Coral reefs wouldn’t be the same without their beautiful fish, many of which have adapted their appearance, body structure and lifestyle to take refuge in the reefs. Recent studies suggest that coral reefs, however, are just as dependent on these fish for key nutrients that help coral grow.
When fish urinate, they release phosphorus into the water. This phosphorus, along with nitrogen excreted as ammonium through the gills of fish, is crucial to the survival and growth of coral reefs.
A new study appearing in Nature Communications has found that in coral reefs where fishing occurs, nearly half of these key nutrients are absent from the ecosystem.
The main reason? There are fewer large-bodied and predator fish to pee nutrients into the water.
“Part of the reason coral reefs work is because animals play a big role in moving nutrients around,” said lead author Jacob Allgeier of the University of Washington’s School of Aquatic and Fishery Sciences.
“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.”
Researchers surveyed 143 fish species at 110 sites across 43 Caribbean coral reefs that varied in the amount of fishing pressure sustained — ranging from marine preserves that banned all fishing to heavily fished reefs where large predator fish are almost gone.
They found that reefs with more large, predator fish had healthy levels of nutrients, while reefs depleted of large fish had nearly 50 percent fewer nutrients, including phosphorous and nitrogen, essential to their survival.
“This study is useful to understand alternative ways fishing is affecting coral reef ecosystems,” Allgeier said.
The researchers determined that, despite the substantial reduction in fish-mediated nutrients, fishing didn’t substantially reduce the number of fish species present. Instead, the large reductions in fish pee were driven by the reduction of large-bodied fish and predator fish such as grouper, snapper or barracuda that occurs through selective fishing practices.
“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.