific (within species) communicatino is critical. In the case of Cape fur seals, communication appears to be coordinated by behaviour, where sudden bursts of speed, bubbles, by a seal (the spotter) are detected by adjacent seals who respond be evoking anti predator behaviours (burst of speed, evasive manovours). As such, the pressence of a predator is effectivly communicated thoughout a raft of seals. 

Increased vigilance

If a predator is present, then it is better to know where it is! This is a a very true statement when it comes to predion avoidance by prey through nature. In a similar manner, it is apparent that it is advantagour for seals to remain in visual contact to a shark that is patrolling the seal colony. Doing so, reduces the chance of the shark instigating a surprise attack. During patrols of the colony, seals would routinely continue to mob and harrass a shark, and keep it in visul range until it had moved into deep water. Upon driving a shark into deeper water, seals routinely swim back to the water directly adjacent to the land and colony. 

Predator removal

Harrassment and mobbing plays a role in driving off a predator. Many predators rely on surprise to be successful in hunting potential prey. Without the element of surprice, predators may be dissuaded to expand effort patrolling and hunting. At Plettenberg bay, seals will often position themselves at the back of the shark, and have observed been nipping at the sharks body and fins. Such behaviour suggests that the seals are trying to make the shark uncomfortable and ultimatly drive it from the area. The success of driving sharks from the area seem limimited, as sharks will frequently respond by swimming into the depth (and losing the pressence of seals) before circling around and continue to partol adjacent to the colony. 

Honest signalling 

Cape fur seals utilise mobbing to communicate to great white sharks that they are healthly, strong and fit individuals. In a similar manner that gazelles will stot or pront infront of lions, the Cape fur seals perform vigiours jums and swimming in the pressence of the shark. Such overt displays are geared towards communicating to the shark that an attempted predator would be a pointless endevour, and highly unlikely to succeed. Thus demotivating the patrolling shark

 

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ENVIRONMENT

White sharks hunting cape fur seals have been traditionally studed in waters adjacent to Seal Islands, around South Africa's western cape. This includes Seal Island, False Bay, Gyser / Dyer Island, Gansbaay, Seal Island, Mossel Bay and Bird Island, Port Elizabeth. In these environments, White sharks patrol at depth (8 - 14m) waiting for seals to exit or return to the island, where they are required to move over channels and dropoffs. The hunting strategy of white sharks in this environment is based upon utilising camoflague offerred by depth to ambush cape fur seals from below. The consequence of such an attack strategy is the remarkable breaching behaviour captured frequently on film. 

At Plettenberg Bay, the environment is considerably different in a number of important manners. These include

SUBSTRATE TYPE

WATER DEPTH

PENSULA GEOGRAPHY extending near the entire 

The robberg pensula stretches 3km our to seal, with the seal colony 

 

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