Orca vs great white: killer duel explained

When it comes to great white sharks and killer whales, a battle to the death isn’t just black and white.
Contributor

Dr Blake Chapman

Contributor

Dr Blake Chapman

Dr Blake Chapman is Australian Geographic’s shark Editor-at-large.

By Blake Chapman March 12, 2020 Reading Time: 4 Minutes

Sometimes, we humans like to think that we have things figured out; that nature can be viewed in black and white.

But then something happens to shake this belief. Something like five great white sharks washing ashore in False Bay, South Africa, dead, and sliced open – as if by an expert surgeon – from just behind their pectoral fins. Oh, and their livers are missing.

Marine experts believe they likely fell prey to orcas (aka killer whales). The main evidence is tell-tale teeth marks, which, gorily, suggest that the orcas performed a tug-of-war while holding onto the shark’s pectoral fins in order to rip the skin, rupture the pectoral girdle, and access the liver.

As violent as this is, there is little question about why the whales would be targeting the shark’s liver. Sharks are different to most fish in that they don’t have swim bladders. Instead, they have extremely large, fatty livers that help them stay buoyant.

A White shark’s liver constitutes a whopping 28 per cent of its total body weight, and 90 per cent of that can be high-energy lipids. In other terms, one adult white shark liver could contain 400 litres of oil and 2 million kilocalories of energy. So basically, a big, fatty, dream meal for other carnivores with high nutritional needs.

Uncommon occurrence

While the grisly False Bay deaths aren’t the first recorded case of orcas predating on white sharks, it’s not a usual practice. In fact, 1997 was the first time anyone witnessed (and documented) orcas killing and eating the liver of a white shark. This occurred in the Southeast Farallon Islands (off the coast of California in the north-eastern Pacific Ocean).

Then, all went quiet again until 2015/16, when two distinct events of orcas preying on broadnose sevengill sharks were reported from Miller’s Point in False Bay over a five-month period. Generally occupying a slightly different environment to white sharks, broadnose sevengill sharks are also apex predators in False Bay.

In fact, white sharks were traditionally considered to be their only real predator. In both of these events, the evidence of orca predation was near identical: a number of shark carcasses, missing livers and orca-like tooth marks on fins.

The second attack also delivered a further blow, the instantaneous abandonment of the area, which was renowned as the largest-known broadnose sevengill shark aggregation site, by the remaining sevengill sharks. The site remained bereft of sevengills for up to a month.

It seemed, then, that the False Bay orcas shifted their attention to white sharks. Researchers had been witnessing a continued decline in white shark abundance at Seal Island (within False Bay) since 2015, but then major dips were noted in 2017 and 2018. These dips then turned into eerily prolonged white shark absences, up to 18 months in some cases.

Actions and consequences

While the disappearance of white sharks may seem like a dream to some, it’s not just the presence-absence status of these sharks that needs to be considered. When the white sharks moved away from Seal Island, sevengill sharks moved in, and then continued to come arrive in increasing numbers.

This may not sound overly significant, but sevengill sharks had never been documented in this location previously, representing a major ecological shift. The consequences of such shifts remain unknown.

Similarly, when orcas arrive in the Farallon Islands, the white sharks disappear. This results in a marked decrease in seal and sea lion predations. Apex, or top-order predators, are known to have both direct and indirect impact on the ecosystems that they occupy; direct through consuming prey, and indirect through potential prey taking less risks and behaving differently when sharks are around compared to when they are not.

Therefore, such dramatic behavioural shifts in normal top predators have varied (and often unpredictable) effects that cascade down the food chain and throughout the ecosystem more generally.

Also, the predation risk orcas place on white sharks doesn’t just affect the sharks that are directly killed. Changes in shark movement and residency in response to the presence of orcas could also impact their fitness if optimal habitats are avoided, and sub-optimal feeding grounds become more competitive.

Does it happen in Australia?

As Australia has both white sharks and the occasional orca pod, should we expect to see this predation and avoidance behaviour here? Potentially, and it’s actually already been observed off south Australia.

But, so far, it seems that these predations are very rare and that only certain orcas target such apex species of sharks. Unwitnessed predation of white sharks by orcas, however, could be occurring more frequently, but outside of our normal surveillance areas.

Unobserved flight responses (to orca presence) could also be an underlying reason for atypical shifts in white shark movement patterns. Tracking data of white sharks in the Farallon Islands showed that the sharks that were displaced from normal aggregation sites by orcas moved to other sites, creating greater-than-average densities in these places, which included locations along the mainland California coast.

The idea that only certain individuals have unique and specialised prey preferences (known as ecotypes) isn’t unprecedented in marine mammals.

Individual (or small groups) of dolphins and sea otters are also known to show distinct and specialised prey preferences and hunting techniques. For example, certain dolphins off Shark Bay in Western Australia have learned to use marine sponges to help them hunt, a behaviour known as ‘sponging’.

Only a select few individuals do this, and it’s thought to be an advantage because those individuals are able to exploit different food items to other dolphins, meaning there is less competition and more food for all.

In the case of the orcas, it’s starting to look like there may be a distinct ‘flat toothed’ orca morphotype that commonly preys on sharks.

For now, we’re still left with lots of questions. But what is clear is that even with orcas, nothing in nature should be simply viewed as black and white!