Coral Reefs Depend on Big Fishes’ Pee

None of us live, or pee, in a vacuum.* The substances that leave our body interact with our environments in a million little ways. Traces of prescription drugs in our waste travel through the waterways and affect the way shrimp live their lives. Someone else’s pee in a public pool can raise your risk of developing asthma. But not all excrement is harmful. Scientists say that fish pee is a necessary—and threatened—component of healthy marine ecosystems. The researchers published their findings in the journal Nature Communications. 

Lead author Jacob Allgeier studies ecology at the University of Washington’s School of Aquatic and Fishery Sciences. “Part of the reason coral reefs work,” he said in a press statement, “is because animals play a big role in moving nutrients around.” 

Researchers have been working since the 1980s to understand the relationship between fish and the health of their homes on the reef. Recent research has suggested a primary link: fish waste. Their pee is rich in phosphorus, and their gills give off ammonium—two chemicals corals need to stay healthy and strong.

Allgeier and his colleagues wanted to test this hypothesis by looking at reefs where fish had been removed. They visited 43 different coral reefs in the Caribbean, ranging from pristine to over-fished. Allgeier caught and measured hundreds of live fish. He placed each one in a bag of water for a half-hour, then analyzed the water once the fish were gone.

Fish in a bag. Image credit: Jacob Allgeier

He found that large, carnivorous fish peed out more phosphorus than smaller herbivores, and that bigger fish almost always gave off more ammonium, making them especially desirable to a community of coral. Unfortunately, these same fish are also quite desirable to fishers, who yank the big ones right out of the ecosystem equation.

“Simply stated, fish biomass in coral reefs is being reduced by fishing pressure,” Allegeier says. “If biomass is shrinking, there are fewer fish to pee.” 

He and his colleagues say these findings should influence future thinking about fishery management. “Fish hold a large proportion, if not most of the nutrients in a coral reef in their tissue, and they’re also in charge of recycling them,” he said. “If you take the big fish out, you’re removing all of those nutrients from the ecosystem.”

*This is not a challenge. Do not pee in your Dyson.

Know of something you think we should cover? Email us at tips@mentalfloss.com.