Poor Knights Islands Marine Reserve: 130,000 sea urchins removed in trial

A diver removes long-spined sea urchins at Poor Knights Marine Reserve last month. Photo / DoC

More than 130,000 long-spined sea urchins were removed as part of a trial to protect one of New Zealand’s most iconic underwater ecosystems.

More than 1.5 million of the urchins are estimated to be in the Poor Knights Islands Marine Reserve, raising concerns about the spread of urchin barrens.

The barrens describe parts of the reef stripped of other life because of increased populations of the native long-spined sea urchin.

The Department of Conservation (DoC), Te Whānau o Rangiwhakaahu, the University of Auckland, and Northland Regional Council undertook a two-week trial last month to stop the spread of urchin barrens.

Dive teams spent nearly 290 hours underwater during 440 dives and worked across 6ha to remove long-spined sea urchins.

Long-spined sea urchin at Poor Knights Marine Reserve. Photo / DoC

DoC marine technical adviser Dr Monique Ladds said the trial was the first coordinated removal effort that specifically targeted long-spined sea urchins in a New Zealand marine reserve.

“The goal is to slow the spread of urchin barrens in the Poor Knights while we continue to investigate long-term solutions.”

According to the department, the urchins graze on a wide range of marine life which threatens the rich biodiversity and communities of fixed marine animals — sponges, corals, and anemones — that cover the vertical reef walls and made the Poor Knights internationally renowned.

The urchins, which are native, have reportedly had a dramatic population boom believed to be because of warmer waters and fewer predators.

DoC said the number of urchins had increased more than elevenfold in the past 25 years.

“Unlike kina, which showed signs of natural decline under marine protection, long-spined sea urchins continued to expand.”

Ladd said last month’s removal work followed successful but smaller trials in 2023 which showed rapid recovery of the kelp and wall communities.

Follow-up surveys in July will help assess the effectiveness of the trial and guide decisions around future management.

“Although removals may help buy time in some areas, we know they are not a sustainable strategy for managing the scale of the problem,” Ladd said.

DoC was continuing to work with iwi, scientists and others to explore future options for protecting ecosystems within the Poor Knights Islands.

“This trial is one way we’re testing what’s possible. When we take action, nature can bounce back,” Ladd said.

Marine reserves managed by DoC are protected areas, and removing or harming marine life without a permit is illegal.