Why healthy coral reefs need fish urine

One fish’s waste is a coral reef’s wealth.

It’s no secret that overfishing can diminish biodiversity in marine ecosystems. But in a study published Tuesday in the journal Nature Communications, researchers found a second consequence: as fish populations dwindle, coral loses an essential nutrient – fish urine.

“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.”

Coral reefs are an important resource for large-bodied fish in the Caribbean. They use the reef for shelter during the day, and as a hunting ground by night. Recent studies have shown that coral reefs rely on fish, too. Fish excrete ammonium, an essential nutrient for coral growth, through their gills. And fish urine contains phosphorus, another key nutrient.

But it was unclear exactly how crucial fish were to the nutrient flow of reef ecosystems. To find out, Dr. Allgeier led a team of researchers to survey nearly 150 fish species at 43 different Caribbean coral reefs. Each site had experienced varying degrees of fishing impact, with some untouched and others decimated.

They found that, at sites where predatory fish thrived, the coral reefs had healthy nutrient levels. 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 new research will allow greater understanding of the different ways fishing affects coral reef ecosystems, Allgeier said, inspiring more nuanced conservation efforts.

In recent years, the roles of animal waste and nutrient flow in marine ecosystems have become increasingly clear. In 2015, Australian marine scientists identified “predictors of resilience” in coral reefs – that is, factors that help reefs survive large bleaching events. Among them, they noted density of coral polyps had a significant impact. These organisms form a close partnership with tiny algae, which line the polyps’ digestive tract. The algae provide organic compounds, which allow the coral to build huge calcium carbonate structures.

And what do the algae get in return? Nutrients, in the form of coral waste.