Overexploitation or overfishing is the removal of marine living resources to levels that can not sustain viable populations. Ultimately, overexploitation can lead to resouce depleation and put a number of threatened and endangered species at risk for extinction.
A greater variety of species at a higher trophic level is exploited in the sea than on land: humans exploit over 400 species as food resources from the marine environment; whereas on land only tens of species are harvested for commercial use. Exploitation of marine biodiversity is also far less managed than on land and amounts to the hunter-gatherers stage that humans abandoned on land over 10,000 years ago, yet exploitation technology is becoming so advanced that many marine species are threatened to extinction. Insufficient consideration has been given to the unexpected and unpredictable long-term effects that such primitive food-gathering practices engender.
The exponential growth in human population experienced in last decades has lead to an overexploitation of marine living resources to meet growing demand for food. Worldwide, fishing fleets are two to three times as large as needed to take present day catches of fish and other marine species and as what our oceans can sustainably support. The use of modern techniques to facilitate harvesting, transport and storage has accelerated this trend. According to the United Nations Food and Agriculture Organization (FAO) over 25% of all the world's fish stocks are either overexploited or depleted and 52% are fully exploited . Thus a total of almost 80% of the world's fisheries are fully to overexploited, depleted, or in a state of collapse. Although, these estimates are considered rather conservative. Recently, a study showed that 29% of fish and seafood species have collapsed (i.e their catch has declined by 90%) and are projected to collapse within by 2048, unless inmediate action is taken . Worldwide about 90% of the stocks of large predatory fish stocks are already collapsed.
All fishing activities, if not conducted in a sustainable non-destructive manner, can lead to overexploitation of marine living resources. Overexploitation of marine resources has major impacts on marine systems as a whole, but target species are generally the most impacted.
Fishing effect can be divided into: direct effect and indirect effects. Direct effects are related to target species and by-catch species. When recruitment of target species is relatively high, the average size of individuals is affected because larger individuals tend to be harvested and populations display signs of growth overfishing. When adult populations are heavily exploited the number and size of the adult population (spawning biomass) is reduced to a point that it has not the reproductive capacity to replenish itself, leading to recruitment overfishing. Direct effects of fishing also include physical disturbance by fishing gear than can cause scraping, scouring and resuspension of the substratum. The effects vary according to the gears used and the habitats fished .
Indirect effects of overexploitation and fishing in general include effect of "goast fishing", trophic cascading effects  and food web-competion.
Overexploitation do not only affect open ocean or pelagic ecosystems, but also coastal and intertidal areas Intertidal limpets in Hawaii (Cellana spp.), the Azores, Madeira and Canaries (Patella spp.) have all shown declines, and in the case of the Azores, dramatic population crashes 
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