Hawaii reefs face added El Niño stress after storm damage

NOAA Coral Reef Watch confirmed the world’s fourth global coral bleaching event on April 15, 2024, after above-average sea surface temperatures spread across the equatorial Pacific Ocean. For Kona divers, that matters because West Hawaii reefs already carry storm damage, and the next warm-water stretch can decide whether popular sites hold color and structure or slide into a flatter, more algae-heavy look.

At the University of Hawaii’s School of Marine Biology Coral Resilience Lab at Moku o Loe in Kaneohe, Professor Eva Majerova and graduate student Alyssa Varela were shown inspecting coral tanks, a reminder that reef recovery now depends as much on lab work as on calm weather. Majerova is a coral molecular biologist focused on how corals acclimatize or adapt to changing climate, including heat stress. That kind of research is aimed at answering a practical question Kona guides already know well: which corals survive repeated hits, and which ones fade after one more hot season.

The warning signs are not theoretical. NOAA said that from January 1, 2023 through September 30, 2025, bleaching-level heat stress affected about 84.4% of the world’s coral reef area. NOAA’s Hawaii Integrated Ecosystem Assessment says sea surface temperature in the main Hawaiian Islands reached the highest recorded monthly temperature in well over a century in 2015, and West Hawaii waters hit 30.3 degrees Celsius, or 86.5 degrees Fahrenheit, that September. Those anomalously warm, prolonged conditions wreaked havoc on coral reefs in West Hawaii.

Scientists in Hawaii have already seen the pattern repeat. NOAA declared the third confirmed global coral bleaching event on October 2, 2015, and later described the 2019 bleaching event in Hawaii as a cautionary tale, noting that bleaching events are increasingly occurring in the islands. A University of Hawaii report tied the third-ever global bleaching event to heat stress from El Niño, which triggered mass coral bleaching and mortality on reefs around the world.

For divers heading out from Kona, the visible changes are likely to show up first on reef sites that have already taken storm hits and have less room to bounce back. Expect thinner patches of live coral, more algae on broken framework, and fewer of the fish and invertebrates that use complex coral heads as shelter. Even when a site stays diveable and the visibility holds, the reef itself may look more tired than it did a season ago, and the best thing a visiting diver can do is avoid adding more pressure while the coral works through yet another recovery cycle.