Invasive sea urchins — eat them to beat them

By Richard Musgrove

An invasion by ugly but tasty sea creatures might save Tasmania’s kelp forests. But first there has to be some over-fishing.

Tasting salty and fishy, and looking like orange-yellow mandarin slices, sea urchin roe is a delicacy, driving a billion dollar global market, with 80% consumed in Japan.

Rising sea temperatures are forcing many marine species south, including the long-spined sea urchins. Sea urchins can reach 10cm or more across and are deep-red to midnight black with long spines which are sometimes weirdly iridescence green. The spines help protect the urchins and wedge them into crevices.

There are prominent urchin habitats on New South Wales’ coastline.   

In recent years the warming East Australian Current has been transporting baby long-spined urchins from New South Wales south to eastern Tasmania’s rocky coasts, with populations rising from a few individuals to about 11 million by 2001-2002,  and about 18 million by 2016–2017.

This expanding pest population is destroying rocky reef ecosystems through ‘barren’ formation.

Barrens form as the sea urchins eat all the plants on reefs, including the kelp, causing what marine biologist Katie Cresswell from the University of Tasmania calls “catastrophic ecological shifts” with hundreds of reef-dependent species disappearing, including commercially-important abalone and rock lobsters and impacting recreational fisheries.

The problem was clearly outlined in this article in Cosmos when researcher at the Institute for Marine and Antarctic Studies, Dr Scott Ling, put out an alert of “clear and present danger,” warning the sea urchin population explosion was likely to lead to a potential halving of productive kelp bed habitats which provide a home for the Tasmanian rock lobsters and abalone.

“It’s like most disaster movies. It starts with a scientist being ignored,” says Ling.

The best way of dealing with the sea urchin threat is to eat them, says Cresswell.

Her study is recommending commercial overfishing, a term usually associated with fishery collapse. But in this case its targeted, with incentives to overfish in areas valuable to the abalone fishery.

“We used modelling to estimate how much worse the urchin problem would be without commercial fishing – and the answer is at least twice as bad,” says Cresswell

“The aim was to see what impact the fishery has had so far in Tasmania, what management measures make sense in different areas, and how we compare to other places in our management of this problem so far.”

“We also investigated different management scenarios to guide informed decision-making including which regions would make more sense to ‘overfish’ using the limited funding for subsidies, and which to allow to operate as a stand-alone sustainable fishery.”

“Subsidies encourage urchin overfishing in areas where there were still healthy kelp reefs and abalone stocks, such as in the southeast of Tasmania, while letting the fishery operate sustainably and without subsidies in areas of high urchin density,” she says.

The now-established commercial fishery has taken about 470 tonnes of sea urchins a year for the last six years, the equivalent of around 1.2 million urchins a year removed from Tasmanian reefs. The fishery has been assessed as ‘sustainable’ since the first assessment in 2019, says the study.

It reports modelling which shows by the end of 2023 fishing season, the sea urchins populations were reduced by about 50% in the northern fishery and 15-30% further south, compared with unfished areas.

“The goal is ‘functional eradication’, which is about setting a target density below one that results in ecosystem destruction like urchin barrens – but not aiming for total eradication which would be extremely expensive, ultimately unsuccessful, and would likely crash the control mechanism itself,” says Cresswell

The Great Southern Reef Foundation looks at the urchin problem in Tasmania.

Full-scale eradication of sea urchins is impractical, says Cresswell, because warming waters constantly bring in more baby urchins and increase their survival when they get there. Size and depth are also issues — the minimum harvestable economic size is 85mm, below which the gonads (the roe) are too small to be marketable, say the researchers. Divers can get down to 26m but urchins do well down to 35m, which means deeper populations constantly replenish the shallows. 

“This study sets our Tasmanian story of this ‘edible pest’ in an international context, among places where invasive or range-extending species have been commercially or recreationally fished,” says Cresswell.

“We recommend a combination of targeted fishing and ongoing research, which are both essential for effectively managing any species that is moving into a new range, is marketable and fishable, and has negative ecosystem impacts,” says coauthor Dr John Keane.

Cresswell says climate change will continue to push more species beyond their historical ranges. “This will impact ecosystems and communities alike, so it’s increasingly important to use modelling to guide flexible management decisions that balance sustainability with economic needs.”

The paper was published in Nature Sustainability.

Read more about these fascinating sea urchins