Fish Pee Key for Coral Reefs

Removing big fish from reefs also removes the nutrients they provide.

Coral reefs have long been known for providing bounteous habitat for numerous fish species. Now, it turns out, the fish return the favor -- by peeing. And the bigger the fish, the greater the impact.

"Part of the reason coral reefs work is because animals play a big role in moving nutrients around," said Jacob Allgeier, a postdoctoral researcher at the University of Washington's School of Aquatic and Fishery Sciences and lead author of a study this week in the journal Nature Communications. "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."

Phosphorus in fish urine and nitrogen excreted through their gills in the form of ammonium are important nutrients for coral reefs to grow. In many reef communities, fish will take shelter in and around coral during the day -- peeing out valuable nutrients -- then forage for prey in and around the reef by night.

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Accordingly, when fish are removed from a coral reef ecosystem by fishing, the amount of nutrients declines, which the Nature Communications paper underlined. Study co-authors Abel Valdivia at the Center for Biological Diversity in San Francisco and Courtney Cox of the Smithsonian Marine Station in Fort Pierce, Fla., surveyed 143 fish species at 110 sites across 43 Caribbean coral reefs that varied in the amount of fishing pressure sustained -- from those that are heavily fished to marine reserves where all fishing is banned.

Heavily fished reefs in many cases had 50 percent fewer nutrients, including phosphorous and nitrogen -- but not because fishing had reduced the overall number of species. Rather, the biggest differences occurred with declines in or absences of large-bodied fish and predator fish such as grouper, snapper or barracuda. The reason, Allgeier has found, is that smaller fish produce fewer nutrients because of their high metabolism and different diet.

"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. He is presently working with researchers from the University of California, Santa Barbara to replicate the research on reefs in the tropical Pacific Ocean.

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