Restoring kelp forests by culling sea urchins makes financial and ecological sense: study

Restoring underwater kelp forests by culling overgrazing sea urchins would deliver significant financial benefits, a new study has found.

Kelp provides shelter and food for countless species while removing pollutants like nitrogen and phosphorous from the water.

But sea urchins that feed on the kelp have reached unsustainable population levels in southern Australia.

In a new study led by RMIT University, researchers found despite the cost, restoring southern Australian reefs from urchin barrens is feasible and beneficial for improved water quality, fisheries and carbon sequestration.

Study lead author, Dr Paul Carnell, said although climate change and human factors were also to blame for disappearing kelp forests, managing overabundant sea urchin populations was key to restoring healthy reefs.

“Managing the sea urchin population is a practical step we can take to ensure the health of our waterways, which are crucial to local ecosystems, tourism and food security,” said Carnell, a senior lecturer at RMIT’s Centre for Nature Positive Solutions.

“Now we have the figures to show this is also a smart economic investment.”

Australia’s kelp forests have been declining, with kelp cover in Victoria’s Port Phillip Bay dropping by 59–98% over the last 40 years, while the native purple sea urchin has become 2.5–4.2 times more abundant.

For the study, published in Ecosystem Services, researchers modelled the costs and benefits of urchin culling at different sites in Port Phillip Bay, factoring urchin density, dive depth, travel time and kelp’s potential to boost nitrogen storage.

Economic modelling showed an AU$50 million investment in targeted culling and kelp restoration would return up to AU$92 million in economic benefits.

The AU$50 million investment would go towards paying commercial divers to cull sea urchins and the cultivation and deployment of kelp to help restore reefs, creating jobs in those sectors.

Most of the AU$92 million gained would be from the value of removing nitrogen from Port Phillip Bay but also includes boosts to recreational fishing and carbon storage to mitigate climate change, Carnell said.

“We know from previous trials in Port Phillip Bay that this is an effective approach to restore these reefs,” he said.

The researchers hope this study will guide the future prioritisation of kelp restoration on reefs that have degraded through sea urchin overgrazing.

The research was led by RMIT with Deakin University, The University of Melbourne, University of Western Australia, and Canopy Economics and Policy. The Victorian Government Department of Environment, Land Water and Planning funded the study.

‘Prioritising investment in kelp forest restoration: A spatially explicit benefit-cost analysis in southern Australia’ is published in Ecosystem Services. DOI: 10.1016/j.ecoser.2025.101739.