Orcas And Sharks: Just Who Is Hunting Whom?

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Thoughts of sharks looming silently from the dusky depths is the stuff of nightmares, and is enough to keep many otherwise stalwart humans from venturing into the ocean…but dolphins and whales don’t have the option to leave. Bite marks and scars on the bodies of cetaceans are testament to the frequency and relentlessness of shark attacks, and shark stomach contents can include bits of marine mammals of all types.

Orca and shark (Visser)

But who would have thought that sharks, in turn, are hunted by dolphins and whales?
Although some dolphins are limited to catching and eating small and young sharks, the orca (killer whale), takes on even the largest sharks using intelligent, cooperative hunting techniques. Even great white sharks are disabled and dispached with little risk because the orcas have learned that these predators are helpless when turned upside down (a hypnotized-like state called ‘tonic immobility’). Mako and other similar sized sharks are driven to the surface then smashed with the orca’s powerful tail.
This account by Jean-Michel Cousteau (http://independent.com/news/2010/feb/27/cousteau-seaworld-tragedy/) describes yet another method, where it almost seems like the orcas are trying to teach him something:

In October 1988, we were going to do a film in Papua New Guinea on blind fish that lived in a cave. In the entrance of the cave, before we started filming, we encountered a male and a female orca.
We spent 10 hours with them. Or, to be correct, they spent 10 hours with us because they could have left anytime they wanted. They did a show for us: They would go and catch a live shark, bring it to us, release the shark and catch it again in front of us. Eventually, they would take each end of the shark and destroy it. Then, very carefully, they’d take the liver, eat it, and abandoned the rest of the carcass. They would do it again and again and again. They were showing off and eating their choice food.
We eventually became so exhausted that we had to return to the ship. We started drinking champagne and celebrating because it was such an amazing experience.

Remarkably, scientists have now discovered that some types of orcas, called ‘offshore’ orcas, catch sharks to such an extent that their teeth are worn down from grasping the rough hide (shark skin is covered in tiny plates made out of the same material as the shark’s teeth). What seems odd about this is that the species of shark that the researchers documented having been attacked is the Pacific sleeper shark – a deep water, opportunistic scrounge with flesh that is inedible to most mammals.

Sleeper Shark (Creative Commons Photo)

The sleeper shark is a huge (up to 23 feet), slow growing fish that has been filmed thousands of feet under water, and mostly this shark eats what it can find on the bottom – flat fish and the carcasses of animals – but it also consumes quantities of octopus and squid. They may move up to the surface at night in pursuit of prey that hides in the depths during the day, and tagging studies show that these sharks move up and down the water column in some circumstances. Even though it’s languid habit of moving slowly and deliberately earned it the name of sleeper shark, it is capable of quick bursts to nab it’s prey. The orcas have to locate and kill the shark in the dark and/or at depth where light doesn’t penetrate (orcas generally dive to only about 300 feet, though they are capable of diving two or three times that depth).
Then there is the problem of the shark’s toxic flesh.
Like it’s cousin the Greenland shark, the Pacific sleeper shark’s muscle contains compounds that affect the nervous system of humans, dogs, and mice (tested in a laboratory), causing disorientation and even death. In order for it to be safe, the meat has to be treated by burying it and allowing it to go through several cycles of freezing and thawing, or by other similarly drastic methods. How the orcas would manage to eat this flesh unharmed is unknown at this time, and it is likely that they don’t eat the flesh at all.
The orcas may kill these sharks in order to eat the liver, but it is hard to imagine that the orcas can find enough of these sharks to sustain them as their major food source if they are just eating the liver. It is also unlikely that the orcas would have evolved the ability to eat the toxic flesh, yet would not have developed teeth that can withstand the abrasive shark skin.
But these sharks are also swimming sushi bars, adapted to store food in their stomachs in an environment where food is intermittent – one stomach of a smallish 12 foot sleeper shark contained 300 pounds of prey items such as octopus that could provide additional nourishment to the orcas.
But still, what about the orca’s worn out teeth? One possibility is that this feasting on Pacific sleeper sharks is a relatively new development, tied to shifting ocean conditions or shark abundance and there is some tentative indication that recently in parts of it’s range the population of these sharks did go through a temporary up-tick. (“On the eastern Bering Sea shelf, Pacific sleeper sharks were not well documented in survey catch until the mid-1990s. Biomass in 1999 was estimated at 2,079 mt and increased to 5,602 mt in 2002. However, biomass estimates have decreased since then and were down to 2,944 mt in 2006“) The more open ocean where these orcas live may leave them more vulnerable to sharks so they may kill sharks in defense of their young animals as well as for food; world wide, orcas are known to attack different types of sharks, including reef, basking, hammerhead, Galapagos, blue, whale shark, and carcharhinid (great whites are included here) sharks.
As researchers study these offshore orcas, we will no doubt continue to be amazed at what they discover, but no matter how it is that sharks fit into the orcas’ lifestyle this is another reminder that we need to protect the sharks from our thoughtless and fear-driven persecution, for we have yet to learn how important this ancient animal is to the mammals we seek to protect.

This is adapted from a more detailed post. Go here for more information, related videos and references.

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