Spawning season gives coral nurseries chance to shine

Aaron Hunt and Brittany Balli from Cayman Eco Divers work on a Grand Cayman coral nursery. – Photo: Charlotte Roslev

Each year, in August or September, the spawning of Cayman’s hard corals on the reefs occur. Operators of coral nurseries along Grand Cayman’s west coast are hopeful that this year’s and future spawning will give threatened coral species a fighting chance on its road to a return of abundant growth.

While some coral nursery programmes involve growing corals on underwater artificial ‘trees’ and then replanting that coral onto the reef, Eco Divers Reef Foundation’s approach involved erecting the trees within a short distance of the reef, so that when the coral spawning occurs, the gametes and eggs released will find their way to the reef and create new coral growth.

Aaron Hunt is one of the founders of Eco Divers Reef Foundation which, in partnership with Divetech, Don Foster’s, Sunset House and Grand Cayman Eco Divers and, under the guidance of the Department of Environment, have been growing staghorn corals on ‘trees’ in the sandflats off the west shore of Grand Cayman since 2017. Currently, there are 14 of the nurseries in existence along that coastline.

The foundation’s plan is “simple, yet effective”, Hunt said.

Spawning season

“Every summer, our corals spawn a few days after the full moon at the end of August or early September. Healthy, adult corals release gamete packets containing reproductive material to seed the nearby reef. Drawing from decades of research, the timing of these spawning events has been able to be observed by our team starting in 2018 and continuing on to today. Our goal is simply to create enough healthy coral to participate in that spawning event every summer,” he explained.

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But managing the nurseries is a year-round effort, with cleaning and maintenance of the coral trees being carried out regularly, as well as work to protect them from
predators or storm damage.

“We also must choose how to stock our nurseries so that multiple corals of the same species are nearby during spawning so that they have the best chance of fertilising the gamete bundles and settling onto the reef as new coral larvae,” Hunt said. “This method is not a quick fix either.”

Aaron Hunt of Eco Divers Reef Foundation inspects one of the coral nurseries off the west coast of Grand Cayman.

He added that annual spawning requires years to see strong results and instead, many programmes opt to transport their nursery corals directly to nearby reefs and plant them there.

But, he said, even with the most intensive of efforts, years of growing and nurturing the coral nurseries can end in defeat, and the typical long-term survival for “outplanted” corals hovers between zero and 20% success – “a grim option for a dwindling species”.

“We cannot fix what we refuse to accept. Maybe there is a reason corals disappeared from a specific area,” he said, “and maybe forcing new corals back to that same spot is a flawed concept. Unless the cause of the original loss is understood and corrected, the results can only ever be disappointing.”

Hunt said, after the spawning occurs, fertilised gamete bundles settle back onto the reef “in a surprisingly close range”.

Volunteers with Cayman Eco Divers perform a weekly clean-up of a coral nursery near Divetech’s Lighthouse Point dive site in West Bay.

He added that studies had shown the gametes can reach the reef from as far as 1.2 miles, but the foundation and local dive operators’ efforts show that the greatest settlement is out to around 550 years (500-600 metres).

“This is the target area that we have based our plan around,” he said. “Our coral nurseries and spawning sites form a network of evenly spaced sites 1,000-1,200 meters apart. As the currents carry coral larvae north one year, they might head south the next. Each site feeds toward both of its neighbours, filling its own target area and scattering larvae loosely into its neighbours’ zone,” he said.

Initial absence of staghorn coral

The foundation’s initial surveys on the west coast displayed a dramatic absence of staghorn coral, Hunt said – “We could swim 10 minutes without finding one and our average ranged from 3-7 staghorn corals in each 500-meter range we examined.”

“After our first year of spawning, we observed 43 instances of [staghorn coral] settled and growing a year later. In our second year, we counted to 200 and accepted that many were overlapping and becoming a jumble of overlapping corals,” he added.

Hunt said the foundation planned to carry out a five-year monitoring of coral spawning and settlement of larvae.

“We are currently in our third year and, considering all the challenges of 2020-2021, we are proud of how well our planning and caution has rewarded us. We represent what is arguably the final chance for humans to reverse the extinction of coral reefs worldwide. Within our lifetime, they may well disappear. It is our goal to prevent that terrible option. So we move thoughtfully, we plan and predict, we listen to new voices and established research. But most important, we continue,” he said.

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