Eight lessons about how this ocean works. Each one answers a single question, and each one is about the coast outside your window rather than an ocean somewhere else.
Lesson 1
Why coral reefs matter
What is a coral reef actually made of?
A reef is built by tiny animals called polyps, each one smaller than a grain of rice. Each polyp builds a hard cup of limestone around itself. When it dies the cup remains, and the next generation builds on top. A reef is therefore a stack of skeletons, built up over thousands of years.
Inside each polyp live microscopic algae. The algae capture sunlight and make sugar, and share it with the coral. In return the coral gives them a safe place to live. Neither does well alone.
When the sea gets too warm, the coral expels its algae and turns bone white. This is bleaching. If the water cools soon enough the algae return; if not, the coral starves. Kenya's reefs bleached badly in 1998 and have bleached again since.
Did you know? A reef covers less than one percent of the sea floor, yet roughly a quarter of all ocean species depend on reefs at some point in their lives.
Lesson 2
Mangroves: the forest in the sea
How can a tree live in salt water?
Most plants die in salt water. Mangroves have three tricks: some block salt at the roots, some push it out through their leaves, and some dump it into old leaves and drop them.
Their arching roots stand above the mud so the tree can breathe when the tide is in. Those same roots make a tangle that young fish and prawns hide in.
Mangroves also protect the land. A belt of mangrove absorbs the force of a storm surge before it reaches houses behind it.
Did you know? Mangrove forests store several times more carbon per hectare than tropical rainforest, most of it locked in the mud below.
Lesson 3
Seagrass: the underwater lawn
Is seagrass the same as seaweed?
No. Seaweed is algae — no roots, no flowers, no seeds. Seagrass is a true flowering plant that returned to the sea about a hundred million years ago. It has roots, it flowers, and it makes seeds underwater.
Seagrass meadows are nurseries. Many of the fish caught on this coast spent their early life hiding in seagrass before moving out to the reef.
Green turtles graze it. Dugongs — very rare now in Kenya — eat almost nothing else.
Did you know? Seagrass grows fast enough that a healthy meadow can bury carbon in its sediment for thousands of years.
Lesson 4
Tides: why the sea comes and goes
Why does the sea rise and fall twice a day?
The Moon pulls on the ocean. Water on the side of Earth facing the Moon bulges towards it, and water on the opposite side bulges away. As Earth spins, each coast passes through both bulges — which is why this coast gets two high tides and two low tides roughly every day.
The Sun pulls too, though less. When Sun and Moon line up, the pulls add together and you get very high and very low tides — spring tides. When they are at right angles the tides are gentler — neap tides.
Fishermen here plan around the tide. Some grounds only fish well on a particular state of tide, and at spring low tide the reef flat is walkable.
Did you know? At a big spring low tide on this coast you can walk out onto reef that is normally under two metres of water.
Lesson 5
Kaskazi and Kusi: the winds that rule the sea
Why does the ocean change through the year?
Two monsoon winds shape life on the Swahili coast. The Kaskazi blows from the north-east roughly from November to March: gentle seas, clear water, good fishing.
The Kusi blows from the south-east roughly from April to October. It is stronger, the sea is rougher, and small boats stay in more often.
For centuries these winds carried sailing dhows between the East African coast, Arabia and India — out on one monsoon and home on the other. The Swahili language and coastal culture grew out of that traffic.
Did you know? Before engines, a dhow captain could not simply come home when he chose. He waited for the wind to change, sometimes for months.
Lesson 6
What makes fishing sustainable?
Can you catch fish and still protect the sea?
Yes — but only if you take less than the sea replaces. Three things decide it: how much you catch, what gear you use, and whether you let fish grow up and breed first.
Gear matters enormously. A handline catches the fish that takes the hook. A bottom trawl drags a net across the seabed and takes everything, breaking the structure as it goes.
Size matters too. A fish caught before it has bred once has contributed nothing to the next generation. Minimum sizes exist for exactly this reason.
Some Kenyan communities close an area for a few months and then reopen it. The animals inside grow, breed, and spill out into the water around. Octopus closures on this coast have worked particularly well.
Did you know? Small, fast-breeding fish like sardines and mackerel can be fished far harder than slow-growing reef fish like large groupers.
Lesson 7
Plastic and the ocean
Where does plastic in the sea come from?
Most of it starts on land. Rubbish dropped in a town washes into a drain, the drain runs to a river, and the river runs to the sea. Very little is dumped directly from ships.
Sunlight and waves break plastic into smaller and smaller pieces. It does not disappear — it becomes microplastic, small enough for a fish to swallow by mistake.
Lost fishing gear is a particular problem. A net that drifts free keeps catching things for months. That is called ghost fishing, and Kenyan fishermen have been testing nets tied with cotton twine that rots and lets the net fall apart.
Did you know? Kenya banned single-use plastic carrier bags in 2017, one of the strictest such bans anywhere.
Lesson 8
A warmer ocean
How does climate change reach the sea?
The ocean has absorbed most of the extra heat trapped by greenhouse gases. Warmer water bleaches coral, and it holds less dissolved oxygen, which fish need.
The sea has also absorbed a large share of the carbon dioxide we have released. Dissolved in water, it makes the sea slightly more acidic, which makes it harder for corals and shellfish to build their skeletons and shells.
Warmer water expands, and ice on land melts into the sea, so sea level rises. For a low coast with towns close to the water, that matters a great deal.
Did you know? Reefs that still have plenty of grazing fish, like parrotfish, recover from bleaching faster than reefs where those fish have been fished out.
