It’s not easy rearing all those urchins at the Oahu-based hatchery but the scientists keep getting better at it.

Kaneohe Bay has suffered from invasive seaweeds since the 1970s.

Two species in particular — Kappaphycus and Eucheuma, often used to make the food-thickening agent carrageenan — have grown so well in the bay that they smothered its patch reefs, killing corals. 

That is until the state decided a decade ago to fight back by deploying sea urchins to eat the seaweed.

Scientists outplanted the 1 millionth sea urchin into Kaneohe Bay this year to eat invasive seaweeds. (Grace Cajski/Civil Beat/2023)

The novel technique has proven successful, and this year marked the 1 millionth native collector urchin outplanted from a hatchery developed by aquaculturist David Cohen for the University of Hawaii Pacific Cooperative Studies Unit.

Officials with the state Department of Land and Natural Resources Division of Aquatic Resources, which houses the Anuenue Fisheries Research Center where the urchin are grown, notes a measured decrease in invasive seaweeds on every reef that has been stocked with lab-reared urchins.

“Not a lot of people are growing sea urchins, so they don’t know how to treat them,” Cohen said. “I’m pretty sure we’re the largest sea urchin hatchery at this point.” 

He explained that sea urchin farming is not easy. The process of growing sea urchins as biocontrol is fastidious and detailed — and a lot of it has been pioneered by this urchin hatchery team. 

Once a month, the team collects roughly 25 wild adult urchins as broodstock. Cohen said that consistently growing offspring of two wild urchins ensures that the hatchery maintains the wild-type, the form of an organism as it appears in the wild. 

They induce the urchins to spawn, and fertilize the eggs of six of the female urchins. After just 24 hours, the urchin larvae are swimming in their tanks. The team siphons off the larvae swimming at the top of the tank, and puts them in larval rearing tanks at specific densities. 

Sea urchin “spat,” miniature adults, grow out in tanks. (Grace Cajski/Civil Beat/2023)

These steps in the process are particularly unique to sea urchins, which have “very ornate, soft-bodied larvae,” said Cohen. “The skill sets that people have developed in large commercial (aquaculture) settings are largely for animals that are a little bit tougher than the sea urchins. It’s really easy to abuse them.”

Originally from Guilford, Connecticut, Cohen said he learned these skills at labs and companies in his home state. 

Gary Wikfors, chief of the National Oceanic and Atmospheric Administration’s Northeast Fisheries Science Centerʻs Aquaculture Sustainability Branch, thinks of Cohen as a product of Connecticut’s shellfish aquaculture community. 

“My impression is that David approaches aquaculture as a hands-on craft, rather than a theoretical, academic topic,” he said. “He has always been curious about how and why things work and apparently has the motivation to do what it takes to succeed.”

After three days, the urchin larvae begin eating. The team feeds them a specific amount of specially cultured phytoplankton. They count, using a microscope, how much the urchins have eaten. This process lasts for the rest of the larval cycle, about 28 days. 

The team then grows biofilm on clear, corrugated roofing material, called plates. They are placed in settlement tanks, as are the larvae. Within five days, the larvae have settled on the plates, and are starting to look like miniature adults, called “spat.” Three weeks later, the team counts how many urchins have settled. 

Kaneohe Bay Oceanfront reef aerial1.
An aerial view of Kaneohe Bay, where sea urchins are being deployed to eat invasive seaweeds. (Cory Lum/Civil Beat/2018)

The spat are moved to grow-out tanks, and once they get to the size of a pencil eraser, the team starts feeding them native limu that they have grown in another part of AFRC. When the urchins reach the size of a dime, they are outplanted in the bay. The facility is in operation 24 hours, seven days a week. 

“We’re pretty much the only ones who are doing this, this way,” said Cohen. 

The hatchery team is constantly refining their methods. Their first successful larval run was in August 2010. From 2012 to 2020, the hatchery averaged 50,000 outplanted urchins per year.

But they’ve recently learned how to keep urchins in the hatchery for longer, which leads to better survivorship overall. “We’ve upped our game,” Cohen said.

In 2020, they outplanted 260,000 urchins, and Cohen estimates that the hatchery could produce 300,000 urchins annually. 

Using native urchins as biocontrol does not address the invasive seaweed issue at its root though. Cohen urged people to think about the downstream effects of their actions: using fertilizers in the area surrounding Kaneohe Bay causes excess nutrients to flow into the ocean, which allows for invasive seaweeds to overgrow.

“The bay is out of balance, so we need to address the upland and clean that up,” he said.

No coral-dominated ecosystem can be healthy with excess competition from seaweeds, and urchins can mitigate the effects of these seaweeds. Upland change cannot happen overnight, so the urchins are playing a crucial role. 

“Herbivorous urchins keep corals healthy,” Cohen said. 

Civil Beat’s coverage of climate change is supported by The Healy Foundation, the Environmental Funders Group of the Hawaii Community Foundation, Marisla Fund of the Hawaii Community Foundation and the Frost Family Foundation. 

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