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
Even the smallest creature
in the ocean is preyed on
by larger creatures. The
smallest creatures, such as
phtyoplankton, form the
base of the food chain and
are eaten by herbivorous
(plant-eating) plankton,
who are in turn eaten by
predatory
zooplankton.
Zooplankton are preyed on
by fish, which then might
end up in man's fishing
nets.
Phytoplankton
Microscopic plants that drift
along in the ocean currents.
Phytoplankton
photosynthesise with
pigments such as chlorophyll,
which are also found in
terrestrial plants.
light
man
0m
phytoplankton
tuna
zooplankton
anchovy
detritus
upwelling
nutrients
detritus feeders
5000m
Herbivorous plankton
The majority have limited
movement but may migrate to the
surface at night to feed.
Most plankton are herbivorous,
but some are scavengers and
some may even cannibalise. May
be found in swarms.
Predatory zooplankton
May be predacious carnivores,
filter-feeding omnivores or
scavengers.
Use a range of feeding
methods from actively hunting
prey and swallowing it whole to
waiting for food to 'float' by
then stinging and entangling it.
Anchovy
Silvery fish with blue-green backs
12-20 cm length
Spawn once a year
Life expectancy of 3 years
Occurs in shoals
Caught near the surface
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
Fisheries yield (millions of tonnes)
12
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.