The colorful fish that dart in, around and above coral reefs depend on the environment's craggy folds for camouflage and protection. And it's not a one-way relationship. The fish return the favor in the form of nutrient-rich urine.

When fish urinate, they release phosphorous into the water. They also excrete nitrogen through their gills, and both are vital for the growth and survival of coral reefs, according to 2012 research from the University of Georgia and Florida International University.

A new study further investigates this concept, confirming that in areas where there is fishing, these nutrients are nearly missing from the ecosystem. The primary reason, the researchers found, was because there are fewer large, predator fish to urinate in the water, releasing those nutrients.

“Part of the reason coral reefs work is because animals play a big role in moving nutrients around,” said lead author Jacob Allgeier, a researcher at the University of Washington’s School of Aquatic and Fishery Sciences, in a statement. (Allgeier was a doctoral student at UGA and was one of the leads on the earlier study.) “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. If you take the big fish out, you’re removing all of those nutrients from the ecosystem.”

Researchers surveyed 143 fish species at 110 sites across 43 Caribbean coral reefs. They found that coral reefs with more large, predator fish had healthy levels of nutrients, while those reefs that had fewer large fish had nearly 50 percent fewer nutrients. Those nutrients included phosphorous and nitrogen, which are vital to the reefs' survival. The study was published in the journal Nature Communications.

“This study is useful to understand alternative ways fishing is affecting coral reef ecosystems,” Allgeier said. “Simply stated, fish biomass in coral reefs is being reduced by fishing pressure. If biomass is shrinking, there are fewer fish to pee.”

During his research, Allgeier caught live fish, put them in plastic bags and studied the nutrients in the water before and after 30 minutes. He found that larger carnivorous fish would urinate more phosphorous than smaller, herbivorous fish.

While coral reefs continue to face environmental pressures all over the world, Allgeier says curbing fishing that targets large predator fish may help the reefs recover and thrive.

This video from the researchers explains the relationship between the coral reefs and the fish:

Mary Jo DiLonardo covers a wide range of topics focused on nature, health, science and anything that helps make the world a better place.

Coral reefs need fish urine to thrive
More fishing means less phosphorus from fish urine, and that hurts the chemistry of a coral reef, says a University of Washington researcher.