Cleaner wrasse females died more often than males during a marine heat event

At Australia’s Lizard Island, female fish mortality more than doubled during the 2024 El Niño-related warming, while male mortality did not rise.

Editorial illustration — not from the study.

Researchers tracked 731 adult cleaner wrasse across eight reefs around Lizard Island on Australia’s Great Barrier Reef. The fish first function as females and some later change sex and become males, allowing the study to compare survival and growth across these life stages.

Males had higher survival than females during normal conditions and during the warming event. Female mortality more than doubled during the event, while male mortality did not change. Growth declined in both sexes, and the researchers suggest that females’ generally faster growth may partly help explain their higher overall mortality.

How survival differed

Terminal-phase males—fish that had changed sex—had higher survival than initial-phase females under both ordinary conditions and the 2024 ENSO event. During the event, female mortality more than doubled, while male mortality was not affected. Growth declined in both sexes during the event. The researchers identify females’ generally faster growth as a partial explanation for their higher overall mortality, but also propose that differences in energy demands or sensitivity to high temperatures could contribute.

Why the pattern matters

The findings challenge the common expectation that males in species with competing males should die sooner, especially during environmental stress. They show that in a sex-changing reef fish, sex and the timing of sex change can shape how populations respond to climate extremes. That matters for understanding how warming and coral loss may affect the makeup and persistence of reef fish populations.

Evidence and limits

The study followed individually recognizable fish across eight reefs for two years, including the 2024 ENSO event, which brought a temporary 1-degree temperature increase, severe coral bleaching and coral mortality at Lizard Island. This is observational evidence from one fish species, one island region and one event, so it does not establish which mechanism caused the higher female mortality. The researchers discuss possible roles for direct heat effects, reduced food availability and females’ energy demands, but these explanations were not directly separated in the study.

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