OCEAN FOOD CHAINS by asafwewe

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									                               OCEAN FOOD CHAINS
  Based on a poster created by Natalie Barnes, a postgraduate student at the Southampton Oceanography Centre,
               with the help of Katie Poneroy and Jo Gill, pupils of St Anne's School, Southampton.



OCEAN PRODUCTIVITY
High oceanic productivity occurs in areas of upwelling in the ocean, particularly along
continental shelves (red areas on map). The coastal upwelling in these regions is the
result of deep oceanic currents colliding with sharp coastal shelves, forcing nutrient-rich
cool water to the surface. Over 90% of the world's living biomass is contained in the
oceans, yet only about 0.2% of marine production is harvested.




                                           Peruvian
                                           upwelling zone




THE PERUVIAN UPWELLING ZONE
The Peruvian upwelling is a 300 x 300 mile area adjacent to the coast and is the most
biologically productive coastal upwelling system on Earth. Carbon levels (an indicator of
production) are tens of times higher than those of the next most productive upwelling
region, the California current.
 HOW THE OCEAN FOOD CHAIN WORKS

                                                                                                                       light
Even the smallest creature
in the ocean is preyed on                                         man
by larger creatures. The                                                                                                                 0m

smallest creatures, such as                                                                                     phytoplankton

phtyoplankton, form the                                           tuna                  zooplankton
base of the food chain and                                               anchovy                         upwelling
                                                                                                         nutrients
are eaten by herbivorous                                                             detritus
(plant-eating) plankton,                                                    detritus feeders
who are in turn eaten by
predatory     zooplankton.
Zooplankton are preyed on
by fish, which then might                                                                                                                5000m

end up in man's fishing
nets.




                                                                                                            Herbivorous plankton
                                          Phytoplankton                                               The majority have limited
                                  Microscopic plants that drift                                       movement but may migrate to the
                                  along in the ocean currents.                                        surface at night to feed.
                                  Phytoplankton                                                       Most plankton are herbivorous,
                                  photosynthesise with                                                but some are scavengers and
                                  pigments such as chlorophyll,                                       some may even cannibalise. May
                                  which are also found in                                             be found in swarms.
                                  terrestrial plants.




                                                                                                      Predatory zooplankton
                                                                                                      May be predacious carnivores,
Anchovy                                                                                               filter-feeding omnivores or
Silvery fish with blue-green backs                                                                    scavengers.
12-20 cm length                                                                                       Use a range of feeding
Spawn once a year                                                                                     methods from actively hunting
Life expectancy of 3 years                                                                            prey and swallowing it whole to
Occurs in shoals                                                                                      waiting for food to 'float' by
Caught near the surface                                                                               then stinging and entangling it.
All life stages filter-feed on plankton
Restricted to cool, nutrient-rich
upwelling zones
Found along the coast of Peru
and Northern Chile
                                          Photo: NOAA
OCEAN FOOD CHAINS AND MAN
Humans form the end link of the oceanic food
chain. In terms of fisheries yield, upwelling zones
are up to 66,000 times more productive than the
open ocean per unit area. Offshore Peru is an
example of an upwelling zone and it is heavily
fished for anchovy. Before 1950, the Peruvian
anchovy were harvested purely for human
consumption but after the second world war,
traditional fishing boats became outclassed in
favour of large, high tonnage ships. Modern,
industrialised fishing vessels are now equipped with
fish-seeking radar, and are highly mechanised
which reduces manual labour costs and increases
fishing efficiency. Today only 5% of the anchovy
catch is used for human consumption, the rest is
used in animal feed.


HOW DOES CLIMATE AFFECT THE FOOD CHAIN?
During El Niño events, the temperature of the ocean surface may rise by up to 3ºC, causing
upwelling to stop. Diatoms and phytoplankton that are normally abundant in upwelling zones
disappear. Anchovies migrate to lower depths where cooler water and some phytoplankton are
available. This makes fish inaccessible to fishing fleet nets and the birds that are dependent
on the anchovies for food. Animals that feed on the anchovy either migrate to find new food
sources or die off.


                                                                    16


                                                                                  El Niño
                                                                    12
                             Fisheries yield (millions of tonnes)




                                                                                       Estimated sustainable yield

                                                                     8


                                                                                                 El Niño
                                                                                       Pre-1950s catch
                                                                     4




                                                                     0
                                                                         1960   1970           1980              1990




AN INFINITE RESOURCE?
The large fish populations associated with upwelling zones have traditionally been viewed as an
infinitely renewable resource. However, the rapid development of the Peruvian anchovy
fishing industry coincided with severe El Niño effects, which nearly destroyed the fishery.
Even such rich environments require careful management to ensure they do not become
depleted.

								
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