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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