A new survey of five Florida Keys' coral reefs shows extensive damage from a long-lasting and severe marine heat wave last year.

Why it matters: Corals are havens for biodiversity, providing shelter for over 25% of ocean animals, and they are major drivers of fishing and tourism revenue. Increasingly, climate change is threatening their viability.

The details: The preliminary results show that less than 22% of the approximately 1,500 staghorn coral surveyed are still alive.

• Of the five reefs surveyed by NOAA's Mission: Iconic Reefs program and the Mote Marine Laboratory and Aquarium, only the two most northern ones, Carysfort Reef and Horseshoe Reef, had any living staghorn coral.
• And of those surveyed, live elkhorn coral was only found at three sites.
• No living staghorn or elkhorn corals were found at sample areas surveyed at Looe Key Reef, located in the lower Florida Keys, NOAA stated in a release.

How it works: Warm water corals mainly thrive in a narrow temperature range.

• 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, including eventual mortality if the extreme temperatures continue.

By the numbers: The coral mortality seen as a result of last year's marine heat wave — which came on earlier — was more severe and lasted longer than usual for this region.

• According to Katey Lesneski, the monitoring coordinator for NOAA's Mission: Iconic Reefs program, the roughly 30,000 staghorn coral outplants at these five reefs (planted between 2020 and 2022) had one-year survival rates from an average of 40% to over 75%.
• "Outplanting" refers to planting coral fragments that were grown in nurseries, back onto reefs.

Yes, but: Scientists have not yet completed their surveys, including reefs where other coral species may have fared better.

Between the lines: During the past year, scientists at NOAA, Mote, universities and other institutions went to extraordinary lengths to save corals and avoid seeing hard-won conservation gains completely wiped out in the Keys.

• This included removing in-watery nursery corals from the hot seas, placing them in nurseries on land, and then re-planting them once water temperatures cooled.
• Coral evacuations such as this are an extreme conservation measure that may not be viable over the long term, particularly if marine heat stress becomes increasingly common, severe and long-lasting, as climate projections show.

The intrigue: Scientists with NOAA's Coral Reef Watch program, which monitors and warns of marine heat waves worldwide, recently added three new alert categories and colors as a direct response to the 2023 marine heat waves seen around the world.

• The new categories include a Bleaching Alert Level 5, which corresponds to a risk of "near complete mortality."
• Last year was a particularly severe year for marine heat waves around the world. The combination of long-term, human-caused climate change and the El Niño climate cycle has contributed to record-high ocean heat content worldwide.
• According to NOAA, the cumulative heat stress on corals in the Florida Keys during 2023 was nearly three times the previous record.