This strange sea animal is on menus, but it is disappearing from our reefs

The alien-looking sea cucumber is a delicacy eaten in soups or consumed for its perceived health and libido benefits.

But a boom in demand since the 1980s has led to overfishing and population collapses in parts of the world, including the Pacific.

Entire species are disappearing from remote Australian reefs, with illegal fishing pointed to as a key culprit.

And there are now concerns that regulators for Australia's legal sea cucumber fisheries are underestimating how long it takes fished stocks to recover. 

A new "first-of-its-kind" genetic study, published in iScience, indicates at least one sought-after sea cucumber species lives longer than we thought.

Study lead author and molecular ecologist Sven Uthicke, from the Australian Institute of Marine Science, said the black teatfish (Holothuria whitmaei) could reach 100 years of age.

It was assumed the species, found in Australia's tropical waters but less so in other Pacific countries, only lived five to 20 years in the wild.

Dr Uthicke said the finding meant fishery management needed to revise whether current catch limits were sustainable.

The age a marine species reaches is a major factor for calculating a fishery's environmental sustainability.

Dr Uthicke said the sustainable catch limit for black teatfish and similar species might be lower than we thought.

What are sea cucumbers?

Sea cucumbers are part of a large group of "spiny skin" animals that includes starfish and sea urchins.

Colours and patterns of their leathery skin range from plain browns to vibrant reds and yellows.

Sometimes they have alien-like mouth tendrils, which are actually modified "feet".

Sea cucumbers are believed to benefit the health of coral and seagrass meadows.

"We don't necessarily understand all the ecological services that sea cucumbers provide, but we do know that they are very important in aerating sediment," CSIRO marine ecologist John Keesing said.

Much is still unknown about these weird animals, with few studies into basic biology like how long they can live.

How old do sea cucumbers get?

In the new study, Dr Uthicke revisited a no-fishing zone near Lizard Island in the Great Barrier Reef 24 years after another study he did into the growth rates of black teatfish.

He was surprised to find 31 out of the 76 sea cucumbers from his earlier fieldwork in 2000.

"We still don't know how fast they grow because the individuals in my study basically did not grow at all over the entire 25 years …  and we didn't find any small ones."

As the research was only conducted in one location, Dr Uthicke said other locations and species could be looked at.

But, he said, regulators could use the current findings to manage stocks more conservatively.

Black teatfish stocks have previously collapsed at the Great Barrier Reef and Zenadh Kes (Torres Strait).

As black teatfish are considered globally endangered, the federal environment department has to sign off on any fishing regimes occurring in Australia.

The species is not caught in the regulated Western Australia or Northern Territory sea cucumber fisheries, but it is caught in Queensland fisheries.

A Queensland Department of Primary Industries spokesperson said a recent stock assessment for black teatfish indicated the species was being fished at sustainable levels.

"Future stock assessments will consider all new, relevant information."

An Australian Fisheries Management Authority spokesperson said the new research would be considered when advice was developed for future management settings in the Torres Strait.

They said a past CSIRO survey in 2019–20 found the black teatfish had significantly recovered from historic overfishing and was safe to be fished under a strict harvest strategy.

"Black teatfish is an important source of income supporting livelihoods and the traditional way of life in the Torres Strait," the spokesperson said.

"Torres Strait Islanders have exclusive access rights to black teatfish, which is estimated to be worth over half a million dollars to the local economy."

How much do sea cucumbers sell for?

Dried sea cucumbers can go for hundreds of dollars per kilogram in domestic markets like Hong Kong.

A similar price bracket to other high-end produce like Wagyu beef and foie gras.

Sea cucumbers move slowly, so they are easy to pluck from the water by the basket-load. 

Once caught, a sea cucumber is gutted, cooked, salted and dried.

"There's very few other animals that eat them," Dr Keesing said.

"That's the reason why they don't bother hiding themselves too much."

This vulnerability is thought to have led to local extinctions at remote Australian reefs.

Where sea cucumbers are disappearing already

A study led by Dr Keesing last year suggested the golden sandfish (Holothuria lessoni), which was once common at Ashmore Reef, has gone locally extinct, with overfishing the most likely cause.

The reef is located about 200km from Indonesia and 300km from the Kimberley region of Australia.

"Reefs like Ashmore Reef to the north are now sort of fished out so they're [illegal fishermen] looking further south," Dr Keesing said.

At the next reef down, Scott Reef, black teatfish and white teatfish (Holothuria fuscogilva) have reportedly collapsed.

Illegal fishermen have been intercepted as far south as Rowley Shoals, islands west of Broome, in recent years by federal authorities.

Another study earlier this year suggested the white teatfish was no longer in Rowley Shoals and three other species, including the black teatfish, were close to dropping under a critical population threshold for remaining viable.

"That level of fishing just over quite short a period of time has had a very significant impact on those populations," Dr Keesing, one of the authors, said.

"If that can be prevented by greater surveillance, greater consequences for being intercepted, then those populations should recover.

"If that's just allowed to continue … then potentially you could have the Rowley Shoals in the same state as Ashmore Reef."