Maximum sustainable Yield

maximum sustainable yield is a term population ecologists and economists use to define the theoretical top catch or yield of a species that can be taken indefinitely without depleting the population. The maximum sustainable yield is assumed to be half of the carrying capacity of a species. Commonly, the maximum sustainable yield is more than the optimum sustainable yield, which ends when yield is no longer economical.

Maximum sustainable yield is a long-term management approach, not a crisis recovery method. The European Union (EU), for instance, uses it for all stocks, not just those that are approaching unsus-tainability. According to the logistic model of growth, population in a new habitat, or one that is depleted will initially experience a slow growth rate, but will grow rapidly once it reaches a foothold level. Rapid growth will slow when the population nears the carrying capacity of the habitat. The goal of maximum sustainable yield is to raise or lower population to the level where the highest growth rate is most likely. The new population level should be capable of indefinite maintenance. Maximum sustainable yield is highly variable, depending heavily on weather-influenced factors. Global warming affects maximum sustainable yield by altering the weather.


A sixth of the Earth's population depends on the sea for over a third of its animal protein (the world average is 16 percent). Fishing and processing employs 200 million people worldwide. The current annual catch is about 88 million U.S. tons, or 80 million metric tons. The total is down from 1989, the peak year with 86 million metric tons. The 1989 catch culminated a 50-year process in which the catch rose by a factor of four. However, the world total conceals the disparities in different parts of the world—the Indian Ocean catch continues to rise, while 13 of the 15 major areas have shown declines (the Atlantic cod catch is down by a factor of almost three since 1970.)

In November 1992, a collection of scientists warned that nature and humanity were on a collision course because of the damage human activity was doing to the environment and resources. They warned that unless change occurred, humanity risked creating a world incapable of sustaining life as we know it. They said that Earth had too many people. The scientists warned of atmospheric damage from ozone depletion, air pollution, and acid rain. They noted critical depletion of the water supply, putting world food production at risk. They cited soil depletion that degraded land and cut food production. They decried the loss of tropical rainforests. They also talked of the destruction of ocean life, particularly the world's supply of food fish. They noted that the world's farmers, industrialists and urbanites were sending pollutants into the oceans, that fishermen were taking more than the maximum sustainable yield, and some fisheries were showing signs of collapsing. They calculated that by 2100, upwards of one-third of all extant species might disappear. Among other steps to prevent the disaster, humanity needed to find alternatives to fossil fuels, become more efficient in using nonrenewable resources, and manage critical resources more effectively. The developed world had to help the undeveloped world to make these changes.

The earlier approach to fisheries management was to keep stocks from falling too low. At the World Summit on Sustainable Development in Johannesburg, in September 2002, EU members agreed to limit fishing to sustainable levels, and to maintain or restore stocks to maximum sustainable yield levels. The goal was to replenish depleted species by 2015. The attendees also committed to act against illegal, unregulated, or unre-ported fishing, another priority. Four years after the agreements, the EU reported that it had many species outside safe limits. According to an International Council for the Exploration of the Sea (ICES) analysis, 81 percent of the stocks in the northeast Atlantic and adjacent waters were over-fished, some as much as five times maximum sustainable yield. The EU implementation of maximum sustainable yield sought to maximize the catch without endangering the stock. The EU noted that small stocks could provide only small yields, because too few adults were available to replenish the stock. When stock was too great, reproduction slowed because of lack of food and decreased competition. The maximum sustainable yield lies between too small, and too large.


Maximum sustainable yield is common in fisheries management. In modern fisheries, maximum sustainable yield is around 30 percent of the unexploited population, but it varies depending on the life history of the species and the fishing method chosen.

The stock of a given type of fish varies over time, from natural causes and human impacts. A fishing fleet that has doubled in the past 25 years suffers when the variation is adverse, and as the catch trends down for commercially valuable varieties, concern arises that the ocean may not be able to supply the protein need of an ever-growing population without conservation and management. There are increasing disputes over who gets to fish for how much in which waters at which time of year—jurisdictional rights versus open access. There are disagreements over how many fish are left. Estimates are notoriously unreliable, because too many uncontrollable factors affect the total. Particularly important are environmental changes, which can affect mortality in larval fish and significantly impact the grown population. Factors include the number of predators, the available nutrients, and the temperature of the water. Water temperature is critical because fish can not regulate their own body temperature. Warming and cooling encourage populations to move to more suitable waters. Turbulence also affects the supply by putting the larvae at a disadvantage against larger and less susceptible predators. Turbulence changes reflect, among other factors, changes in regional climate.

The EU implemented maximum sustainable yield through long term plans prescribed by the Common Fisheries Policy, plans that define the fishing rate and the rate of annual adjustment. The approach was gradualist to avoid short-term harm to the fishing industry, while the changes took place. The EU also authorized closing fishing areas and restricting types of fishing gear on a case-by-case basis. Before implementing any plan, the EU prepared a

Scientists have warned for over a decade of a critical depletion of food fish production due to the destruction of ocean life.

social and economic impact statement. Maximum sustainable yield was assumed to keep stocks from collapsing, reducing discards of too small/too low value/caught beyond quota fish. It was also expected to reduce by-catches and cut down on impact on the environment. After the initial reduction to allow fish to grow, fishing could resume with presumed lower costs and greater profit, as catches of larger/better fish cut down fishing time and fuel consumed. The EU, in 2006, was dependent on imports for 60 percent of its fish products.

climate change

The Intergovernmental Panel on Climate Change (IPCC) released a study on probable impacts of greenhouse warming. Acknowledging that impacts vary from species to species, and that climate change models are only approximate, the IPCC predicted that global warming will have a small impact if global warming impacts are restricted to changes in surface temperature.

Warming will have a greater impact if local economic impacts of shifting fisheries are taken into account. The global total catch should remain about the same because the fish can move easily. The regions where fishing is viable will change, as will the species available, because some species are more susceptible to warming than others. A more significant impact could be the climate change that could alter winds, thereby changing currents and modifying the flow of nutrients and larval fish. Warming's rising waters would alter the area near shore or in river mouths, changing the ecosystem and modifying the types of marine life.

Determining maximum sustainable yield can be inaccurate when insufficient data are available. Miscalculation of the sustainable yield of roughy occurred in New Zealand when initial quotas assumed that the species lived a relatively short life and bred quickly. In fact, the roughy lives 30 years and breeds slowly. By the time this was known, the available roughy stock was mostly gone. That climate change affects maximum sustainable yield is indisputable, but exactly what the impacts will be is disputed, because sufficient data are not yet available.

sEE ALsO: Marine Mammals; Oceanic Changes; Ocean Component of Models; Oceanography; Pollution, Water.

BIBLIOGRAPHY. European Union, "Questions and Answers on Maximum Sustainable Yield," (July 5, 2006); Ron Nielsen, The Little Green Handbook (Picador, 2006); B.J. Rothschild, "How Bountiful are Ocean Fisheries?" Consequences (v.2/1, 1996); United Nations Development Programme, World Resources 2000-2001 (World Resources Institute, 2000); World Transformation, "World Scientists' Warning to Humanity," (November 18, 1992).

John H. Barnhill Independent Scholar

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Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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