Just yesterday we told you why you shouldn’t pee while taking a dip in a chlorinated pool. But the rules are different for fish. It’s not just that they don’t have much alternative; the urine of big predatory fish is a vital nutrient source for coral reefs.
Coral reefs are complex ecosystems, and their diversity suffers without any of their components. When overfished, for example, the coral can become overgrown with algae. A paper in Nature Communications has demonstrated that fish are essential to coral reefs for other purposes as well, particularly maintaining stocks of phosphorus and organic nitrogen.
"Part of the reason coral reefs work is because animals play a big role in moving nutrients around,” said lead author Dr Jacob Allgeier of the University of Washington 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."
Allgeier and his co-authors studied 110 sites at 43 Caribbean reefs. Some reefs were in areas where fishing is banned, others have limited restrictions, while still others were almost entirely depleted of large predatory fish. The reefs were tested both for fish numbers and for the concentrations of nutrients.
Big fish pee more than small ones, and reefs without them had barely more than half the essential nutrients of those where fish are numerous. The finding is the missing piece to 30-year-old research showing that coral grows twice as fast where fish stocks are healthy.
Relevant information for future Finding Nemo sequels, perhaps?
A Nassua grouper, center, one of the larger fish that keeps reefs fertilized. Craig Layman
Allgeier's general conclusion was anticipated, although the extent was unknown. However, Allgeier found something less expected. Fishing didn’t affect the number of fish species present on reefs, and this diversity didn’t matter for nutrients. What was important was the weight of fish for a particular area, known as the biomass, much of which is concentrated in big fish such as grouper, snapper, and barracuda.
"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. Moreover, he found that carnivorous fish have more phosphorus in their urine than herbivores.
The problem is that simply retaining diversity of fish species is not enough to keep coral reefs at full functionality. The authors argue any attempts to preserve healthy reefs, and probably other ecosystems, require an understanding of the ways animals affect the availability of nutrients.
The limited size of coral reefs makes them a relatively easy environment to study, and the finding invites further research on more dispersed ecosystems, such as the role whales and other big sea creatures play in fertilizing the open oceans with their waste.
In the meantime, the work further builds the case for limiting fishing on coral reefs to protect these threatened ecosystems.
Bags like this were used to measure how much nitrogen and phosphorus fish excreted. Jacob Allgeier.