Metrics

Bleach Score

A 1–6 Likert scale where 1 = no bleaching.

A 1–6 Likert scale where 1 = no bleaching, 6 = fully bleached.

Raw value	Meaning
1	No bleaching
3	Partial
6	Fully bleached

What bleaching indicates: Coral bleaching is a stress response — typically to elevated sea surface temperatures. Bleached corals have expelled their symbiotic algae (zooxanthellae). Prolonged bleaching leads to death. Bleach scores spike during thermal events (e.g. El Niño) and recover when temperatures normalise.

Note on direction: Bleach and health scores share the same 1–6 scale and direction — lower is better in both cases.

Provenance

The 1–6 bleach scale is adapted from the CoralWatch Coral Health Chart (Siebeck et al. 2006). CoralWatch's brightness scale maps lower numbers to lighter (more bleached) corals and higher numbers to darker (healthier) corals. Coral Gardeners inverts this direction so that 1 = no bleaching and 6 = fully bleached — keeping the convention consistent with the health score, where lower numbers always mean better condition.

Methodology note: The 2025 Methodology Manual references bleaching monitoring (ReefApp every 2 weeks during bleaching events, and % bleaching at bommie level) but does not define the 1–6 scale or its provenance. The scale definition is operational practice. The scale is planned to move from 1–6 to 1–5.

Siebeck, U.E., Marshall, N.J., Klüter, A. & Hoegh-Guldberg, O. (2006). Monitoring coral bleaching using a colour reference card. Coral Reefs, 25, 453–460. doi:10.1007/s00338-006-0123-8