Great Barrier Reef Bleaching Across 344,000 Square Kilometers as Biologists Reseed the Reef, Hectare by Hectare

The title does not exaggerate: the Great Barrier Reef indeed spans about 344,000 square kilometers, the area of the protected marine park off the coast of Queensland. Yet that figure misleads anyone who looks closely. Within this vast expanse, the coral reefs themselves account for only roughly seven percent of the park’s total surface, with the rest made up of sandy bottoms, seagrass beds, and mangroves. The real target for biologists who reseed coral is measured in tens of thousands of square kilometers, not hundreds of thousands. It’s already a gulf between ambition and the resources deployed.

One Night per Year, a Cloud of Gametes to Harvest

Mass spawning on the Great Barrier Reef typically occurs over two to three main nights, spanning two or three lunar cycles, toward the end of spring and into the austral summer. On those evenings, hundreds of coral species release their eggs and sperm simultaneously, packaged in tiny spheres. Most species are hermaphroditic and project their gametes into the water column, where the packets drift to the surface before breaking apart and fertilizing. It is this brief instant that diving teams, nets in hand, watch for to capture the cloud before it disperses.

Once collected, the larvae spend a few days in tanks. The Commonwealth Scientific and Industrial Research Organisation (CSIRO), together with Southern Cross University, has developed a method tested at Lizard Island in 2024: clear plexiglass boxes, called larval seedboxes, filled with larvae and then placed on the degraded seabed.

Each campaign yields tens of millions of coral larvae derived from mass cultures. The larvae exit the boxes at their own pace, drift with the current, and settle naturally onto the substrate. A single box can thus seed more than two hectares of reef. In 2018 already, the project led by researcher Peter Harrison of Southern Cross University aimed to restore hundreds of square meters, with the ultimate goal of reaching square kilometers. Seven years later, that objective remains an horizon, not a done deal.

What the Counters Say, One Year and Two Years After

The success figures exist, and they are scientifically solid. The first large-scale trial of seedboxes showed coral settlement up to 56 times higher, across thousands of square meters of reef. It is a clear, measured result, published.

But the path forward remains challenging. On the traditional attachment tiles, the newly planted recruits endure rapid mortality of about 80% during the first year, with survival continuing to decline afterward. A 2025 study on seeding devices found that after a year and a half to two years, yield varied greatly depending on site and species, reaching at best 32% at Moore Reef and dropping to 13% elsewhere. Another study conducted the same year on Acropora digitifera corals noted that after one year, more than 60% of devices still contained living coral, with yields of 83–91% on sites most exposed to swells.

These rates are not bad. They are even encouraging for a relatively young technique. The problem is not biology; it is the scale at which it is applied.

The Denominator That Crushes the Arithmetic

Against those hectares patiently seeded, the area that bleaches is counted in thousands of reefs. The 2024 bleaching event was the fifth mass bleaching episode since 2016 and the seventh since 1998, with 75% of reefs exposed to heat stress capable of causing bleaching. Of 1,080 reefs surveyed, 792 showed some level of bleaching, and 49% displayed high to extreme levels. The following year offered little real respite.

A new mass bleaching episode struck the Great Barrier Reef in 2025, the sixth since 2016, less widespread than 2024 but marking the second time the reef endured two consecutive years of such a shock. Aerial surveys covered 281 reefs from Torres Strait to the reef’s center, revealing widespread bleaching.

Relying on a simple rule of three here would be dishonest. Seedling programs report treated surfaces that vary greatly depending on whether they count the zone where the boxes are placed or the area recolonized a year later, and no official synthesis totals these efforts across the entire Great Barrier Reef. What can be said without trembling is that the scale gap between the hectares seeded each season and the thousands of square kilometers of reefs affected by a single bleaching event spans several orders of magnitude.

Buying Time, Not Saving the Reef

The researchers steering these programs do not claim to reverse the trend on their own. Peter Harrison, who launched one of the earliest larval seeding efforts, has himself warned against excessive optimism. He noted that his project would not “save” the Great Barrier Reef, specifying that the approach aims more to buy time for coral populations to survive and adapt until emissions are capped and the climate stabilized.

This is where the technique finds its real utility, more modest than restoration might suggest. It enables targeting strategic areas, a popular diving site, a natural nursery of larvae, a reef that feeds its neighbors through currents. It also allows selecting heat-tolerant colonies and multiplying them, a path being pursued by several Australian teams alongside pure seeding. Gaining two or three decades while ocean temperatures continue to rise is no small feat for an ecosystem that takes generations to rebuild on its own.

The next spawning rendezvous will come in a few months, as it has every year for millennia. The nets will be stretched again, the seed boxes filled again. The question is no longer whether the technique works; it does. It is how much time remains to deploy it before the sea takes back, each summer, what the previous winter had left standing.

Sources: la1ere.franceinfo.fr | lepetitjournal.com