Great whites back in the bay

Great white sharks were once again sighted in Gansbaai today, after a spate of attacks by orcas sent them relocating to safer waters for almost a month. ‘Our WSCD research vessel had the pleasure of sighting two great white sharks on Wednesday and three sharks yesterday,’ said Mary Rowlinson, a marine biologist and manager of the White Shark Diving Company Research and Volunteer programme. ‘Activity was low but they are making a slow return.’

Conservationists expressed concern earlier this month when three great white sharks washed up dead on Western Cape shores in a matter of days – an unheard-of event in this part of the world. Orcas (Orcinus orca) are recognised as the only natural predator of the white shark, but fatal encounters between these two apex predators are rare, and have been documented on only a few occasions in recent decades. Nevertheless, white sharks have evolved to avoid the threat orcas pose – evidence shows that white sharks will avoid areas where they perceive orca vocalisations (real or recorded).

Necropsies revealed that the livers of all three of the white sharks found in the Cape had been removed – a behaviour that has been known to occur with sevengill sharks in False Bay, as well as sevengills off the coast of California. The removal of the livers, along with bite marks and pectoral damage found on two of the great whites, led experts to surmise the most likely cause of death was predation by a pair of orcas that have been sighted in the bay on several occasions since early February.  The liver of the first shark found had been so cleanly removed that the she bore no visible bite marks – perhaps an indication of how expert orcas have become at this form of predation.

Orcas have previously been intermittent visitors to our coastline, but in recent years the length and frequency of their stays seems to have increased, and this is the first time great whites had been washed up in such numbers.  Bizarrely, just a week before the great whites were killed in Gansbaai, orcas killed 4 gray whales in a spate of attacks on the California coast, an event that veteran orca researcher Nancy Black described as equally ‘unprecedented’ in terms of the speed and precision with which the kills were executed.

The orcas in South Africa are transient pods which come up from the Marion Island at this time of year, says Lucy Babey, Head of Science and Conservation at ORCA (UK). ‘These are opportunistic feeders and stomach content analysis confirms this, with the remains of pinnipeds (seals), cetaceans (dolphins), fish and elasmobranchs (including numerous shark species) being commonly found.’ It is this opportunistic streak that makes orcas so successful as a species, she explains. ‘Though different pods of orcas have been seen to specialise in different prey, orcas are largely generalists – for example in the UK pods will swap from fish prey to seals during the seal breeding season. This, along with their adaptability and cohesive hunting nature, has led them to thrive.’

Scientists are unclear as to exactly why orcas seem to be predating more heavily upon atypical species, or spending longer periods of time in areas they formerly only rarely visited. These orcas could be capitalising on the great white sharks in the area for the high energy food reward they offer, says Babey. ‘As recent events have shown, it is primarily the liver that the orcas target, due to the high fat and calorific content indicating that the energy expenditure of the kill is worth the reward. Other orca ‘ecotypes’ (tribes) that predate on shark species around the world also target the liver and other energy rich internal organs, such as the heart.’

‘The consumption of only the sharks’ liver raises some interesting questions to the purpose of the predation,’ adds Ichthyologist, Dr Neil Deacon. ‘This organ is comparatively large in sharks and is uniquely rich in vitamin A and squalene  – an oil that is generally found in very low concentrations in nature has been long valued by people for its medicinal properties, which include therapeutic effects on some cancers. This could imply that the orcas may be suffering from a nutritional deficiency, as taking on such a formidable predator as a great white, particularly a large sub-adult such as the female in this recent case is very risky, especially if one considers that white sharks are evolved to prey on mammals,’ says Dr Deacon.

Still, it begs the question – why wouldn’t orcas search out more placid shark species, such as seven gilled sharks, to satisfy any nutritional deficiency? ‘Conceivably, the killing of white sharks by orcas could also be competition between two pinnacle predatory species,’ says Dr Deacon. This behaviour is not uncommon in the animal kingdom, particularly when there is an overlap of prey, as with these two species, which both feed on seals.  Lions kill cheetahs, and eagle-owls will almost go out of their way to kill falcons or any other diurnal bird of prey if given the opportunity.  Typically the victor of these inter-specific interactions will feed on their kill to some extent.  An underlying concern is that this is an aberrant behaviour amongst a few orcas that are killing white sharks just because they can.  Ultimately, white sharks may be no match for the superior intelligence of orcas, just as they have been no match for man.’

As orcas work together in a pod and pass on hunting techniques, so that more individuals learn this behaviour, these incidents could become more common, says Babey. That possibility is extremely worrying,’ adds Dr Deacon. ‘The great white population can’t afford to lose three sharks every other fortnight.’

The results of a five-year study released University of Stellenbosch in 2016 indicated that only 353 to 522 great whites still exist on the South African coastline – a critical mass that could face extinction if measures are not taken to ensure their protection. ‘Three lives in a population already this low is not good news,’ said Dr Sara Andreotti, one of the study’s lead researchers.

‘These deaths are a big blow to our population, particularly given that all three animals were beyond the juvenile stage,’ adds Rowlinson. ‘We can only hope that this most recent male found will be the last dead shark we see for a while and that this does not become a learnt behaviour amongst the orcas.’

These events have further highlighted the need for the increased protection of great white sharks –those magnificent, yet vulnerable, apex predators that are so vital to the health of our oceans.  Even if orca predation upon great whites sharks is a natural phenomenon, it is still a result of man’s actions that their populations are so precarious.

Timeline

4/05/2017

‘This shark had been in the bay before and was recorded in database as an estimated 4m in length, she adds, but when measured on land it was 4.9m, which shows how much we tend to underestimate the size of these amazing animals.

5/05/2017

3.4 m mature male found dead – heart, liver and claspers missing.

7/05/2017

4.4 m male stranded – liver missing.