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Fish come in so many different shapes and sizes, how can we tell what is actually a fish?

For a start, fish don't have legs and they live in water.

Also, they don't breathe air.

They have gills that allow them to breathe underwater.

Surprisingly most fish lay eggs.

Not hard like a chicken, but soft and lots of them.

Fish all have scales, even when they're newborn and tiny.

But even though they may be small to start with, they can grow to be huge.

No matter how big some fish may think they are, don't forget, sharks are also fish!

VOICEOVER: "Fish all live in water - some in the salty sea, and others in fresh water lakes and rivers.

They breathe through gills, little openings on the side of the head. Most fish are covered in scales that protect them and help them swim.

Salmon, born in British rivers, swim downstream to the sea and onto cooler water as far away as Canada. That's an amazing journey! Eventually, they'll go back to their own river, swimming and leaping back up to where they began life. Now, it's their turn to lay eggs that will hatch into young fish.

Tuna fish live in salty water. They're big. Sometimes even bigger than a man! And they're fast swimmers - just as well because sharks like to eat them.

Sharks hunt for food. They are predators. Inside their strong jaws are lots of strong pointy teeth. Pike are predators too and live in lakes. They have long, pointed heads and will lie, still and waiting, ready to catch a fish that's swimming close by.

Flat fish, like turbot and plaice, are good at hiding - they're camouflaged against the sandy bottom of the sea. A flat fish's mouth and both of its eyes are on one side of its body so it can see what's above it.

Angler fish live in deep water. they have what looks like a tiny fishing rod dangling over their mouth. Other fish come to see and get eaten.

Swimming together in shoals makes it harder for fish to be caught by predators but not by fisherman. Herring and mackerel swim in shoals.

Some fish never swim in the sea, or lakes, or rivers. They're pets, just like this goldfish."