The ocean is home to more animals than anywhere else on Earth — and it is changing faster than at any time in human history. This is a short, free guide to the world's fish, the threats a warming climate brings, and why they're worth protecting.
Scientists have described more than 35,000 species of fish — from tiny gobies smaller than your fingernail to whale sharks longer than a bus. They live everywhere the water goes: in sunlit coral gardens, in rushing mountain rivers, and in the crushing dark of the deep sea. Coral reefs alone cover less than 1% of the ocean floor, yet they shelter around a quarter of all marine fish species. Fish feed billions of people, hold ocean food webs together, and are woven into cultures all over the world.
When we burn fossil fuels, most of the extra heat and much of the carbon dioxide don't stay in the air — they go into the sea. That quietly rewrites the conditions every fish depends on.
The ocean has absorbed more than 90% of the excess heat trapped by greenhouse gases. Surface waters have warmed roughly 0.13°C per decade for a century and could rise 1–4°C by 2100. Fish are cold-blooded, so even small shifts change where they can survive, feed, and breed.
The ocean has soaked up roughly a third of our carbon dioxide. That CO₂ reacts with seawater and makes it more acidic — surface waters are already about 30% more acidic than before the industrial era. Acidic water makes it harder for corals, shellfish and plankton — the base of the fish food web — to build their shells and skeletons.
Warm water simply holds less dissolved oxygen — and the open ocean has lost an estimated 2% of its oxygen since the mid-20th century, with low-oxygen "dead zones" expanding. Fish breathe that oxygen through their gills, so warmer, breathless water pushes them out of once-thriving habitats.
When water stays too warm, corals expel the colourful algae that feed them and turn ghostly white. The 2023–2025 global bleaching event — the most intense on record — impacted about 84% of the world's reefs, and over 40% of coral species are now at risk of extinction. Lose the reef, and the countless fish that depend on it lose their home.
These changes don't stay in the background — they reshape fish behaviour, size, and survival in ways scientists are already measuring around the world.
This isn't only a story about wildlife. Around 4.3 billion people depend on fish for a significant part of their protein, and hundreds of millions rely on fishing for their livelihoods. Fish also help regulate the ocean that produces much of the oxygen we breathe and absorbs the carbon we emit. Protecting fish and protecting people turn out to be the same task — and it starts with understanding what's at stake.
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