Sustainability 3: The Food Web


Our cultivated clams and oysters, just like those in the wild, feed on naturally occurring phytoplankton. Phytoplankton, from the Greek words phyto meaning plant and plankton meaning wanderer or drifter, are microscopic, photosynthetic organisms that float freely in the water.  They are primary producers that take the energy from sunlight and carbon dioxide and create organic compounds and oxygen through the photosynthetic process.    They are at the base of a complex food web in the estuarine ecosystem that includes clams and oysters.   A food web represents all the trophic connections between different organisms in an ecosystem.  An understanding of these complex interactions can demonstrate how energy and nutrients move through the system. How does this relate to sustainability?  As you move through the food web there are different trophic levels from primary producers (plants) to primary consumers (herbivores) and secondary consumers (carnivores).   The total energy consumption per pound of protein produced increases exponentially with only about 10% of the energy consumed converted to biomass between trophic levels.  Cultivating herbivore species, like clams and oysters, reduces the total amount of energy required per pound of production compared with eating a carnivorous fish, like a salmon.  Also since clams and oysters eat naturally occurring plant food rather than feeds, there is no additional energy needed to grow them when compared with the production of species that eat energy intensive farmed feeds.  Making food production more sustainable involves moving toward cultivating species that are low on the food chain, like bivalves, that are feeding on a natural food source.   While we cultivate clams and oysters from hatchery raised seed they are still feeding on wild, naturally occurring phytoplankton species.  They are therefore "free ranging", in as much as a clam or oyster ranges, grazing on all of the foods that the wild population of shellfish would feed on.  

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