It is argued that globalization and industrialization of commercial fishing in the last half century has lead to significant depletions in many of the worlds fish stocks. This is due not only to the direct extraction of the target species, but rather to the impact of fishing on the marine ecosystem and its biodiversity.
Most commercial fish are at the top trophic levels of their food webs. These trophic levels range from 3.0 for sardines, which feed on zooplankton, to the larger predatory fish such as cod and tuna that feed on other predators or planktivores. These predators are the furthest removed from the algae trophic level of 1 that drives the marine wood webs. There has been an observed decrease of 0.05 to 0.10 of trophic levels per decade in the fisheries landing signifying the removal of the largest fish from the oceans. This has been further demonstrated by the reported decline in the biomass of predatory fish in the North Atlantic by 2/3 in the 2nd half of the 20th Century.
The targeting of large fish versus a random sampling of the population can have a significant impact on the overall species. First, large females can produce a significant portion of the eggs for the following years age class. For example a large female snapper (12.5 kg) my produce up to 9,300,000 eggs, which is the same amount that would be produced by 212 smaller females (1.1kg). Second, the targeting of larger individuals may be reducing the genetic diversity of the population by removing individuals with traits for fast biomass growth.
Finally, commercial fishing has simplified and shortened the food web making fish populations more susceptible to environmental changes. In the past there was greater ability for predators to switch prey when one prey population had been depleted, but now there are fewer alternatives. In addition fewer age classes make each species more vulnerable and dependent on new recruitment for the maintenance of their biomass levels.
//Nature (2002). Towards sustainability in world fisheries. Nature, 8 Aug. 2002. Web. Dec. 2009. <http://www.nature.com/nature/journal/v418/n6898/full/nature01017.html>.//