Coral reefs suffer when deprived of a vital, life-giving ingredient: fish pee

Everything is connected.
Everything is connected.
Image: AP Photo/David J. Phillip
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The beauty and vibrance of coral reefs relies on a delicate balance of ecological factors. One of the most important factors, it turns out, is fish pee.

Just like crops grow better when sprinkled with manure, underwater fields of coral also need fertilizer to flourish. A team of American researchers published a study this week in Nature Communications, revealing the crucial role that urine plays in keeping coral healthy.

When fish urinate, they release phosphorus into the water, more so for carnivorous fish than smaller herbivores. Phosphorus, together with nitrogen excreted as ammonium through gills, are two key ingredients that coral needs to grow.

The researchers examined 110 sites across 43 Caribbean coral reefs—ranging from pristine, protected marine preserves to heavily fished sites—and studied 143 fish species in the process. At each site, divers counted the fish and identified the species living there. They took various fish and put them in a plastic bag for half an hour, measuring the nutrition levels in the water before and after. Nearly 73,000 fish later, they found that the nutrient concentration in reefs where fishing took place was 50% lower than in non-fished sites.

However, fishing didn’t significantly reduce the number of species living at coral reefs—that is, decrease biodiversity. Rather, fishing removed some large, mostly predatory fish (and their phosphorus-rich pee) from the ecosystem, which had an outsize impact on nutrient levels. Big fish like grouper, snapper, and barracuda usually spend the day relaxing among the coral—peeing away vital nutrients all the while—and go out to hunt for prey by night.

The study points out that even though maintaining biodiversity is essential, sustainable fishing practices should take into account the species that significantly contribute (with their pee) to the reef’s nutrition cycle.

Coral reefs are extremely valuable breeding grounds for thousands of species, buffers against storms, and big tourist attractions. They are in more trouble than ever. In addition to ocean acidification and rising sea temperatures that causes bleaching, we can now add a shortage of fish pee to the growing list of threats to these vital habitats.