More than 130,000 long-spined sea urchins have been removed during a trial operation to protect one of New Zealand’s most iconic underwater ecosystems – Tawhiti Rahi/Poor Knights Islands Marine Reserve.
The Department of Conservation (DOC), working alongside Te Whānau o Rangiwhakaahu, the University of Auckland, and Northland Regional Council, undertook the trial over two weeks in May. Dive teams spent nearly 290 hours underwater during 440 dives, covering five sites across six hectares, to halt the spread of urchin barrens – barren reefs stripped of life by increasing populations of the native long-spined sea urchin.
Despite being native, the long-spined sea urchin has seen a dramatic population boom – increasing more than 11-fold in the past 25 years. Warmer waters and fewer predators are thought to be key factors, and even in the fully protected marine reserve, where their numbers are now estimated to exceed 1.5 million.
Unlike kina, which have shown signs of natural decline under marine protection, long-spined sea urchins have continued to expand. They graze not just on kelp but on a wide range of marine life, threatening the rich biodiversity and the colourful communities of fixed marine animals – like sponges, corals, and anemones – that cover the vertical reef walls and make the Poor Knights internationally renowned
“This is the first coordinated removal effort specifically targeting long-spined sea urchins in a New Zealand marine reserve,” says DOC Marine Technical Advisor, Dr Monique Ladds.
“The goal is to slow the spread of urchin barrens in the Poor Knights while we continue to investigate long-term solutions. Follow-up surveys in July will help assess the effectiveness of the removal and guide future management decisions.”
The removal work follows successful but smaller trials in 2023 which showed rapid recovery of the kelp and wall communities.
“This is not a long-term fix,” says Dr Ladds.
“Although removals may help buy time in some areas, we know they are not a sustainable strategy for managing the scale of the problem. We’re continuing to work with iwi, scientists, and partners to explore future options for protecting these ecosystems at the Poor Knights and elsewhere. This trial is one way we’re testing what’s possible. When we take action, nature can bounce back.”
Marine reserves managed by DOC are protected areas, and removing or harming marine life without a permit is illegal.
