Where Fish Pee, Corals Grow

By Nathaniel Scharping | August 18, 2016 3:16 pm

You can’t see it, but these fish are spreading nutrients all over. (Credit: Tischenko Irena/Shutterstock)

There’s something in the water around coral reefs. And that something is fish pee. Although you may cringe at the thought of swimming through clouds of urine, coral reefs wouldn’t be the same without fishes’ urinary benevolence.

Fish excretion may be the furthest thing from your mind as you swim through a vibrant coral reef, but it a plays a vital, though underappreciated, role in supporting the diversity of life under the sea. That’s because it contains two important elements: nitrogen and phosphorous. Fish spread them around by ingesting plants and other fish, extracting the nutrients through the digestive process and then excreting them as they swim. From there, the nitrogen and phosphorus go on to feed the algae that sustain corals.

Overlooked Element

University of Washington ecologist Jacob Allgeier has been studying the importance of fish pee in the Bahamas for several years now and has published papers showing that coral reefs, which get little outside nutrient input, thrive only when these nutrient levels remain within a fairly narrow band. He focuses on the role fish play in this process — specifically how overfishing and changes to fish populations negatively impacts reefs by upsetting the delicate chemical equilibrium that the critters help maintain.

In his latest study, published Tuesday in Nature Communications, Allgeier narrowed his focus to figure out which kinds of fish were important to reefs. So he captured hundreds of fish and temporarily held them in his version of make-shift test tubes — water-filled plastic bags. His goal was to find out how many nutrients different kinds of fish put into the water. He looked for concentrations of both nitrogen and phosphorus, testing the water before and after a fish had been held in the bag for half an hour. In total, Allgeier gathered data from 143 species across 43 Caribbean reefs.

Top of the Food Chain

He concluded that when it comes to reefs, it’s the big fish that matter most — the predators on the top of the fish food chain. These scaly sea fauna, including groupers, snappers and barracuda, tend to produce more phosphorus than small fish, meaning that removing them from the reef can cause a harmful shift in the ratio of nutrients in the water.

Unfortunately, they also happen to be the most desirable to fishermen in the region. In the same study, Allgeier compared heavily fished reefs to those protected by marine reserves and found that overfished reefs had half as many essential nutrients as those that were protected. The reefs’ distance from populated areas was another important factor in determining whether nutrient levels were affected.

In oceans where reefs are already under threat from warming waters, rising acidification and even viruses, further altering their intricately composed environment comes as just another straw on their backs. We might not ever really think of, or see, fish pee, but take it away, and a stage in the cycle of reef life goes with it.

MORE ABOUT: animals, ecology
  • http://www.weddymoons.com/ Rick S Garrett

    There you go with Global Warming again. If GW is the culprit then why aren’t coral reefs dying everywhere? If fish pee is an essential ingredient to healthy reefs then it seems like overfishing is a better place to start. After all, a specific reef can be overfished without affecting the reefs around it, but GW would affect all reefs would it not?

    • lump1

      Well, different reef species respond differently to higher water acidity. But still I think you make a good point: Right now, our overfishing is causing more environmental harm than our CO2 release. And it’s not like we don’t know how to reverse the damage we’ve done to the seas. We just need tighter quotas, enforcement with more teeth and larger no-fishing zones. We’re being far too blase about it, maybe because people just want to whine about the harm we do rather than solve problems. It doesn’t help that we don’t have the attention span for more than one environmental problem, and that some people have been made to think that because of CO2, the seas are fucked anyway, so what’s the point of trying?

      • OWilson

        It’s a vicious circle.

        We have to act NOW! Before it’s TOO LATE! We are running out of time! We are approaching a TIPPING POINT, we have PASSED the tipping point.

        All this urgency designed to loosen purse strings, just like those TV ads.

        The TV ads eventually go away and life goes on, but with climate it is important that we move the goalposts to the NEXT TIPPING POINT, otherwise people would just paraphrase you:

        “We are fucked anyway, so what’s the point!

        So, Doomsday always has to be around the corner, to raise any money at all, no matter how many tipping points are passed.

    • http://www.mazepath.com/uncleal/qz4.htm Uncle Al

      We must allocate unlimited budgets (Carbon Tax on Everything) to create megagallons of nature-equivalent fish whiz to Save the Earth and its little children. I’m thinking past expiry date Kraft Cheeze Whiz (thank you Edwin Traisman!) diluted with Brazilian Guanabara Bay juice (chunkies to be emulsified), then add a shot of Gatorade shaken not stirred/

      Who hears the Anthozoa when they cry?

    • OWilson

      They should do a study on Marine Biology’s impact on reefs.

      Thousands of scientists on huge fully equipped cruisers, dragging anchors across reefs, diving in gear, capturing fish, injecting poison in starfish to kill them by the thousands (we have to kill them to save them).

      They do more harm in a week than my seldom driven SUV could do in a lifetime!

      • http://www.mazepath.com/uncleal/qz4.htm Uncle Al

        Further the daily struggle! Hasten to join shock brigades of exemplary labor to end Klimate Kaos!

        A Progressive stance declares something must be done, now!, at large scale with generous public funding. A paternalistic hegemony demands something “appropriate” must be engaged in racist genderist violation of scientific social constructionism’s “all knowledge is subjective and based on one’s position in society” ((re Dorothy Smith and Standpoint Theory). No microaggressions, no trigger events are allowed in a moral culture of preening academic intellectual vacuity.


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