It happened over 300 years in Holland

skeletons like cod and salmon. There are over 20,000 species of fish. Their total
population greatly exceed that of whales.
(6) The largest of the baleen whales is the Blue Whale. It can be more than 30 m
long and weigh as much as 130 tonnes. That’s as much as the combined weight of
30 average-sized elephants! Toothed whales are smaller but vary in size, from 1.3
m long in dolphins to the 18 m Sperm Whale which can weigh almost 70 tonnes.
Oddly enough, the largest fish in the world is called the whale shark. It is up to
14 m long and weighs up to 15 tons. The average size is 7.6
m in length. The smallest fish, called a goby, is only
slightly more than a centimetre long.
Life Under Water
(7) Whales are mammals which 40
million years ago returned to the sea
and evolved a fish-like form. Their hind
limbs disappeared and their front limbs
slowly changed into flippers. Although they breathe air, whales cannot survive on
land. Whales that accidentally get stranded on a beach soon die because the
great weight of their bodies crushes their vital internal organs.
(8) Whales have a thick layer of fat called blubber under their smooth, almost
hairless skin. This fat helps them retain their body heat. Fish have very little fat.
This is why they are prized as food. To keep them from freezing in cold water
environments, fish blood contains an anti-freeze substance. To protect them from
salt, ocean fish are covered in a slimy material. Some freshwater species are like
this too.
"There She Blows!"
(9) Whales can dive deeply and stay underwater a long time. The Sperm Whale,
for instance, can dive down more than 2000 metres, depths that would crush a
submarine. It can remain below the water’s surface for up to 90 minutes! Like
land mammals, whales have lungs and must come to the surface to breathe
through their nostrils. A whale's nostrils is called a blowhole and is located on
the top of the head. A surfacing whale exhales warm moisture air from its
blowhole in a huge fountain. This was mistaken by old-time whalers for a spout
of water. It is more like a spout of steam, like when you warm breath hits cold air
when you exhale. Whale species can be identified by the size and shape of its
"blow"! Fish, on the other hand, breath by removing oxygen from water through
special organs called gills. The quickly suffocate when removed from the water.
Eat and Be Eaten
(10) Baleen whales sieve plankton, squid and small fish from seawater with their
baleen. Toothed whales prey on a variety of fish and squids. Whales swallow their
food whole and have a muscular compartment to their stomach which crushes
what they have eaten. Fish feed on plankton and other fish too. They will also
feed on the remains of other creatures.
Songs of the Sea
(11) Most whales are social animals. They like to live and travel in groups called