When a whale dies it becomes a stinky feast for sharks, tiny shore animals and scientists

By Zoe Kean, ABC Science

Earlier this week, a dead whale surrounded by hungry sharks washed up into Adelaide's port.

South Australian authorities are still deciding about what to do with the carcass, but in the meantime scientists have taken samples for research.

Originally thought to be a humpback whale, the identity of the carcass is still a mystery, but at this time of year there are plenty of whales near the Australian coast.

Currently, southern right whales are breeding in the warmer waters of the Great Australian Bight before travelling back to Antarctica.

Humpback whales are also about to start their annual migration down the east and west coasts of Australia from calving grounds in the tropics.

Most whales will make it from their breeding grounds back to their summer destination south of Australia, but sometimes a whale will perish along the way.

We're more likely to see carcasses wash up when whales are travelling closer to the coast, shark and whale scientist James Tucker said.

"Stranding will increase along migration routes due to recovering populations [after whaling]," Dr Tucker said.

They can also collide with ships.

The poor condition of the whale carcass that washed up in South Australia suggests it was likely hit in international waters and carried on the bow of the ship for some distance, according to a spokesperson for South Australia's Department for Environment and Water.

"Whale deaths and carcasses are an unusual occurrence in South Australian waters; however, this is not an uncommon incident globally.

"Whales can be hit by large ocean-going vessels and then carried on the bow of ships until they reach their harbour destination."

Dead whales offer a feast for sea creatures

Although it's a gory sight, a dead whale offers the feast of a lifetime for sea creatures - and scientists.

From flying drones over sharks feasting on bloated carcasses, to analysing the chemicals whales leak when they are buried in the sand, Dr Tucker has been up close and personal with whale remains as they are nibbled and chomped to oblivion by a long guest list of marine diners in the sea and on the shore.

"It's like the beach has won the lottery," Dr Tucker said.

"It's a huge amount of food and nutrients in a relatively barren environment."

But leaving a whale to decompose on a beach can become a stinky, shark-attracting and sometimes explosive problem.

Just how the rotting carcass is dealt with has consequences for us and the myriad of creatures that depend upon its wealth of energy and nutrients.

Sharks are one of the first animals to arrive at the buffet when a whale dies at sea.

A whale's skin and blubber is so tough that Dr Tucker always has someone on hand to continuously sharpen his knives when he is dissecting a dead animal.

While this blunting flesh might be daunting to some, sharks come prepared.

"[Tiger sharks] can move their top jaw separately from the bottom jaw, and they saw back and forth through the blubber, which is amazing," he said.

From his drone footage, Dr Tucker has found that bull sharks, tiger sharks and white sharks are the heavy hitters at the dead whale buffet.

Smaller shark species will take a bite, but it is a shark-eat-shark world out there so "when those big boys come in, the smaller sharks tend to disperse pretty quickly," he says.

"The more we understand about large predators, the more we understand how much of a role scavenging plays in their diets."

All this feasting gets messy.

"As sharks munch away on [a whale], they tear off sections, so a lot of stuff will float off and become food for smaller organisms," Dr Tucker says.

The sharks create holes in the blubber, allowing smaller animals, such as seabirds, to get at the flesh beneath.

These birds will perch themselves on the floating, gas-filled carcass and peck away at their gory life raft.

In most cases, the sharks will eventually burst the carcass and the whale will sink to the deep-sea floor, becoming a "whale fall".

In the depths, a hyper-specialised group of scavengers will pick over it for decades.

However, in some cases, winds or ocean currents will drive the carcass into shore before it sinks.

It is then met in the shallows by a host of new creatures ready to take a bite.

In the shallows: sharks, scientists, and all manner of critters

As the carcass drifts into shore, large terrestrial scavengers descend: scientists.

"You have the scavengers coming in, but you also have the researchers; all these specimens [they collect] are feeding into their research projects," said Sandy Ingleby who is the collection manager of mammals at the Australian Museum.

"The specimens are like little time capsules. They are physical embodiments of the conditions in the environment at the time they were collected.

"For the next 100 years or more, people can come and use these samples for their research."

With sharks circling, scientists need to get inventive to score their samples safely.

To get blubber specimens from a fin whale carcass floating in a bay in New South Wales in 2018, one of Dr Ingleby's colleagues enlisted a pole-mounted saw normally used for lopping trees.

"He leant out [from the rocks] with his colleagues holding him so he wouldn't fall in," she said.

It's not only scientists who sniff out the boon offered by a carcass, explains Shane Ahyong, an invertebrate expert at the Australian Museum and University of New South Wales.

"There's what is called a scavenging guild, and they are like your clean-up team," Professor Ahyong said.

This cluster of hungry creatures includes crabs, fish, worms and a host of small crustaceans called isopods and amphipods.

"They will turn up and start shredding and stripping anything they find; it's gruesome, but it's quite amazing."

If you have ever dug your hands into the sand at the beach and been greeted by hopping sand-coloured creatures, you have met amphipods.

"Sometimes they're called sand hoppers, or sand fleas," he says.

"The ones that are slater-like are isopods; you don't often see them, but if you throw a bit of dead meat in the water, they will show up.

"The other big component of the scavengers are the beach worms, and they come and attack from below."

On the sand: the leftovers

Eventually waves push the carcass onto the beach and above the tide line, where a new group of scavengers will greet the increasingly putrid feast.

Diners include ghost crabs, rose beetles, flies, more birds and in some areas, goannas.

"The ghost crabs are big enough to start picking at things and the rose beetles get into small gaps and can find their way into almost anything," Professor Ahyong said.

It's at this stage scientists return, sometimes taking more than the lion's share.

"[The museum] usually end up getting something from each [carcass]," Dr Ingleby said.

"Usually the skull, sometimes a flipper or a foetus."

Occasionally Dr Ingleby and her team will take an entire carcass and compost it in a secret bushland location.

After composting, the skeleton will be ready for display at the museum.

But most carcasses are not destined for display or analysis.

Leaving a whale carcass on the beach to decompose has the benefit of continuing to nourish the surroundings.

But as the carcass breaks down, it oozes a decomposition product called leachate.

If this gets into water, it may lure sharks.

This raises the question of how to dispose of the whale.

"If it's in a location with very little foot traffic, where there are not many water users and the risk to people is low, I'm always going to recommend the most natural option," Dr Tucker said.

What happens when we remove whales off beaches?

While leaving a whale to rot benefits the beach, carcasses are often removed by being towed back out to sea, buried or chopped up and taken to the tip.

Dr Tucker said towing rarely happens because the floating carcass can become "navigational hazards" to boats.

Beach burials take care of the smell and the sight of decaying whale, but there are concerns this method could attract sharks to the beach.

However, research by Dr Tucker suggests this method can be safe if the whale is buried in the dunes far away from streams or ground water.

"It's a natural option, and if it's done correctly it takes away most of the risk, but it still allows that nutrient to be on the beach."

Heavy machinery digging deep hole in the sand on Port Macquarie beach with dead humpback whale at the bottom.

The last option - being chopped up and taken to the tip - removes any risk of attracting sharks.

But does removing a whale carcass unbalance the ecology of a beach?

In the deep sea, intricate and specific ecosystems spring up around whale falls.

If whales no longer rained from above, these species would likely go extinct.

This is not true for the animals that flock to a dead whale at the beach, Professor Ahyong said.

Beach scavengers tend to be generalists, having evolved to take advantage of whatever turns up, he explains.

While whale carcasses "provide a huge energy input" for these scavengers, Professor Ahyong said sandy beaches are facing a host of threats more serious than removal of a single animal.

These threats include development, pollution, runoff from cities and beach grooming.

Beach grooming involves removing (for aesthetic purposes) all the seaweed and dead animals that have washed up onto the beach, thereby removing the food source for all the scavenging denizens.

"It's a way of starving the beach," he said.

- ABC