Around the world and at every commercial white shark cage diving location a familiar and disturbing pattern has emerged. Great white sharks washing up on beaches – dead. The bite marks on the sharks, together with confirmed sightings in the area days before indicate that orcas, Orcinus orca, were responsible for precisely targeting great white sharks. Orcas? yes!
Guadalupe Island Orca Free Zone?
The great white shark, Carcharodon carcharias, is considered the most voracious apex predator in marine ecosystems worldwide, playing a key role in controlling ecosystem dynamics. At Guadalupe Island, since 2000, America’s Shark Boat has witnessed this dynamic first hand. Great white sharks essentially rule the waters Guadalupe from June – January each year with arrivals coinciding with seal breeding and pupping.
It is difficult for our shark cage diving crews and our shark divers to imagine a great white as prey at Guadalupe Island. And yet, earlier this year the carcasses of five great whites washed ashore along South Africa’s Western Cape province. Ranging in size from 9 feet to a whopping 16 feet, the two females and three males all had one thing in common – holes puncturing the muscle wall between the pectoral fins. Strangest of all, their livers were missing. This is the hallmark predation strategy of the orca.
Why White Sharks?
There’s no doubt that orcas are using highly specialised hunting strategies to target the liver of great white sharks only; the real question is: why? Shark livers are large, typically accounting for 5% or more of a shark’s total body weight. They are oil rich, with a principal component, squalene, serving as an energy store and providing buoyancy in the absence of the swim-bladder found in teleosts (bony fish).
Analysis of white shark livers in particular shows an extremely high total lipid content, dominated by triacylglycerols (>93%). This results in an energy density that is higher than whale blubber. For the sharks this serves as an energy storage unit to fuel migrations, growth and reproduction (Pethybridge et al 2014). For the orcas this is like eating a deep fried Mars Bar with added vitamins. Generally speaking, livers contain vitamin C, vitamin B12, folate, vitamin B6, niacin, riboflavin, vitamin A, iron, sodium and of course fat, carbohydrate and protein energy sources.
Since the attraction of this delicacy to the orca is clear, how exactly does an orca go about removing a great white shark’s liver? The evidence we have shows that it is done with some precision – the shark carcasses were not obliterated.
Guadalupe Island White Shark Population
While orca white shark predation has not been recorded at Guadalupe Island for the past 17 years, neither have visits from orcas to the island which could explain a lot in terms of the white shark population’s size (over 208 animals documented) and seasonal groupings. Shark divers are thrilled with younger males who arrive at the island en mass from June through October. These frisky smaller males range from 9-12 feet. Later in October the much larger females and males arrive pushing out the smaller sharks, and by November we see sharks like Deep Blue, the largest recorded white shark in the Pacific at 20 feet long!
America’s Shark Boat will be back at Guadalupe Island in 2018 celebrating our 18th shark cage diving season here.We’ll continue to track individual sharks with an eye towards orcas, now we know they have a taste for great white sharks!