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Food Webs
Ecologist Charles Elton created the concept of the food web in 1927. A food web maps the relationships between the species that share a particular ecological community. In other words, it shows which species are eating each other. Unlike food chains which only show simple connections between plants, herbivores and predators, a food web shows the relationships of all the species in a community. All species depicted in the food web are divided into levels. The plants make up the primary producer level. Herbivores make up the primary consumer level and predators that eat the herbivores make up the secondary consumer level.
Marine Food Webs
The primary producers in the marine food web are tiny organisms called "phytoplankton." The phytoplankton use energy from sunlight to convert carbon dioxide in the water into carbohydrates and other nutrients. Tiny animals called "zooplankton" then feed on the phytoplankton. The zooplankton become food for predators such as shrimp, krill, baby fish and jellyfish and small fish such as herring or sardines. These small predators then become the prey for top predators including adult jellyfish, squid, octopuses, tuna, mackerel, sharks, marine mammals and birds. Finally these predators become prey for human beings.
Overfishing
According to the website Overfishing a Global Disaster, as of 2006 fishing fleets around the world are about two to three times larger than needed for the present fish catch. These huge fleets have damaged the supply of fish. About 52 percent of the world's fish stocks are fully exploited while more than 25 percent of the world's fish stocks are either over-exploited or depleted.
Overfishing and Food Webs
Overfishing causes a problem sometimes called "fishing down the food web." As commercially valuable fish such as cod or tuna disappear from the ecosystem due to overfishing, smaller predatory fish fill the niche left behind by tuna or cod. As valuable fish disappear, commercial fishermen go after lower-level prey fish, these predators then lose their food supply. For example, when cod populations declined in the North Sea, fisherman began taking pout. Pout prey on krill and copepods. Krill and young cod compete for copepods. As pout were removed from the ecosystem, there was nothing to keep krill populations in check. The krill population increased, the krill ate up the copepods and juvenile cod populations decreased.