Everything you wanted to know about whale poo, but were afraid to ask

Whales fertilized ocean ecosystems, until we hunted them to near-extinction

Whale poo. If you’re a normal person, you’ve never thought about it. If you’re an ocean ecologist, you’re obsessed with it.

Gross though it might seem, whale poo turns out to be an absolutely central part of healthy ocean ecosystems. And the absence of whale poo has set the oceans into a kind of death spiral from which they’ve yet to recover.

The basic mechanism is straightforward. Many whales feed at depth, diving hundreds or even thousands of meters for a meal. But they tend to poo closer to the surface. The whale gut, then, acts as a nutrient conveyor belt, bringing key nutrients up from deeper in the water column and depositing them near the surface.

This is true of all kinds of nutrients, but the one that ocean ecologists get excited about is iron. Because iron is often scarce in surface waters, especially as you get farther from land. But iron is also absolutely central to the biology of phytoplankton: the photosynthesizing microorganisms at the base of the ocean food web. Everything in the ocean lives off of phytoplankton, directly or indirectly. As phytoplankton populations go, ocean ecology goes.

For a hundred years, people hunted large whales to near extinction in blissful ignorance of the crucial role their poo played in keeping oceans productive. What scientists now call the “whale pump” is a thing of beauty: whales are central to the production of their own food. Think of it as the original circular economy: whales eat the organisms that eat the organisms that eat their poo. But then, it’s not just whales: everything in the ocean depends on those poo-munching phytoplankton.

The problem, of course, is that idiot-old people went and hunted many whale species to near extinction. Sperm whales, in particular, were singled out for their unique oil. This was especially unfortunate, because these deep-feeding, shallow-pooing species are especially prodigious iron recyclers in the Southern Ocean. Industrial whaling broke the back of the whale pump, and the result is dramatically declining phytoplankton biomass.

A landmark study in 2010 found phytoplankton populations in long-term decline, and while there’s some evidence of recovery in more recent decades, overall phytoplankton populations remain far below 1950 levels. The steepest falls seem to be in the Southern Ocean, precisely in areas where deep-diving sperm whales were most important to iron cycling.

When industrial whaling was banned in the 1970s, the hope was that whale populations would recover quickly. They haven’t. In 2022, there were less than half as many sperm whales as in 1710. The breakdown in the whale pump is one important reason why populations haven’t really bounced back. There’s just not enough phytoplankton to support strong ocean food webs anymore, because there aren’t enough whales pooing to fertilize it.

If you read this substack, you know exactly where my mind is going here. We have a responsibility, as a species, to undo some of the ecosystem damage whaling did. Phytoplankton concentrations are far too low to allow whale populations to rebound because of the damage we did. This is our mess, it’s up to us to clean it up.

Ocean iron fertilization is conservation work. Replacing iron in the ocean isn’t some unprecedented intervention, it’s about making amends for past ecological wrongs. If we hadn’t hunted sperm whales to near extinction, we wouldn’t need to fertilize southern ocean gyres to restore their population.

But we did.

So we do.