Page 222 - (DK) Ocean - The Definitive Visual Guide

220 INTRODUCTION TO OCEAN LIFE

Ocean Migrations DISCOVERY
TRACKING

FEEDING AND BREEDING ARE THE MAIN REASONS that animals migrate. They move Until recently, any journey made
by a marine animal, such as the
from one place to another, often at the same time of day or year, and usually follow leatherback turtle shown below,
the same, well-defined routes. Migratory species include many of the larger marine was poorly understood because
tracking devices used on land were
animals, such as whales and turtles, but smaller creatures, such as squid and plankton, inappropriate for use in water.
also make spectacular journeys in order to survive and reproduce. Animal migration This changed when satellite-
tracking devices became available.
in the oceans is more complicated than on land because animals can move both Attaching one to a turtle does not
horizontally and vertically through the water column. impede or harm it in any way but
it can still pose problems. Yet
turtles are threatened in the wild,
Types of Migration so knowing where a female goes
after laying her eggs is vital to
The driving force behind any animal migration is survival. conservation work.
Individuals must eat to live, and some will travel long ADULT
distances to find food. Such journeys often coincide with POPULATION TRACKING TURTLES
A tracking device is being attached to this
peak production times of plankton and other food sources return of adults migration of
after spawning young adults leatherback turtle on Juno Beach, Florida. In
in particular places, such as sites of seasonal upwellings. case an opportunity does not arise to remove it
A shorter, more regular feeding migration is made daily by manually, parts of the harness are designed to
gradually disintegrate.
plankton and active swimmers such as squid (see p.221). spawning
migration
A species’ survival depends on reproductive success.
Gathering together and breeding in a few places at the SPAWNING NURSERY
same time optimizes conditions for offspring survival. SITE migration AREA
Breeding grounds where food is abundant and conditions as plankton
favorable are used repeatedly, with individuals often
returning to their birthplace to breed. Some shore animals MARINE MIGRATORY CYCLE
Some marine organisms migrate to a specific spawning site
also migrate up and down the beach, following the
to release their eggs. The eggs hatch into larvae that join the
outgoing tide to feed and avoiding immersion by plankton and drift in the currents to another nursery area,
returning before the tide turns. where they feed and mature before joining the adult population.

Migrating between

MIGRATING TERN PACIFIC
migration Salt and Fresh Water
route ATLANTIC OCEAN OCEAN
Although some marine species can cope with a great
PACIFIC
INDIAN
ARCTIC TERN MIGRATION OCEAN range of salinity and temperature, only a few move
OCEAN
This small bird flies from the Antarctic to between fresh and salt water at particular stages of
the Arctic to breed and then returns south, their lives. Some, such as salmon, start and finish their
a round trip of nearly 22,000 miles (35,500 km).
Terns spend 90 days at the nesting grounds each lives in fresh water and spend the rest of their time in
year. The rest of the time is spent mostly on the wing. the ocean. Such fish are described as anadromous.
summer distribution winter distribution Eels, on the other hand, start and finish their lives in
the sea, but spend 10 to 14 years in fresh water while
LOBSTER
MIGRATION maturing. Fish such as these are termed catadromous.
Caribbean spiny At maturity, both of these fish return to their
lobsters migrate birthplace to breed, after which they die. Changing
in single file across from fresh to salt water or vice versa would be fatal
the sea floor in
winter, seeking to most fish, but various physiological adaptations,
warmer water, and including the way their kidneys function, allow both
return to shallow anadromous and catadromous fish to make the
water in summer. transition without experiencing any ill effects.

SALMON RETURNS TO FRESHWATER SPAWNING GROUNDS

OCEAN LIFE 1 ready to return to the river where it was born to 2 grounds, the salmon may swim 2,175 miles 3 eggs in the gravel on the riverbed, the male 4 gravel until they absorb their yolk sacs and

Newly hatched salmon live among the
In migrating upstream to its spawning
At three to five years of age, a coho salmon is
As soon as the female has deposited her
become fry. They then begin their journey
swims over them and releases his sperm,
(3,500 km) against the water current,
spawn. Some mature at only two years,
negotiating several waterfalls and rapids.
returning to their home river as “jacks.”
downstream to the sea.
optimizing the chance of fertilization.