Marine biologists have found a surprising relationship between the amount of fish urine and the health of coral reefs.

Scientists have long understood that overfishing can affect biodiversity in our oceans, but now a second consequence has been discovered. This Tuesday a group of researchers published the results of their latest study in the journal Nature Communications, which showed that as fish populations fall corals are deprived of an essential nutrient, namely fish urine.

Fish Urine, Coral Reefs
Photo by USFWS Pacific

Fish recycle nutrients in marine ecosystems

“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.”

In the Caribbean, coral reefs are important for the survival of large-bodied fish, which use them to shelter during daylight hours and hunt during the night. More recently it has been shown that coral reefs benefit from the presence of fish, which excrete ammonium through their gills. Coral use ammonium for growth, while fish urine contains another key nutrient, phosphorus.

This latest study explores how vital fish are in maintaining the flow of nutrients in coral reef ecosystems. In order to do so Dr. Allgeier and his team surveyed almost 150 fish species at 43 different Caribbean coral reefs, some of which had been almost destroyed by fishing and others that were relatively untouched.

Fish urine provides nutrients to coral reefs

The study shows that those sites where predatory fish were present in large numbers had coral reefs with healthy nutrient levels, while sites with fewer fish showed a lack of up to 50% of nutrients.

“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.

A paper published in science in 1980s revealed that fish-heavy coral reefs grew twice as fast as those where there were no fish. This study inspired Allgeier to look into the reasons behind the relationship between fish and coral.

Coral reefs are a prime example of a delicate ecosystem, with high levels of biodiversity but few spare nutrients. This means that efficient nutrient transfer is essential for the growth of coral, and this cycle is largely controlled by fish.

Scientists still working on understanding coral reefs

Allgeier says that the research will grant scientists a better understanding of how fishing impacts coral reef ecosystems. This could in turn inspire more finely tuned conservation efforts.

Over the last few years various pieces of research into animal waste and nutrient flow in marine ecosystems have been published. Marine scientists in Australia worked out so-called predictors of resilience in coral reefs in 2015, described as the factors that can help reefs to survive bleaching events.

One factor is the density of coral polyps. These organisms work in conjunction with tiny algae which lines the digestive system of the polyps, providing organic compounds that the coral use to build calcium carbonate formations. In return the algae receive nutrients in the form of coral waste.

It must be hoped that research continues to inform our actions towards coral reefs with the ultimate aim of protecting them as sites of incredible biodiversity.