When breeding season arrives, male three-spined sticklebacks change colour – their bellies going fiery red – and make elaborate nests to attract the ladies, which they’ll defend with their lives.
Hundreds of adult salmon journey from the Atlantic Ocean, upstream, to the exact place of their birth to spawn. They have to get through rushing rapids and leap up waterfalls in a single bound. Some become so tired and hungry (they don’t have time to eat) that they simply give up and die after mating.
And who needs sharks when there are pike about? These ‘water wolves’ (that’s what their Latin names mean) are wicked freshwater predators and can grow as big as an adult human! They like to lie in wait for their prey, patiently holding fire until the right moment to strike when they show super speed. They eat mainly fish, but are known to enjoy the odd water vole, duckling or even their own young!
Fish take in water across their bodies. Most ocean-going fish have less salt in their bodies than in the sea, so they have to lose water to their salty surroundings. Freshwater fish have more salt in their bodies than in the freshwater they swim in, so they take on water. If they didn’t get rid of it by making really weak wee, they’d swell up and burst like a balloon!