Coral reefs may be feeling the effects of overfishing much more severely than we previously thought, a new study suggests. The team found that fish urine is critical to maintaining coral healthy and well fed, while a dwindling fish population might spell doom for a reef environment.

Coral aren’t that good at searching for food, seeing as they’re completely immobile. So they do the next best thing and filter the waters around them, waiting for all the nutrients they need to come to them. But researchers recently found that one very important nutrient source of corals that’s having a hard time coping with human activity — the fish that live in coral reefs.

“Part of the reason coral reefs work is because animals play a big role in moving nutrients around,” Jacob Allgeier, an ecologist at the University of Washington, said in a statement.

“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.”

“Moving nutrients around” is a nicer way of saying that the corals feed on fish pee. It’s a mutually-beneficial arrangement. Large bodied fish in the Caribbean use the reef for shelter during the day and as hunting grounds by night. They naturally excrete ammonium through their gills, which is an essential nutrient for coral growth. And their urine contains phosphorus, another key nutrient.

But to find out exactly how important fish were in the nutrient balance of a reef ecosystem, Dr. Allgeier led a team of researchers to survey nearly 150 fish species at 43 different Caribbean coral reefs. Each site had felt the effects of fishing differently, with some left untouched while others were completely decimated. The researchers found that in reefs where predatory fish thrived, the corals showed signs of having a bountiful diet. Reefs with fewer fish lacked necessary nutrients by as much as 50 percent.

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

The findings will help us better understand how different ways fishing affect coral reef ecosystems, Allgeier said, so we can devise more efficient conservation practices.

The full paper, “Fishing down nutrients on coral reefs” has been published online in the journal Nature.

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