Overexploitation or overfishing is the removal of marine living resources to levels that can not sustain viable populations. Ultimately, overexploitation can lead to resource depletion 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.
Overexploitation do not only affect open ocean or pelagic ecosystems, but also coastal and intertidal areas . For example, 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 owing to food gathering 
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 .
Trawling for demersal species is having a major effect on the habitat for species other than target species. It has been estimated that all of the sea bed of the North Sea is trawled over at least twice per year and the gear is getting heavier over time . Trawls have destroyed long-lived species of molluscs and echinoderms in the North Sea. Since these species play important functional roles in biogeochemical cycling the consequences may be far-reaching.
Fishing not only has direct effects on target populations but also results in indirect effects such as effect of "goast fishing", trophic cascading effects  and food web-competion.
Trophic cascading effects has been observed when top-level predators are removed resulting in indirect effects throughout the ecosystem . On many temperate reefs shifts from macroalgae-dominated habitats to habitats grazed by sea urchins, termed ‘urchin barrens’, have been linked to the over-harvesting of top predators , . Perhaps the best known example of this is the interaction between sea otters, sea urchins and kelp . The importance of the sea otter-urchin-kelp trophic cascade was demonstrated after sea otters were wiped out by harvesting for their fur, allowing their prey, sea urchins, to overgraze kelps and dominate many benthic ecosystems. After the repopulation of areas by otters, kelp and its associated communities became much more abundant.
Ghost fishing result from fishing nets that have been left or lost in the ocean by fishermen. These nets, often nearly invisible in the dim light, can be left tangled on a rocky reef or drifting in the open sea. They can entangle fish, dolphins, sea turtles, sharks, dugongs, crocodiles, seabirds, crabs, and other creatures, including the occasional human diver. Acting as designed, the nets restrict movement, causing starvation, laceration and infection, and — in those that need to return to the surface to breathe — suffocation.
Conservation tools and techniques for preserving marine biodiversity conservation tend to combine theoretical disciplines, such as population and community marine biology, with practical conservation strategies, such as setting up marine reserves or marine protected areas (MPAs). Other techniques include developing sustainable fisheries, which involves establishing fishing quotas and restoring the populations of endangered species through artificial means.
Another focus of conservation effort is to halt human activities that are detrimental to the marine environment through policies or legislations, at an international, European and/or regional level.
Education of the general public about conservation issues is key in the process of conservation of the marine environment. There are many marine conservation organisations throughout the world that focus on funding conservation efforts, educating the public and stakeholders, and lobbying for conservation law and policy (see below).
Marine Reserves and Marine Protected Areas
Very little (less than 1%) of the earth oceans are protected, compared to 12% of the land surface. Highly-protected Marine Reserves are areas of the sea where human disturbances are minimised, allowing to maintain the natural biodiversity or, more often, to recover it to a more natural state. In Europe there are very few Marine Reserves, they are small and almost all are in the Mediterranean. While, there are many Marine Protected Areas (MPAs) in Europe, these areas only have some extra regulations or planning procedures, but they do not fully protect marine biodiversity. Inside Marine Reserves all extractive and potentially-disturbing human activities are prohibited. Marine Reserves are of crucial importance to science and education, essential for conservation, useful in resource management and necessary for reversing the effects of overexploitation. The benefits of marine reserves has been shown in New Zealand where twenty marine reserves have been created over the last 25 years. For example, inside Marine Reserves the abundance of species of ecological and commercial importance, such as the snapper Pagurus auratus and the spiny lobster Jasus edwardsii are almost 9 and 4 times, respectively, more abundant inside reserves than in adjacent unprotected areas. Furthermore, the mean body size of these animals is significantly large inside reserves than in outside . There is an urgent need for a representative, replicated, networked and sustainable system of highly-protected Marine Reserves .
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