There’s no polite way to say this, so I’m just going to come out with it: coral reefs run on poop. OK, maybe ‘run’ is a bit of an exaggeration but, certainly, faeces, from marine animals such as fish and seabirds, plays a significant role in supporting coral reef ecosystems.
Yes – poop. Fish poop. Bird poop. It's all part of the invisible machinery that helps coral reefs function.
Let’s start with fish. Every day, thousands of reef fish cruise the coral gardens, nibbling, grazing, or chomping their way through algae, plankton, and even coral. What goes in must come out, and when it does, it’s often in the form of little nutrient packages that fertilise the reef. Some fish, like parrotfish, even poop out sand. (In fact, in some places like Hawaii, a significant portion of the beach beneath your towel is technically parrotfish poo. See my blog post The Sand Poopers.)
But it’s not just about fertilising the reef or topping up the sandbanks. Fish poo – particularly from herbivores – is food for other species. Recent research has shown that certain reef fish will actually eat the faeces of their neighbours. It might sound gross, but in a nutrient-limited environment like a coral reef, nothing goes to waste. It’s all part of a tight recycling loop that keeps the whole system going.
Birds get in on the act too. Seabirds feed far out at sea and return to their island rookeries to roost, depositing nitrogen-rich guano on the land. When it rains, that guano trickles into the sea, supercharging nearby coral reefs with extra nutrients. This isn’t just theory – scientists have found that reefs near seabird islands are healthier, with faster-growing corals and more abundant fish. The whole reef community gets a lift from these nutrient deliveries.
So, we can see that fish poop supports the reef – fuelling coral growth, feeding other animals, even helping shape the sandy beaches we love. while bird guano, delivered from far-off feeding trips at sea, brings in nutrients that supercharge the ecosystem. It’s a kind of natural fertiliser conveyor belt, running from the sky to the sea.
However, where there are seabirds there are often rats – introduced into the mix often by accident, via passing ships or visiting humans. They feast on eggs, chicks, and even adult birds, reducing the numbers of seabirds. With fewer seabirds, the flow of guano slows. And without that nutrient boost, the nearby reef ecosystems begin to suffer. Studies have shown that fish biomass can be 50 per cent lower around rat-infested islands compared to rat-free ones. That’s a big drop for something most people have never even heard of.
Some of the most productive and biodiverse reefs are those near seabird islands – provided the birds are left undisturbed. When rats invade and drive birds away, the nutrient flow stops. It’s a reminder that reef health isn’t just about what happens underwater – it depends on the whole system working together.
Here on Norfolk Island, we have two other islands in our remote group, Nepean Island and Phillip Island, plus a number of small islets that are remnants of the island’s volcanic origins immediately off the coast, particularly along the northern and western shore. These are all important safe havens for seabird breeding colonies. So far they have remained rat and cat free, making them important not just for the seabirds themselves but also for the ecosystems that are interconnected with them, including our inshore coral reef area.
From the fish that poop sand, to the birds that fertilise the sea with guano – and even the fish that eat other fish’s droppings – the reef is a tightly connected web. And while it might not be glamorous, poop is one of the most important links in that web.
So next time you’re snorkelling and a fish lets one go right in front of you … take it as a sign that the reef is doing what it’s meant to.
Above: a banded scalyfin (Parma polylepis) lets one fly. This damselfish is part of the reef’s recycling crew, farming algae and fertilising the neighbourhood as it goes.
Good poop, bad poop
I can hear the questions about the difference between the faeces of fish, birds, human and cattle, so it would be remiss of me not to address this. Here’s a very quick summary.
You’d think that if a bit of bird poo is good for corals, then a bit of human waste might be too. After all, it’s all just nutrients, right? Not quite. It turns out that not all poop is created equal.
Fish and bird waste are part of the natural cycle. This waste is released in small amounts and taken up quickly by plants, corals, or other animals. But human sewage and livestock runoff come in much larger quantities, and often during storms or floods when septic tanks overflow or paddocks are flushed into the sea. These pulses overwhelm the reef.
They also contain things reef ecosystems haven’t evolved to deal with: pathogens that can infect corals, pharmaceuticals that disrupt fish behaviour, and nutrients at concentrations high enough to fuel algal blooms that smother coral and suck the oxygen out of the water.
In short, fish poop supports the reef. Our poop, when it reaches the ocean untreated or in excess, unravels it.
Want to dive deeper?
Grupstra, C. G. B., et al. (2023). Consumer feces impact coral health in guild-specific ways. Frontiers in Marine Science, 10, 1110346. https://doi.org/10.3389/fmars.2023.1110346
van der Heijden, L. H., et al. (2022). Feces consumption by nominally herbivorous fishes in the Caribbean: Coprophagy as a nutrient cycling mechanism. Coral Reefs, 41(1), 1–12. https://doi.org/10.1007/s00338-022-02228-9
Lavers, J. L., et al. (2018, July 12). Rats are damaging coral reefs by attacking seabirds. ABC Science. https://www.abc.net.au/news/science/2018-07-12/pests-rats-damaging-coral-reefs-great-barrier-reef/9957628
Sidley, P. (2018, July 12). Bird poop helps keep coral reefs healthy, but rats are interfering. Science News Explores. https://www.snexplores.org/article/bird-poop-helps-keep-coral-reefs-healthy-rats-are-interfering
National Oceanic and Atmospheric Administration. (n.d.). How does land-based pollution threaten coral reefs? NOAA National Ocean Service. https://oceanservice.noaa.gov/facts/coral-pollution.html
Reef Builders. (2023, April 28). New study finds herbivore fish poo is bad for corals (sometimes). Reef Builders. https://reefbuilders.com/2023/04/28/new-study-finds-herbivore-fish-poo-is-bad-for-corals/