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Overfishing is a situation where one or more fish stocks are reduced below predefined levels of acceptance by fishing activities. More precise definitions are provided in biology and bioeconomics. Biological overfishing occurs when fishing mortality has reached a level where the stock biomass has negative marginal growth (slowing down biomass growth). Economic or bioeconomic overfishing in addition to the biological dynamics takes into consideration the cost of fishing and defines overfishing as a situation of negative marginal growth of resource rent. A more dynamic definition may also include a relevant discount rate and present value of flow of resource rent over all future catches.
Overfishing therefore may be sustainable, but in a non preferable way. Ultimately overfishing may however lead to depletion in cases of subsidised fisheries, low biological growth rates, critical biomass levels, etc.
Glaring examples exist of the outcomes from overfishing in areas like the North Sea and the East Coast of Canada and the United States. The result has been not only disastrous to fish stocks but also to the fishing communities relying on the harvest.
The ability for nature to restore the fisheries is also dependent on whether the ecosystems are still in a state to allow fish numbers to build again. Dramatic changes in species composition may establish other equilibrium energy flows which involve other species compositions than before.
The "United Nations Convention on the Law of the Sea" treaty deals with aspects of overfishing in articles 61, 62, and 65.
- Article 61 requires all coastal states to ensure that the maintenance of living resources in their exclusive economic zones is not endangered by over-exploitation. The same article addresses the maintenance or restoration of populations of species above levels at which their reproduction may become seriously threatened.
- Article 62 provides that coastal states: "shall promote the objective of optimum utilization of the living resources in the exclusive economic zone without prejudice to Article 61"
- Article 65 provides generally for the rights of, inter alia, coastal states to prohibit, limit, or regulate the exploitation of marine mammals.
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