Scientists start returning rescued corals after deadly Florida heat wave

• Coral reefs provide shelter for over 25% of ocean animals, and are vital parts of marine-based economies. For example, in Florida they are a major driver of tourism.

How it works: Warm water corals have a narrow temperature range in which they thrive.

• When water is too warm, they expel algae that lives in their tissues, which causes them to turn white (an event known as coral bleaching.) Bleached corals are more susceptible to further heat stress.

Zoom in: The return of 5,000 rescued corals after being held for three months is a sign that water temperatures have dropped somewhat, but don't yet signal an "all-clear."

• They may be more susceptible to disease outbreaks in the next three to six months, according to Cynthia Lewis, director of the Florida Institute of Oceanography's Keys Marine Laboratory, operated by the University of South Florida, told Axios in an interview.
• Ocean temperatures surrounding South Florida and the Florida Keys reached historic levels this spring and summer, and hit records unusually early in the warm season.
• Some locations recorded water temperatures in the high 90s°F to low 100s°F in July, with the dangerously hot water persisting from spring through early fall.
• In total, the Keys Marine Lab housed more than 5,000 rescued corals in tanks for three months.

Yes, but: Physically removing vulnerable corals from the ocean during heat waves is not a viable long-term conservation strategy, Lewis noted.

• Scientists are seeking to maintain the genetic diversity of the reefs, and eventually breed the species that are most heat resistant.
• "We're now trying to breed stronger corals. It's like taking the most resistant resilient ones and putting them together and breeding them and then growing those corals," Lewis said.
• "You need that genetic diversity and to roll the genetic dice and you may have some offspring that are just more resistant to the new system that they find themselves in."
• She noted that there are myriad threats to these reefs besides warming oceans, such as water pollution.

What they're saying: "This is death by 10,000 cuts," Lewis said.

• "If we had not brought any of these corals then some of these species probably would have gone extinct this summer, never to be seen again on the reef. Certainly some of these genotypes would have, and the reefs are going to die without our help."

The big picture: This year so far features the hottest ocean temperatures on record globally, by far, with a record-high percentage of the oceans roasting in marine heatwaves.

• Warm water corals are considered especially vulnerable to climate change. The U.N. climate science panel warned in 2018 that if warming exceeds 1.5°C compared to preindustrial levels, the likelihood of widespread bleaching and mortality would dramatically increase.
• Currently, the world is on track for more than 2°C of warming by 2100, a shift that would cause warm water coral reefs to "mostly disappear," scientists concluded.

By the numbers: Scientists have not fully accounted for the damage from the 2023 marine heat wave. Across NOAA's Mission: Iconic Reefs program in the Keys, a partial survey hints at the likely toll, however.

• According to Katey Lesneski, the research and monitoring coordinator for Mission: Iconic Reefs, a mid-August survey of certain outplanted elkhorn and staghorn coral at the project's seven reefs found that of the corals that recently died, 32.7% of staghorn and 45.4% of elkhorn most likely died due to heat stress.
• Of the corals that were still alive, about 89% of elkhorn corals surveyed and 91% of staghorn coral surveyed were exhibiting signs of heat stress.

What's next: NOAA plans to conduct another survey in January to get a fuller picture, since the marine heat wave lasted beyond mid-August.