[bai-giai-den-roi-cham-com]-Luyen-doc-reading-voi-bo-giao-trinh-oxford-level-5

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Many animals migrate to breed. Some make long and
difficult journeys to get to special breeding places.
Most water animals live either in salt water or in
fresh water. Salmon are unusual. Salmon eggs hatch
in freshwater streams and then the salmon migrate
down rivers to the ocean. When they get to the ocean,
salmon often make long journeys to places where
there is lots of food. They stay in the ocean for up to
six years. Amazingly, the salmon then swim back up
rivers into the streams where they hatched. Some
salmon travel hundreds of kilometers. They use smell
to find their way home. They go back to breed, and
most species of salmon then die.
. Before sockeye salmon breed,
they change color from silver to red.
Dangers for Salmon
It's tiring and dangerous for salmon to migrate up
rivers, because the water moves them in the opposite
direction. Because salmon usually migrate at the same
time each year, many predators, like birds, bigger fish,
and bears, wait for them. Salmon are also in danger
when trees are cut down, or when rivers are polluted,
because this damages their habitats. Dams can stop
salmon migrating, or they can kill or hurt them. Dams
are often built with fish ladders so that salmon can
jump up from pool to pool.