Food Webs In The Ocean: Who Eats Whom And How Much?

Over 100 food webs have been published for marine ecosystems to describe the transfer of food energy from its source in plants, through herbivores, to carnivores and higher order predators. The webs suggest that the lengths of the chains that form food webs are typically short (3-4 links), and that ecosystems with long food chains may be less stable than those with shorter food chains. Stomach contents have been the primary means for determining what marine organisms eat. More recently developed techniques include faecal analysis and fatty acid signatures from blood or fat samples. Consumption has been estimated from the volume of food found in stomachs, from the feeding rates of captive individuals and from bio-energetic modelling. Consumption of marine organisms, expressed as a percentage of an individual's body weight per day, ranges from about 4-15% for zooplankton, to 1-4% for cephalopods, 1-2% for fish, 3-5% for marine mammals and 15-20% for sea birds. Immature age classes consume about twice as much (per unit of body weight) as do mature individuals. Furthermore, consumption is not constant throughout the year, but varies with seasonal periods of growth and reproduction. Most groups of species consume 3-10 times more than they produce, and export or pass up the food web about 70-95% of their production. Marine organisms tend to be larger at successive trophic levels and are limited in the sizes of food they can consume. Humans are one of the few species that can prey upon almost any level of the food chain and any size of prey.Food web analysis and estimates of consumption are essential for understanding which ecosystems can support additional species, and which may be less stable and susceptible to species loss through the synergistic effects of fishing or culling. They are also critical tools for understanding changes in ecosystem dynamics as highlighted by a case study from the eastern Bering Sea.