OCEAN FOOD CHAINS

Ocean Productivity
High oceanic productivity occurs in areas of upwelling in the ocean, particularly along continental shelves (red areas on map below). 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.

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.

Food Chains
Phytoplankton

Microscopic plants that drift along in the ocean currents.
Phytoplankton photosynthesise with pigments such as chlorophyll, which are also found in terrestrial plants.


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 and 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

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 consuption but after the second world war, traditional fishing boats became outclassedin favour of large, 350 ton capacity ships. Modern, industrialised fishing vessels are now equipped with fish-seeking radar, and are higly mechanised which reduces manual labour costs and increases the 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 Nino 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 the fish inaccessible to the nets of the fishing fleet 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.

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 Nino effects, which nearly destroyed the fishery. Even such rich environments require careful management to ensure they do not become depleted.

This poster was created by Natalie Barnes, a postgraduate student at the Southampton Oceanography Centre, with the help of pupils of St Anne's School, Southampton. Visit our website at www.soc.soton.ac.uk

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