Coral spawning on the Great Barrier Reef: when to see it

Coral spawning on the Great Barrier Reef happens once a year, a few nights after the November full moon, and this year that moon rose on 8 November. Corals release tiny bundles of eggs and sperm all at once, after dark, and the water fills with floating egg and sperm bundles. The best nights to look are usually 10 to 14 November off Far North Queensland, though some corals spawn earlier or later in the season.

This guide explains how the timing works, what you will see and how to book a night trip.

Sources for this section: Dive Planit, 2021 coral spawning event; timeanddate.com, moon phases for 2022.

When does coral spawning happen?

Marine biologist and Reef Teach guide Gareth Phillips gives a simple rule. Spawning generally happens two to six days after a full moon in November, when the water has stayed above 27 degrees Celsius for a month. It also needs little tidal movement.

The full moon in November 2022 falls on 8 November. Count two to six nights from there and you reach the nights of 10 to 14 November. That is a guide, not a promise. Phillips says scientists can predict a timeframe, but much about the event is still a mystery, including what sets off the shared release.

The timing moves from year to year. In 2020 the full moon came late in November. Some corals spawned earlier in the month, but most released their bundles over the weekend of 4 to 6 December. In 2021, operators in Far North Queensland confirmed the main spawning nights as 23, 24 and 25 November.

Sources for this section: Travmedia, Genetic gold-mine of tough corals (6 December 2020); Dive Planit, 2021 coral spawning event; Dive Planit, #spawningGBR (2017); timeanddate.com, moon phases for 2022.

What you will see

People often call it an underwater snowstorm. Each coral colony lets go of small bundles that hold both eggs and sperm. The bundles drift up toward the surface, and a thick brown slick can form there as fertilisation begins.

Most corals, about 75 per cent, are hermaphrodites, which means each polyp is both male and female. They release eggs and sperm together into the water. Other corals, called brooders, work differently. A male releases sperm, which swims to a female coral that holds ripe eggs, and the eggs are fertilised inside. The larva that results can float for several days, or up to two months, before it settles on the sea floor and starts a new colony.

Marine biologist Stuart Ireland has filmed spawning in Tropical North Queensland for more than 20 years. After the 2020 event he said he saw soft corals, hard corals, massive corals, branching corals and even mushroom corals spawning on one night. Whale sharks are also known to enjoy what one writer called coral caviar.

Sources for this section: Travmedia, Genetic gold-mine of tough corals (6 December 2020); Dive Planit, #spawningGBR (2017).

Why it happens at night

The bundles are food for many reef animals. Phillips explains that spawning mostly happens at night, when the plankton eaters are asleep. That gives the eggs and sperm a better chance to meet and survive.

Corals also cannot move to find a mate, so a mass release on the same night is how they reach each other.

Sources for this section: Travmedia, Genetic gold-mine of tough corals (6 December 2020); Dive Planit, 2021 coral spawning event.

Why it matters for the reef

David Wachenfeld, Chief Scientist at the Great Barrier Reef Marine Park Authority, has called mass coral spawning the single most important annual event in the Reef's recovery. The Reef had three mass bleaching events in five years, in 2016, 2017 and 2020. Wachenfeld described the corals that survived as a genetic gold-mine of tough corals that have shown they can live through marine heatwaves.

Each new generation carries some of that toughness. That is why scientists, divers and tour guides watch the November moon so closely.

Sources for this section: Travmedia, Genetic gold-mine of tough corals (6 December 2020); Dive Planit, 2021 coral spawning event.

How to see it and book a night dive

You need to be in the right place on the right night, so most visitors join a special spawning trip. Dive operators along the Reef run them each year. They range from an overnight dive tour to multi-day liveaboard trips.

Here is how to plan one.

1. Ask for the dates early. In 2021 the Far North Queensland dates were confirmed on 27 September, about two months ahead, and trips were listed straight away.
2. Pick the trip length. The 2021 choices included a double night dive for $270, a 3 day, 2 night trip with Divers Den for $850 twin share and a 3 day, 2 night trip with Pro Dive Cairns for $915 twin share. These are last year's prices, so check what each operator charges now.
3. Book two or three nights. The spawn does not always fall on one night, so a trip that covers a few nights gives you more chances.
4. Expect to dive or snorkel in the dark. Ask each operator what experience and certification it needs before you pay.

For 2022, most spawning trips from Cairns are likely to be tied to the nights after the 8 November full moon, so check with operators now for any places still open. If you miss it, the next November full moon is the usual window, and a late full moon can push spawning into December, as it did in 2020.

Sources for this section: Dive Planit, 2021 coral spawning event; Travmedia, Genetic gold-mine of tough corals (6 December 2020).

Report what you see

If you see spawn, you can help. The Marine Park Authority's Eye on the Reef app lets reef visitors record reef health and animal sightings. In 2017 the Citizens of the Great Barrier Reef group used the #spawningGBR hashtag and the app to gather spawning reports from divers, operators and research boats. Reports show scientists where and when spawning is happening and how the Reef is doing.

Planning more time in Queensland? Our Cairns to Port Douglas drive guide covers the road between two Reef gateways, and the Airlie Beach travel guide covers the Whitsundays side.

Sources for this section: Dive Planit, #spawningGBR (2017).

Sources

1. Travmedia, "Genetic Gold-Mine of Tough Corals" Reproduce and Ring in New Life for the Great Barrier Reef, 6 December 2020: au.travmedia.com
2. Dive Planit, Everything you need to know about this year's coral spawning event, 27 September 2021: diveplanit.com
3. Dive Planit, The Great Barrier Reef gets it on, 17 November 2017: diveplanit.com
4. timeanddate.com, Moon phases, November 2022: timeanddate.com