Every year, sometime between October and December, the Great Barrier Reef stages one of the most extraordinary biological events on Earth. Hundreds of coral species synchronize their reproduction, releasing billions of egg-and-sperm bundles into the water in a mass spawning event that turns the ocean into what looks like an underwater snowstorm. I’ve been lucky enough to witness it multiple times over the past fifteen years, and it still takes my breath away.

The 2025-2026 spawning season is now behind us, and the data we’ve collected tells a mixed but cautiously encouraging story. Here’s what happened, what it means, and why mass coral spawning remains one of the most important indicators of reef health we have.

How Spawning Works

Mass coral spawning is triggered by a combination of environmental cues—water temperature, lunar cycle, daylight hours, and possibly chemical signalling between colonies. Most species spawn after the full moon in late spring, when water temperatures have been rising steadily for several months. The timing has to be precise because eggs and sperm are viable for only a few hours in open water.

Broadcast spawning corals (the majority of species on the GBR) release their gametes into the water column, where fertilisation occurs externally. The fertilised eggs develop into larvae called planulae, which drift in currents for days to weeks before settling on hard substrate and, if conditions are right, growing into new coral colonies.

The synchronisation is remarkable. Individual colonies within the same species spawn within minutes of each other. Different species spawn on different nights over a period of several days, reducing competition for settlement space and hybridisation. The whole process is orchestrated by environmental signals that coral scientists still don’t fully understand.

What We Saw This Season

Our monitoring covers a stretch of reef between Cairns and Townsville, focusing on five sites we’ve tracked since 2018. This season’s spawning was notable for several reasons.

First, the timing was close to predicted. The main spawn event for Acropora species—the dominant reef-building corals—occurred four nights after the November full moon, which is within the normal window. Climate disruptions can shift timing, so “on schedule” is actually good news.

Second, the intensity was strong at three of our five sites. When I say intensity, I mean the visible density of spawn slicks and the quantity of bundles observed during dive surveys. At our northern sites, the water was thick with bundles—one of the stronger spawning events I’ve seen there in several years.

Third, and more concerning, our two southern monitoring sites showed noticeably lower spawning activity than previous years. These are sites that experienced moderate bleaching during the 2024-2025 marine heatwave. Bleached corals that survive often redirect energy toward recovery rather than reproduction, which means reduced spawning the following season. This is expected but still worrying when you see it in the data.

Why Spawning Intensity Matters

Coral reefs are under pressure from warming waters, ocean acidification, storm damage, crown-of-thorns starfish, and water quality issues. Their primary mechanism for recovery is recruitment—new coral larvae settling and growing into colonies that replace dead or damaged ones.

If spawning is weak, fewer larvae are produced. Fewer larvae mean fewer potential recruits. Fewer recruits mean slower recovery from disturbance events. It’s a straightforward chain, but one that the Australian Institute of Marine Science tracks closely because it directly predicts the reef’s capacity to bounce back.

A single strong spawning event doesn’t guarantee good recruitment. Larvae have to survive days or weeks drifting in the plankton, avoid predation, find suitable settlement substrate, and survive their vulnerable early weeks as tiny settlers. Mortality at each stage is enormous. Out of millions of larvae produced by a single colony, only a handful might survive to become visible juvenile corals.

But without spawning, there are no larvae at all. Spawning intensity is the first step in a long chain of recovery processes, and when it drops, everything downstream suffers.

Temperature and Timing Concerns

One of the questions we’re tracking is whether warming waters are shifting the timing of spawning events. There’s some evidence from other reef systems that thermal stress can disrupt the environmental cues that trigger synchronised spawning. If different colonies within a species spawn at slightly different times, fertilisation rates drop because eggs and sperm don’t encounter each other in sufficient concentrations.

On the GBR, timing hasn’t shifted dramatically—yet. Our data shows year-to-year variation of a few days, which is within normal range. But the cumulative effect of rising baseline temperatures is something we’ll need to watch over the coming decades. Even a small desynchronisation could significantly reduce reproductive success.

Water temperature during the weeks before spawning also affects gamete quality. Corals stressed by heat in the months leading up to spawning may produce fewer or lower-quality eggs. This is harder to measure than spawning intensity itself, but laboratory studies have shown that heat-stressed corals produce eggs with less lipid content—meaning less energy reserves for the developing larvae.

The Bright Spots

Not everything in the data is concerning. Several observations from this season gave me genuine optimism.

Juvenile coral density at our northern sites has been trending upward over the past three years. This suggests that spawning events from 2022-2023 successfully produced larvae that settled and survived. We’re seeing small Acropora colonies—5 to 15 centimetres—in areas that were heavily bleached just a few years ago. Recovery is slow but it’s happening.

We also documented spawning from several coral species that we’d classified as significantly depleted at those sites. When species that were barely hanging on manage to spawn, it means they’ve recovered enough energy to invest in reproduction. That’s a meaningful threshold.

Diversity of spawning species was comparable to our baseline data. This matters because reef resilience depends on species diversity. A reef dominated by one or two fast-growing species is less robust than one with a diverse community of corals with different growth strategies, thermal tolerances, and ecological roles.

What Happens Next

The larvae from this season’s spawning are now out in the world. Over the next few months, we’ll be surveying our sites for evidence of coral settlement and early recruitment. Settlement tiles—small ceramic plates we deploy at monitoring sites—will give us quantitative data on how many larvae settled and where.

The real test is survival through the coming summer. Juvenile corals are vulnerable to temperature stress, predation, competition from algae, and physical disturbance. If summer 2026-2027 brings another marine heatwave, many of this season’s recruits won’t make it.

But if conditions are moderate—and the current La Nina forecast suggests they might be—this year’s cohort could contribute meaningfully to reef recovery. Coral reefs have incredible regenerative capacity when they get a break from repeated disturbance. The question, as always, is whether they’ll get that break.

I’ll be back on the reef next month to begin settlement surveys. The spawning snow globe was beautiful this year. Now we wait to see what it produced.