Coral polyps live in a narrow thermal window with symbiotic algae (zooxanthellae). When water
gets too warm they expel the algae — bleaching — and coral cover declines; low pH slows the
calcification corals need to build new skeleton. Fish populations track coral cover with a lag,
since reef structure is their habitat and nursery.
Bleach risk = max(0, T − 29.5) × 18 [Degree Heating Weeks proxy]
dCover/dt = growth×(1−bleach) − fishingPressure×decay − bleach×lossRate
dFish/dt = k×(Cover − Fish/capacity)
- Water temperature — above ~29.5°C, bleaching risk rises sharply and coral cover erodes.
- Ocean acidity (pH) — lower pH (more acidic) slows coral growth/calcification rate.
- Fishing pressure — higher pressure removes herbivorous fish that keep algae off coral, accelerating decline.
- Protect (MPA) — instantly drops fishing pressure to simulate a marine protected area.
- Restore coral — adds a one-time coral-cover boost representing active reef restoration/coral gardening.
Real-world relevance: this loop mirrors why marine protected areas and coral restoration
programs (e.g. the Great Barrier Reef and Caribbean reef-rebuilding projects) focus on both cutting
local stressors like overfishing and buying time against global warming-driven bleaching.