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Overfishing urchins: A paradoxical path to marine sustainability

In our warming oceans, marine species are moving into new areas and "re-engineering" or often destroying those ecosystems, but scientists say the paradox of overfishing sustainably can help.

A new study in Nature Sustainability this week reveals that establishing a commercial fishery to intentionally overfish an unwanted yet marketable species, like the Longspined Sea Urchin (Centrostephanus rodgersii) in Tasmania, can be a sustainable option to protect vital marine ecosystems.

"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," said lead author, Dr. Katie Cresswell from the University of Tasmania's Institute for Marine and Antarctic Studies (IMAS), who conducted this research with CSIRO, Australia's national science agency.

"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.

"In Tasmania we're leading the way, with sectors working together towards an aligned goal. We have a unique and quite remarkable scenario here where commercial divers, researchers and government all work together to try to control the densities of this destructive urchin—and where government-supported incentives have driven the establishment of a commercial fishery targeting the species," Dr. Cresswell said.

"We used modeling to estimate how much worse the urchin problem would be without commercial fishing—and the answer is at least twice as bad.

"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."

Beyond the implications of subsidizing a commercial fishery to counteract the impacts of climate change, the study also explores the conflict between exploitation and conservation, challenging traditional views and presenting practical pathways to sustainability.

"Our monitoring showed a consistent and increasing influx of urchins, which has been recently investigated and confirmed in Dr. Cresswell's upcoming larval modeling report—so we knew eradicating the urchins along entire coastlines in their extended range was out of the question," said IMAS fisheries scientist and co-author, Dr. John Keane.

"Well before 'eating the problem' became a catch phrase, we were exploring ways to effectively reduce the urchin population here in Tasmanian waters—and developing an industry around urchin roe was a clear option.

"With limited resources to control problematic species, a sustainable control option is vital— and a commercial fishery with subsidies varying across locations currently provides that," he said.

Researchers say that without continued support of government, much of the urchin control to date could quickly unwind.

"Without urchin subsides, we're likely to see fishing contract to northeast of the state, and barrens expand significantly in valuable abalone fishery habitat in the southeast," Dr. Keane said.

"Meanwhile, more investment in product and market development would make the urchin industry more profitable, and that would make fishing at lower densities feasible, further enhancing ecosystem protection."

Dr. Cresswell said using modeling to establish whether the fishery had impacted urchin populations in different areas, then estimating how far the commercial harvest needed to be pushed to protect these areas, was the key to finding the balance.

"An example of this is to use a subsidy to 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 said.

"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.

"That is the fishery, and it is our main affordable management tool, meaning that when populations increase again in five to 10 years, the cost of getting the industry back up on its feet would be massive."

While the study focuses on Tasmania, the findings have broad implications for managing both range-extending and invasive species around the world.

"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," Dr. Keane said.

Dr. Cresswell said 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 modeling to guide flexible management decisions that balance sustainability with economic needs."

Fast facts:

• The larvae of Longspined Sea Urchin (Centrostephanus rodgersii) drifted south from NSW on a strengthening East Australian Current and settled in our warming Tasmanian waters. Adults of this species were first reported on mainland Tasmanian reefs at St Helens in 1978. The larvae drift for around 100 days.
• These voracious feeders have transformed over 15% of our healthy kelp beds into barren rock, called urchin barrens.
• The effect on Tasmanian reefs is profound, with more than 150 species moving out or lost in areas where barrens form, and the productivity of reef-based fisheries like abalone and rock lobster significantly reduced.
• Longspined Sea Urchins spawn in late winter, with larvae only able to develop in water temperatures above 12°C. Once much colder, ocean temperatures in Tasmania are now well above that threshold.
• IMAS and CSIRO researchers have been investigating options to control the urchin population, including working with government, industry, researchers and communities to assess and help direct commercial fishing efforts to protect important areas, expand urchin roe processing capability, progress an urchin waste to fertilizer initiative, and more.
• The commercial fishery has removed an average of 470 metric tons a year for the past 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.
• Urchin roe is considered a delicacy in many countries around the world. Most Tasmanian urchin roe is exported internationally, and there is high demand worldwide for this sustainable wild-caught Tasmanian product.
• Urchins are hand-harvested by scuba divers in Tasmania, down to approximately 26 meters. IMAS researchers have found evidence of kelp regrowth in areas where urchin numbers are lowered through fishing.
• Harvest incentives are funded by the State Governments Abalone Industry Reinvestment Fund which is .
• A , which recommended the government consider making an immediate national investment into Centrostephanus control, is still under consideration.