Coral Reefs Thrive on Nutritious Fish Urine

August 17, 2016 | Erica Tennenhouse

Coral reef with tropical fish
Photo credit: Fascinating Universe/Wikipedia (CC BY-SA 3.0)

Fewer large fish means less fish pee for coral growth.

For the colorful fish that inhabit coral reefs, day-to-day life consists largely of swimming, feeding, and excreting. The reef environment doubles as both home and latrine for these inhabitants. But it is a vital arrangement, as their urine contains phosphorus, which, along with the nitrogen excreted as ammonia from their gills, helps the corals grow.

According to a new study published in Nature Communications, when fishing occurs in coral reefs, nearly 50 percent of these key nutrients are depleted from the ecosystem.

The researchers compared the amount of nutrients stored in fish tissue, and the amount available to corals via excretion, across 110 coral reef fish communities in the Caribbean with varying degrees of fishing pressure.

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Heavy fishing did not appear to reduce the number of fish species present, but it did lead to substantial reduction in nutrients. The researchers determined that targeted removal of larger-bodied fish and species higher up in the food chain was causing the scarcity of fish urine.

"Simply stated, fish biomass in coral reefs is being reduced by fishing pressure,” study lead author Jacob Allgeier, from the University of Washington's School of Aquatic and Fishery Sciences, said in a press release. “If biomass is shrinking, there are fewer fish to pee."

Though coral reefs are well supplied with solar energy, they tend to be starved for nutrients, like phosphorus and nitrogen, leading to heavy reliance on the nutrients stored and cycled by local fish.

"Part of the reason coral reefs work is because animals play a big role in moving nutrients around," Allgeier commented. "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."

He proposes that curbing fishing practices that target large predatory fish could boost the recovery of declining coral reefs in the Caribbean and worldwide.

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