Overfishing is the human act of extracting aquatic (that is, marine and freshwater) fauna from natural water bodies at a rate greater than the reproductive and recruitment functions can replace that extraction. While there is some evidence that localized overfishing may have occurred in prehistoric eras, the bulk of overfishing has taken place in the last 150 years as the human population has expanded greatly and fishing technologies have enabled harvesting of many species at rates not imagined in earlier times. For over a century man's role in the depletion of certain regional fisheries has been noted. A functional definition of overfishing is sometimes given as the reduction in catch per unit effort by fishermen. Typically the concept of overfishing is linked to an individual aquatic species, and this issue is most often discussed within a specific marine or lacustrine province (e.g.North Sea, Pyramid Lake, Monterey Bay). In some cases such as the North Sea, overfishing involves complex multi-governmental issues, where conflicting goals exist among different nations.
The worldwide problem of overfishing can be linked to several fundamental issues: (a) the rapid expansion of the human population creating extraordinary demand for food in all forms; (b) the finite, even if large, carrying capacity of each of the world's fisheries; the Tragedy of the Commons phenomenon encountered in many ways, whereby unlimited free access to a finite enviornmental resource ultimately degrades that resource; and (d) unrelenting pollution of the world's marine and freshwater resources, that acts to reduce fishery productivity.
Often the human impacts of fishery exploitation are masked by fishermen switching to the next bountiful species, after a prime fish species has been commercially exhausted. As this process continues through the marine food web, a serial depletion often occurs, such that considerable ecosystem damage has occurred before the commercial fishing industry notices that material exhaustion of numerous species has been realized. Often, this phenomenon is termed fishing down the food web, leading to a result of dominance by species considered "trash fish" by commercial fishermen.When the process of serial depletion is occurring, commercial and recreational fishermen often concentrate on fish species which in earlier decades were considered discards. For example, in the Indian Ocean off of South Africa the fishery for a number of desirable species was rendered commercially extinct as early as the mid 1970s. The Red Steenbras (Petrus rupestris) and Seventyfour (Polysteganus undulosus), as well as other endemic coral reef fishes are among the overfishing casualties of this Southern African fishery.
A fishery may be depleted by natural factors including parasitic outbreaks, ocean climate alterations, meteorological anomalies and even volcanic eruptions into aquatic basins. While these effects may contribute to fishery composition changes, they have only rarely been attributed to the collapse of a fishery, but rather are natural oceanographic temporal oscillations.
Overfishing in marine waters is evident in almost all seawater basins in the world. Also, the phenomenon is evident in many freshwater bodies. Notably data is remarkably sparse within the freshwater bodies of China, and some researchers have invoked an urgent appeal for more detailed documentation on freshwater fisheries of mainland China.
The Bluefin tuna, (genus Thunnus) harvested since ancient times from broad ranging marine basins, has vanished effectively from the North and Caspian Sea. Presently the species in in severe decline in the heart of its ancestral distribution within the western Mediterranean; Bluefin tuna harvested in the western Mediterranean declined from 14,699 tons in 1995 to 2270 tons in 2006
The sardine fishery on the Atlantic coast was overfished as early as 1962, when a peak in that fishery off the shores of Namibia and South Africa was observed. This purse seine exploitation was chiefly a result of recruitment overfishing (e.g.excessive taking of juveniles).
Economists present a simpler accounting approach to the subject of overfishing, in which two common models are utilized:
Maximum stock model: In this formulation, economists view the maximum stock that a fishery can contain without radical change to the ecosystem. This model is termed alternatively the overfishing input model.
Maximum catch model: Inherent is this formulation, economists premise that the maximum size of a fishery is that which can be sustained with any level of fishing effort. This model is known also as the overfishing output model.
There are numerous variations to these basic two modeling approaches, depending upon the nature of the age structure known about the fish species of interest. The basic thrust of many models is to determine the level of fishing effort that produces an optimal yield without placing the fishery in a downward spiral, from which the fish biomass may not recover.
See main article: Bycatch
Overfishing can be occasioned in the form of bycatch, the unintentional taking of fish or other aquatic species. Bycatch effects are particularly important in the case of endangered species, that may be protected by statute, but that can become ensnared in nets or other fishing gear as a by-product of a fishing enterprise targeting other species. Victims of bycatch are frequently important species such as top level predators as sharks, or other groups of threatened aquatic species including marine mammals and sea turtles. Some of the issues are technology related, since most fishing methods are not highly targeted as to the intended catch. Other issues are legal and regulatory, since it is difficult to fine the fisherman for accidental catch, even if the intention is re-release; moreover, significant mortality is often incurred in the process of the catch, even if the fisherman is willing to release the bycatch. Also, bycatch may refer to the unintended taking of juvenile fish; again, considerable mortality may occur, even if re-release is intended for juveniles.
Overfishing typically leads to massive dislocation impacts in ecosystem stability, biodiversity, food production and employment. Presently, worldwide losses greater than $50 billion USD each year are attributed to depleted fish stocks and poor fisheries management. Alternatively viewed, half the world's fishing fleet could be scrapped with no change in fish catches. Destabilization of aquatic ecosystems often are driven by overfishing especially when the fishery depleted is top level predator or keystone species. In turn, ecosystem services can be compromised severely when dramatic declines in individual fish species populations are observed
Ecosystem impacts are often complex, since chain reaction effects are often set in motion with the removal of a large biomass of higher level aquatic species. These chain consequences may entail loss of substantial biodiversity and induce large scale disease and parasitic imbalance. For example, certain outbreaks of parasitism are linked to sharp declines in individual fish populations; as a specific instance, increased incidence of schistosomiasis in Africa has been linked to declines of fish species that prey upon snails carrying the disease-causing parasites.
Often overfishing eliminates major jellyfish competitors, inducing potential jellyfish population explosions. Expansion of jellyfish populations then threaten fisheries, as they compete with fish for food, eat fish eggs, and poison or swarm fish, and can survive in oxygen depleted environments where fish cannot; as a consequence, jellyfish explosions can create considerable decimation to commercial fisheries.
In many cases adverse impacts to marine biodiversity are occasioned by cumulative impacts from two or more stressors. For example, chemical water pollution, thermal pollution, acid rain, sedimentation or other impacts may be present where overfishing is occurring. In these cases the habitat destruction can be substantial, since the physical or chemical environment is being altered at the same time as removal of significant biomass.
As recently as 1951, approximately 55 percent of the world's fisheries were not yet being commercially exploited by humans. The human population explosion led to a rapid decline in the number of undeveloped fisheries, such that by 1970 there were no fisheries in the planet's waters that were unexploited. In the case of natural fisheries (i.e. excluding aquaculture), fish catch per person peaked somewhere between the years 1971 and 1977, depending on the method of data analysis and curve fitting employed. The peak catch amounted to 16.4 kilograms per person per year; correspondingly by 1998 the annual catch had declined sharply to a level of 9.3 kilograms per person. Ron Greene has termed this situated with the world's fisheries as: "We are witnessing a global fish crisis". This dwindling of aquatic food supply comes on top of terrestrial crop production, that also has peaked in many countries.
Attempts at regulation
Numerous schemes of regulation at all levels of government have attempted to redress the collapse of fisheries. Most of these efforts have achieved at best limited success. At the international level the United Nations Convention on the Law of the Sea agreed in 1982 has two relevant articles (61 and 62) that encourage each nation to foster and protect its marine fisheries. Although the preponderance of nations have ratified this treaty, the abiltiy of individual nations to patrol nationally claimed waters effectively is marginal. Thus these international efforts as well as such efforts as the European Union's controls on fishing have not totally addressed and acted on the problem.
Many conflicts ensue from these well meaning efforts, since nations are far from agreement on territorial waters claims. Furthermore, one nation may have an effective program of protection in its own waters, only to find that other countries' protections are ineffective within the migratory range of the species at issue.
Initial implementation of national controls focussed on enacting limits to the fishing season. Generally, these efforts were unsuccessful, since fisherman simply invented more effective methods of coping with a shorter season. In addition, controls on only part of the migratory range of a species does not address the likelihood that fishermen will simply exploit a species in another nation's waters, that has looser restrictions or lax enforcement.
A later concept of regulation has been adoption of individual fishing quotas, whereby a country grants or sells a prescribed catch entitlement, after establishing a sustainable yield for a given fishery. Some of the first nations to apply such procedures have been New Zealand, the Netherlands and the United States. By 2008, only about ten percent of the world's fisheries came under such individual fishing quota systems, and results from these programs have been mixed. The deficiencies of these programs include the lack of ability of one nation to control the entire migratory range of certain species, as well as laxness in enforcement and squabbling within a nation or between nations regarding the quota numbers and pricing schemes. Nevertheless, the theory of individual fishing quotas has merit in that it creates stakeholders in the fishery future by effectively privatizing each fishery enterprise. As a result, while other forms of regulation have failed miserably, the individual fishing quota system has had some successes.
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