At first glance, the parrotfish seems like the goofiest animal to grace Hawaii’s coral reefs, with its mouth full of beak-like teeth. But the reef wouldn’t be the same without this fish, which nibbles fast-growing algae off the corals. This grazing allows sunlight to reach the symbiotic algae that live within the coral polyps, letting them produce energy. In the process, the parrotfish gnaws off some of the coral’s calcium carbonate skeleton and poops it out as sand—some 800 pounds of the stuff each year per fish. This builds Hawaii's famous beaches.
But this intricate relationship between reef species is now strained, thanks to human waste. Hawaiian cities have centralized wastewater treatment infrastructure, but the state also has some 88,000 cesspools—pits that readily leak sewage into the sea. Also, septic tanks retain solid human waste but release nitrogen-rich liquid. All of this stimulates algae. (It’s similar to the way runoff from farms using nitrogen fertilizers can cause blooms in nearby waterways.) If there aren’t enough herbivorous fish off the Hawaiian coast due to overfishing, there’s nothing to keep the green stuff in check. It chokes out the corals, starving them of energy, and prevents baby corals from taking hold.
This ecosystem is already under threat as ocean temperatures rise. When corals get stressed, they release the symbiotic algae that give them energy and color. That phenomenon is called bleaching.
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