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Life
under the ocean waves
Preview
On the surface, the oceans give few clues as to the huge range
of life forms that dwell beneath, and the complex relationships
between them.
In
1978, New Zealand declared a 200 nautical mile Exclusive Economic
Zone (EEZ) and has exclusive management rights to this area.
Because of our mid-ocean position and outlying islands, our
EEZ is very large - in fact it is the fourth largest in the
world, and equal to about 15 times our land area. Within this
vast area of ocean is everything from subtropical to sub-Antarctic
waters; from shallow estuaries teeming with life to barren
undersea trenches up to 10,000 metres deep.
The marine environment
The marine environment is constantly changing. Some changes
may take thousands of years, like the coming of an ice age,
while others are immediate, like a rainstorm that dumps fresh
water and silt into a seawater tidal pool. When the environment
changes, so do the inhabitants - either by adaptation
or succession.
Adaptation
is a slow process that occurs from generation to generation
as a species or population changes to adapt to slow changes
in the environment. Succession generally happens over a short
period of time - sometimes days, sometimes years.
In
succession, one population is replaced by another population
which is better suited to the environment. For example, if
strong waves knocked mussels off a rock, whatever shellfish
was in larval form in the water at the time may take over
the bare spot.
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Changes
can occur to:
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The
shape of the ocean floor
Bathymetry of the seafloor around New
Zealand |
New Zealand's marine environment is largely shaped by the contours
of the ocean floor. By far the most important areas are the
continental shelves and slopes, the relatively shallow areas
surrounding land masses. The continental shelf goes from the
shore down to about 200 metres, the continental slope continues
down to about 4000 metres: fishing goes down to about 1500 metres.
Continental
shelves occupy less than 10 percent of the world's oceans,
but support up to 80 percent of the world's biomass.
In
some places around New Zealand, the continental shelf is extremely
narrow, such as on the southwest coast of the South Island.
In other places there are large undersea plateaux, such as
the Chatham Rise to the east of the South Island. The large
plateaux tend to be important fisheries areas.
Overall,
however, our Exclusive Economic Zone (EEZ) is not as naturally
well endowed with fisheries resources as those of many northern
hemisphere countries, where there are often much wider continental
shelves and plentiful nutrient supplies to the ocean from
major rivers.
Seasons
The seasons influence life in the ocean just as much as they
do on land. Because water heats and cools more slowly than land,
the maritime seasons lag a month or two behind. The main difference
between the seasons, apart from the overall change in temperature,
is in the vertical temperature patterns of the water.
| 1 |
In
winter the water is mixed to a more uniform temperature
and may be warmer away from the land. Fish tend to be
more dispersed. Little plankton is produced and some of
the surface-feeding pelagic species migrate northwards.
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| 2 |
Spring
brings more light and a good supply of nutrients in the
water, which encourages phytoplankton (plant plankton)
to grow. The water changes from the clear blue of winter
to greenish as the plankton bloom. Some fish species take
advantage of this by congregating to spawn so that their
larvae can feed on the plankton. More fish are seen in
surface waters. |
| 3 |
In
summer there is a warm upper layer and cooler layers below.
The zooplankton (animal plankton) begin to feed on the
phytoplankton. Further up the food chain, small fish and
invertebrates like squid and jellyfish feed on the zooplankton.
They, in turn, are preyed on by larger fish which have
moved from their spawning grounds to areas where they
can thrive. |
| 4 |
In
autumn, the water begins to mix and cool. There are smaller
plankton blooms and fewer nutrients available. Young and
adult fish begin to move out of harbours and bays. The
water changes from green back to a clear blue as the plankton
levels reduce. |
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Tides
The rise and fall of tidal waters affects fish and shellfish
living on the coastline. Animals living in these areas have
adapted to wide temperature variation as well as rapid changes
of other conditions. For example, a pool exposed to the sun
for a three hour period and warmed to 21º C, may drop within
seconds to 10º C when the first wave of the incoming tide
floods it.
Currents
A current is water moving continuously in a certain direction.
Without currents, the seas would stagnate. Currents circulate
food, nutrients and oxygen and play a major role in the reproductive
success of many fish. Maturing adults migrate into the current
to their spawning grounds. Eggs and larvae are then carried
by the current to their nursery grounds.
Oceans
tend to have currents along their edges and fish are often
plentiful at the point where currents meet. Coastal and tidal
currents are important to fishers and divers when they carry
fish into bays and harbours.
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main water masses influence New Zealand's marine environment.
Subtropical water comes across from the west as the Tasman
Current, splitting into a number of smaller currents and
eddies around the coast. The other main water mass is
the cooler sub-antarctic water flowing to the south of
New Zealand. |
Main
ocean currents
around New Zealand |
The
line along which these two water masses meet is known as the
Subtropical Convergence Zone. Convergence
zones are highly productive for sea life, because the eddies
and upwellings bring more nutrients to the surface. Plankton,
the base of the marine food chain, is in greater abundance
in these areas. This draws greater concentrations of fish.
This is especially so along the east coast of the South Island.
There
is another convergence zone in the northern Tasman Sea where
warmer subtropical water from the north meets the Tasman Current
moving from the west.
Upwellings
An upwelling occurs when the wind blows surface water out to
sea and deeper water moves up to replace it. This can be a local,
temporary situation, or cover large areas and be fairly permanent.
Upwellings
are usually good for fish productivity as the subsurface water
is rich in nutrients. They can have adverse effects if they
occur at the wrong time and place and upset spawning; upwelling
water could be too cold and it could be a long time before
plankton could develop.
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From
left to right: Sea surface temperature maps from August
(winter) and February (summer)
Click for larger images
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Salinity
Salinity is the salt content of the ocean. It does not vary
drastically or change rapidly within a single water mass, except
at river mouths, where fresh and salt water meet. Around New
Zealand there is a change in salinity across the Subtropical
Convergence Zone, with a general increase in salinity from south
to north.
Oxygen
Marine animals need oxygen to survive. In most sea and fresh
waters there is enough oxygen. In periods of high temperatures,
shallow still water, such as ponds and estuaries, may become
short of oxygen. Dumped waste material, such as sewage and industrial
by-products, removes oxygen from water. A crude measure of the
quantity of oxygen needed to react with waste material and incorporate
it harmlessly into the environment is the biological oxygen
demand (BOD). Areas with a high BOD will not support much life.
Light
Water absorbs and reflects sunlight. The depth to which light
penetrates depends on the clarity of the water.
The eyes of fish are well adapted to their environment and they
can identify objects and movement. Fish can react to seasonal
changes in light level and day length and this affects their
breeding behaviour.
Fish camouflage is also influenced by how much light is in the
water. Surface dwelling fish are often greenish-blue above and
white below, so that they merge with their background when seen
from above or below; deepwater fish are more uniformly black
or dark red.
Temperature
The ocean has the most stable temperature environment in the
world. Except for those that live in tidal pools, fish do
not need to have a wide tolerance to temperature change. But
patterns of water temperature do influence where fish live
and how plentiful they are.
Some
fish, for example, skipjack tuna, migrate following warm water
south to spend summer and autumn in New Zealand waters, and
then move further north in our winter as temperatures cool.
Many fish, especially pelagic species, concentrate at temperature
"fronts" where there is a rapid horizontal change
in temperature.
The
marine food chain
Phytoplankton are at the base of the food chain, they
are very simple forms of life stimulated by light and nutrients
in the water to grow and multiply.
The
second level of the food chain is Zooplankton such
as the tiny shrimp-like krill, which abound in Antarctic waters.
Animal plankton "graze" on the phytoplankton.
The
Zooplankton is in turn are eaten by invertebrates,
such as squid, and small fish like pilchards and sprats. At
the fourth level are the most spectacular hunters, the pelagic
fish like tuna, kahawai and kingfish. Streamlined for speed,
these fish move in large schools, some migrating many thousands
of kilometres each year.
This "mako shark" is among the
predators at the top of the marine food chain |
At
the very top of this branch of the food chain are marine
mammals such as dolphins and seals, sharks and humans.
Also connected to the food chain are the bottom dwellers.
Life on the sea bed in the continental shelf regions is fed
a constant drizzle of dead and decaying matter from
the upper levels. This supports a complex community of plant
and animal life and bacteria, which is fed upon by bottom
feeding, or demersal fish, and species such as rock lobster.
The
bacteria on the sea bed perform an essential role by
breaking down organic matter and unlocking nutrients
which can then be released into the water for re-use by a
new generation of plankton.
Marine food chain
Click for larger image |
The
continental shelf
Life on the continental shelf sea-bed varies according to the
type of substrate. Burrowing animals such as worms or surf clams
live in sediments - mud or sand. Other creatures, such as like
mussels, barnacles or sea anemones, need something firm to cling
on to, so prefer a rock or gravelly bottom. Some free-roaming
creatures, such as rock lobsters and starfish, can move across
all types of surface.
| Sea
bottom dwellers have two main feeding methods. Sea creatures
like mussels, scallops and oysters are filter feeders,
trapping food particles as they pump the water through
a filter. They prefer clean, moving water, which keeps
the food particles in suspension. |
Pacific Oysters |
Others,
such as some worms and urchins, search for particles of food
in the sediments on the bottom . They prefer quieter waters
where the particles can settle.
There
are plenty of scavengers and carnivores on the sea bed. The
best known of these are crabs, snails, starfish and rock lobsters.
Many of our well known commercial fish species are bottom-feeders,
such as ling, elephant fish, gurnard, tarakihi and snapper.
The
deep sea
On the basis of area, some 70 percent of New Zealand's Exclusive
Economic Zone (EEZ) is a deep sea zone. Life in these regions
is relatively sparse, and the creatures living at great depths
face special difficulties.
There
is little or no light for plant growth, and food particles
drifting down from above are few and far between. Some species
grow very slowly and have a long life span.
Black Oreo Dory - A deepsea species |
Some
fish, squid and prawns overcome the shortage of food by swimming
up to shallower depths to feed at night. Many deepwater fish
look bizarre, with huge eyes and teeth and sometimes a special
phosphorescent light to lure their prey. Most deepsea fish,
squid and prawns are dark coloured, but many have rows of
light organs. Orange roughy are a bright colour in daylight,
but at depth they appear black. Commercial species trawled
from the deep ocean around New Zealand include orange roughy,
hoki, hake and the oreos.
For
more information click on any link below.
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