Sep 6

Fish Turds

Category: scuba

Here’s an essay I wrote as part of the National Aquarium in Baltimore‘s volunteer interview process.  I’m working on being one of their volunteer divers, and if I get in, I’ll be scuba diving in the big exhibits regularly, speaking with visitors, and doing a lot of scut work. I figure that I should aim for an audience of sixth-graders escorted by adults.

Let’s talk about fish turds.  That’s right, piscine poop, A.K.A. fish feces. 

Why talk about such a dirty subject?  It’s nasty, and we usually avoid it.  But it’s super important, and a great way to illustrate how everything that lives is connected.

Inventive creatures like us who live inside have great ways of making poo go away.  Flush – it’s gone – it’s outta your life, don’t have to think about it.  One of the outstanding traits of fish is that they tend to live outside (except here in the aquarium) – perhaps you’ve noticed.  So they don’t have toilets.  They just have to go wherever they are.

Where does their poo go?  It sinks – of course!  But most of it doesn’t hit the bottom.  As fish poo sinks, it gradually disappears, and the deeper you go, the less poo is left.  By the time it reaches the bottom, almost none is left.  Why?

Now you might not want to think about it, but they say that one man’s trash is another man’s treasure… and as it turns out, poo is nutritious.  In the sea, little is wasted.  Fish are an enterprising lot.  If there’s free food sinking out there, someone’s going to eat it.  It’s called coprophagy – which means “poo-eating” – and in the ocean, it’s all the rage. 

Now you might not want to eat it, but fish eat a lot of things that we wouldn’t want.  Like other living, wriggling fish, for instance.  And then they poo them out. And their poo has lots of edible stuff left in it – fats and proteins that other fish need.  In fact, predatory (meat-eating) fish poo is more popular than herbivorous (plant-eating) fish poo, because it’s more nutritious.  Some fish even make a living by following other fish around, eating their poo.

Most of the poo that doesn’t get eaten by fish will get consumed by snails, shrimp, crabs, and even coral.  Some of it dissolves into the water, where it gets eaten by microscopic animals called zooplankton, so that almost none of it ever really lands on the bottom, and if it does, it doesn’t last long.  When you dive into a healthy coral reef and swim around near the bottom, you see lots of animals, but no turds – they’ve all been eaten!

          So life goes on the reef, with lots of happy animals eating each other’s poo.  But the story of poo doesn’t end there.  Parrotfish eat coral, which gets ground up inside and pooed out as sand.  This poo-sand is clean and white, and accounts for a large amount of the nice tropical sand that everyone loves.  An individual parrotfish can make hundreds or even thousands of pounds of sand every year!  And they don’t necessarily poo it out where they eat, either – since they swim around, they can leave it somewhere else, and can distribute the sand to new areas.  The same thing is true for many fish – they eat nutrients in one area, and poo them out in another, allowing poo-eating creatures to follow and set up homes in new places.

          Coprophagy makes life possible.  Without it, the ocean would become choked with fish turds, and many animals would run out of food or die of the pollution.  And that would make the beaches of the world much less fun.

Deloach & Humann, Reef Fish Behavior (1st ed), pg.280


Helfman, Collette & Facey , The Diversity of Fishes(1st ed), pg. 440


Robertson, Fish Feces as Fish Food on a Pacific Coral Reef


Bailey & Robertson, Organic and Caloric Levels of Fish Feces Relative to its Consumption by Coprophagous Reef Fishes


U.N. Atlas of the Oceans,


Smriga, Sandin & Azam, Microbial Communities in Coral Reef Fish Feces


Brabrand et al, Can Iron Defacation from Fish Influence Phytoplanton Production and Biomass in Eutrophic Lakes?




3 Comments so far

  1. spleeness September 8th, 2008 4:22 PM

    That’s my honey! Now let’s talk about it some more when we go out to eat. You can update me after you finish explaining the mummification process of extracting brains through the nose. I never forgot that (and neither did anyone withing a 5 foot radius)!

  2. Niffer September 8th, 2008 10:11 PM

    This is awesome! I’ve always wanted to be one of those guys behind the window! I thought they had an excellent job… just swimming with the fish. It sounds like fun!

    Are you serious about the Parrot Fish pooping sand? I thought sand was just the ultimate end for rocks that erode away until they are sand. I’m thinking your story sounds a little fishy.

  3. Dan September 9th, 2008 11:28 AM

    I’m totally serious about the parrotfish pooping sand. I’ve witnessed it many times. One of them will dart by (they’re football-sized, or larger) and squirt out a jet of sand. I even have a blurry picture of this happening, I will post it later. Parrotfish and their relatives are responsible for a great deal of the sand in tropical areas – I don’t know what percentage, but an adult can produce a ton per year. Imagine all those fish, for eons, producing a ton per year.

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