# Maximum sustainable yield

In population ecology and economics, maximum sustainable yield or MSY is theoretically, the largest yield (or catch) that can be taken from a species' stock over an indefinite period. Fundamental to the notion of sustainable harvest, the concept of MSY aims to maintain the population size at the point of maximum growth rate by harvesting the individuals that would normally be added to the population, allowing the population to continue to be productive indefinitely. Under the assumption of logistic growth, resource limitation does not constrain individuals' reproductive rates when populations are small, but because there are few individuals, the overall yield is small. At intermediate population densities, also represented by half the carrying capacity, individuals are able to breed to their maximum rate. At this point, called the maximum sustainable yield, there is a surplus of individuals that can be harvested because growth of the population is at its maximum point due to the large number of reproducing individuals. Above this point, density dependent factors increasingly limit breeding until the population reaches carrying capacity. At this point, there are no surplus individuals to be harvested and yield drops to zero. The maximum sustainable yield is usually higher than the optimum sustainable yield and maximum economic yield.

MSY is extensively used for fisheries management. Unlike the logistic (Schaefer) model,[1] MSY has been refined in most modern fisheries models and occurs at around 30% [2] of the unexploited population size. This fraction differs among populations depending on the life history of the species and the age-specific selectivity of the fishing method.

However, the approach has been widely criticized as ignoring several key factors involved in fisheries management and has led to the devastating collapse of many fisheries. As a simple calculation, it ignores the size and age of the animal being taken, its reproductive status, and it focuses solely on the species in question, ignoring the damage to the ecosystem caused by the designated level of exploitation and the issue of bycatch. Among conservation biologists it is widely regarded as dangerous and misused.[3][4]

## History

The concept of MSY as a fisheries management strategy developed in Belmar, New Jersey, in the early 1930s.[5][6][7] It increased in popularity in the 1950s with the advent of surplus-production models with explicitly estimate MSY.[1] As an apparently simple and logical management goal, combined with the lack of other simple management goals of the time, MSY was adopted as the primary management goal by several international organizations (e.g., IWC, IATTC,[8] ICCAT, ICNAF), and individual countries.[9]

Between 1949 and 1955, the U.S. maneuvered to have MSY declared the goal of international fisheries management (Johnson 2007). The international MSY treaty that was eventually adopted in 1955 gave foreign fleets the right to fish off any coast. Nations that wanted to exclude foreign boats had to first prove that its fish were overfished.[10]

As experience was gained with the model, it became apparent to some researchers that it lacked the capability to deal with the real world operational complexities and the influence of trophic and other interactions. In 1977, Peter Larkin wrote its epitaph, challenging the goal of maximum sustained yield on several grounds: It put populations at too much risk; it did not account for spatial variability in productivity; it did not account for species other than the focus of the fishery; it considered only the benefits, not the costs, of fishing; and it was sensitive to political pressure.[11] In fact, none of these criticisms was aimed at sustainability as a goal. The first one noted that seeking the absolute MSY with uncertain parameters was risky. The rest point out that the goal of MSY was not holistic; it left out too many relevant features.[10]

Some managers began to use more conservative quota recommendations, but the influence of the MSY model for fisheries management still prevailed. Even while the scientific community was beginning to question the appropriateness and effectiveness of MSY as a management goal,[11][12] it was incorporated into the 1982 United Nations Convention for the Law of the Sea, thus ensuring its integration into national and international fisheries acts and laws.[9] According to Walters and Maguire, an ‘‘institutional juggernaut had been set in motion’’, climaxing in the early 1990s with the collapse of northern cod.[13]

## Modelling MSY

### Population growth

The key assumption behind all sustainable harvesting models such as MSY is that populations of organisms grow and replace themselves – that is, they are renewable resources. Additionally it is assumed that because the growth rates, survival rates, and reproductive rates increase when harvesting reduces population density,[5] they produce a surplus of biomass that can be harvested. Otherwise, sustainable harvest would not be possible.

Another assumption of renewable resource harvesting is that populations of organisms do not continue to grow indefinitely; they reach an equilibrium population size, which occurs when the number of individuals matches the resources available to the population (i.e., assume classic logistic growth). At this equilibrium population size, called the carrying capacity, the population remains at a stable size.[14]

Figure 1

The logistic model (or logistic function) is a function that is used to describe bounded population growth under the previous two assumptions. The logistic function is bounded at both extremes: when there are not individuals to reproduce, and when there is an equilibrium number of individuals (i.e., at carrying capacity). Under the logistic model, population growth rate between these two limits is most often assumed to be sigmoidal (Figure 1). There is scientific evidence that some populations do grow in a logistic fashion towards a stable equilibrium – a commonly cited example is the logistic growth of yeast.

The equation describing logistic growth is:[14]

${\displaystyle N_{t}={\frac {K}{1+{\frac {K-N_{0}}{N_{0}}}e^{-rt}}}}$ (equation 1.1)

The parameter values are:

${\displaystyle N_{t}}$=The population size at time t
${\displaystyle K}$=The carrying capacity of the population
${\displaystyle N_{0}}$= The population size at time zero
${\displaystyle r}$= the intrinsic rate of population increase (the rate at which the population grows when it is very small)

From the logistic function, the population size at any point can be calculated as long as ${\displaystyle r}$, ${\displaystyle K}$, and ${\displaystyle N_{0}}$ are known.

Figure 2

Differentiating equation 1.1 give an expression for how the rate of population increases as N increases. At first, the population growth rate is fast, but it begins to slow as the population grows until it levels off to the maximum growth rate, after which it begins to decrease (figure 2).

The equation for figure 2 is the differential of equation 1.1 (Verhulst's 1838 growth model):[14]

${\displaystyle {\frac {dN}{dt}}=rN\left(1-{\frac {N}{K}}\right)}$ (equation 1.2)

${\displaystyle {\frac {dN}{dt}}}$ can be understood as the change in population (N) with respect to a change in time (t). Equation 1.2 is the usual way in which logistic growth is represented mathematically and has several important features. First, at very low population sizes, the value of ${\displaystyle {\frac {N}{K}}}$ is small, so the population growth rate is approximately equal to ${\displaystyle rN}$, meaning the population is growing exponentially at a rate r (the intrinsic rate of population increase). Despite this, the population growth rate is very low (low values on the y-axis of figure 2) because, even though each individual is reproducing at a high rate, there are few reproducing individuals present. Conversely, when the population is large the value of ${\displaystyle {\frac {N}{K}}}$ approaches 1 effectively reducing the terms inside the brackets of equation 1.2 to zero. The effect is that the population growth rate is again very low, because either each individual is hardly reproducing or mortality rates are high.[14] As a result of these two extremes, the population growth rate is maximum at an intermediate population or half the carrying capacity (${\displaystyle N={\frac {K}{2}}}$).

### MSY model

Figure 3

The simplest way to model harvesting is to modify the logistic equation so that a certain number of individuals is continuously removed:[14]

${\displaystyle {\frac {dN}{dt}}=rN\left(1-{\frac {N}{K}}\right)-H}$ (equation 1.3)

Where H represents the number of individuals being removed from the population – that is, the harvesting rate. When H is constant, the population will be at equilibrium when the number of individuals being removed is equal to the population growth rate (figure 3). The equilibrium population size under a particular harvesting regime can be found when the population is not growing – that is, when ${\displaystyle {\frac {dN}{dt}}=0}$. This occurs when the population growth rate is the same as the harvest rate:

${\displaystyle rN\left(1-{\frac {N}{K}}\right)=H}$

Figure 3 shows how growth rate varies with population density. For low densities (far from carrying capacity), there is little addition (or "recruitment") to the population, simply because there are few organisms to give birth. At high densities, though, there is intense competition for resources, and growth rate is again low because the death rate is high. In between these two extremes, the population growth rate rises to a maximum value (${\displaystyle N_{MSY}}$). This maximum point represents the maximum number of individuals that can be added to a population by natural processes. If more individuals than this are removed from the population, the population is at risk for decline to extinction.[15] The maximum number that can be harvested in a sustainable manner, called the maximum sustainable yield, is given by this maximum point.

Figure 3 also shows several possible values for the harvesting rate, H. At ${\displaystyle H_{1}}$, there are two possible population equilibrium points: a low population size (${\displaystyle N_{a}}$) and a high one (${\displaystyle N_{b}}$). At ${\displaystyle H_{2}}$, a slightly higher harvest rate, however there is only one equilibrium point (at ${\displaystyle N_{MSY}}$), which is the population size that produces the maximum growth rate. With logistic growth, this point, called the maximum sustainable yield, is where the population size is half the carrying capacity (or ${\displaystyle N={\frac {K}{2}}}$). The maximum sustainable yield is the largest yield that can be taken from a population at equilibrium. In figure 3, if ${\displaystyle H}$ is higher than ${\displaystyle H_{2}}$, the harvesting would exceed the population's capacity to replace itself at any population size (${\displaystyle H_{3}}$ in figure 3). Because harvesting rate is higher than the population growth rate at all values of ${\displaystyle N}$, this rate of harvesting is not sustainable.

An important feature of the MSY model is how harvested populations respond to environmental fluctuations or illegal offtake. Consider a population at ${\displaystyle N_{b}}$ harvested at a constant harvest level ${\displaystyle H_{1}}$. If the population falls (due to a bad winter or illegal harvest) this will ease density-dependent population regulation and increase yield, moving the population back to ${\displaystyle N_{b}}$, a stable equilibrium. In this case, a negative feedback loop creates stability. The lower equilibrium point for the constant harvest level ${\displaystyle H_{1}}$ is not stable however; a population crash or illegal harvesting will decrease population yield farther below the current harvest level, creating a positive feedback loop leading to extinction. Harvesting at ${\displaystyle N_{MSY}}$ is also potentially unstable. A small decrease in the population can lead to a positive feedback loop and extinction if the harvesting regime (${\displaystyle H_{2}}$) is not reduced. Thus, some consider harvesting at MSY to be unsafe on ecological and economic grounds.[15][16] The MSY model itself can be modified to harvest a certain percentage of the population or with constant effort constraints rather than an actual number, thereby avoiding some of its instabilities.[15]

The MSY equilibrium point is semi-stable – a small increase in population size is compensated for, a small decrease to extinction if H is not decreased. Harvesting at MSY is therefore dangerous because it is on a knife-edge – any small population decline leads to a positive feedback, with the population declining rapidly to extinction if the number of harvested stays the same.[15][16]

The formula for maximum sustained harvest (${\displaystyle H}$) is one-fourth the maximum population or carrying capacity (${\displaystyle K}$) times the intrinsic rate of growth (${\displaystyle r}$).[17]

${\displaystyle H={\frac {Kr}{4}}}$

### For demographically structured populations

The principle of MSY often holds for age-structured populations as well.[18] The calculations can be more complicated, and the results often depend on whether density dependence occurs in the larval stage (often modeled as density dependent reproduction) and/or other life stages.[19] It has been shown that if density dependence only acts on larva, then there is an optimal life stage (size or age class) to harvest, with no harvest of all other life stages.[18] Hence the optimal strategy is to harvest this most valuable life-stage at MSY.[20] However, in age and stage-structured models, a constant MSY does not always exist. In such cases, cyclic harvest is optimal where the yield and resource fluctuate in size, through time.[21] In addition, environmental stochasticity interacts with demographically structured populations in fundamentally different ways than for unstructured populations when determining optimal harvest. In fact, the optimal biomass to be left in the ocean, when fished at MSY, can be either higher or lower than in analogous deterministic models, depending on the details of the density dependent recruitment function, if stage-structure is also included in the model.[22]

### Implications of MSY model

Starting to harvest a previously unharvested population will always lead to a decrease in the population size. That is, it is impossible for a harvested population to remain at its original carrying capacity. Instead, the population will either stabilize at a new lower equilibrium size or, if the harvesting rate is too high, decline to zero.

The reason why populations can be sustainably harvested is that they exhibit a density-dependent response.[15][16] This means that at any population size below K, the population is producing a surplus yield that is available for harvesting without reducing population size. Density dependence is the regulator process that allows the population to return to equilibrium after a perturbation. The logistic equation assumes that density dependence takes the form of negative feedback.[16]

If a constant number of individuals is harvested from a population at a level greater than the MSY, the population will decline to extinction. Harvesting below the MSY level leads to a stable equilibrium population if the starting population is above the unstable equilibrium population size.

### Uses of MSY

MSY has been especially influential in the management of renewable biological resources such as commercially important fish and wildlife. In fisheries terms, maximum sustainable yield (MSY) is the largest average catch that can be captured from a stock under existing environmental conditions.[23] MSY aims at a balance between too much and too little harvest to keep the population at some intermediate abundance with a maximum replacement rate.

Relating to MSY, the maximum economic yield (MEY) is the level of catch that provides the maximum net economic benefits or profits to society.[24][25] Like optimum sustainable yield, MEY is usually less than MSY.

### Limitations of MSY approach

Although it is widely practiced by state and federal government agencies regulating wildlife, forests, and fishing, MSY has come under heavy criticism by ecologists and others from both theoretical and practical reasons.[16] The concept of maximum sustainable yield is not always easy to apply in practice. Estimation problems arise due to poor assumptions in some models and lack of reliability of the data.[9][26] Biologists, for example, do not always have enough data to make a clear determination of the population's size and growth rate. Calculating the point at which a population begins to slow from competition is also very difficult. The concept of MSY also tends to treat all individuals in the population as identical, thereby ignoring all aspects of population structure such as size or age classes and their differential rates of growth, survival, and reproduction.[26]

As a management goal, the static interpretation of MSY (i.e., MSY as a fixed catch that can be taken year after year) is generally not appropriate because it ignores the fact that fish populations undergo natural fluctuations (i.e., MSY treats the environment as unvarying) in abundance and will usually ultimately become severely depleted under a constant-catch strategy.[26] Thus, most fisheries scientists now interpret MSY in a more dynamic sense as the maximum average yield (MAY) obtained by applying a specific harvesting strategy to a fluctuating resource.[9] Or as an optimal "escapement strategy", where escapement means the amount of fish that must remain in the ocean [rather than the amount of fish that can be harvested]. An escapement strategy is often the optimal strategy for maximizing expected yield of a harvested, stochastically fluctuating population.[27]

However, the limitations of MSY, does not mean it performs worse than humans using their best intuitive judgment. Experiments using students in natural resource management classes suggest that people using their past experience, intuition, and best judgement to manage a fishery generate far less long term yield compared to a computer using an MSY calculation, even when that calculation comes from incorrect population dynamic models.[28]

For a more contemporary description of MSY and its calculation see [29]

#### Orange roughy

An example of errors in estimating the population dynamics of a species occurred within the New Zealand Orange roughy fishery. Early quotas were based on an assumption that the orange roughy had a fairly short lifespan and bred relatively quickly. However, it was later discovered that the orange roughy lived a long time and had bred slowly (~30 years). By this stage stocks had been largely depleted.

## Overfishing

All around the world, from the arctic to the tropics, there is a crisis in the world's fisheries.[30] Until fairly recently it was assumed that our marine resources were limitless.

In recent years however, an accelerating decline has been observed in the productivity of many important fisheries.[31] Fisheries which have been devastated in recent times include (but are not limited to) the great whale fisheries, the Grand Bank fisheries of the western Atlantic, and the Peruvian anchovy fishery.[32] Recent assessments by the United Nations Food and Agriculture Organization (FAO) of the state of the world's fisheries indicate a levelling off of landings in the 1990s, at about 100 million tons.[33]

In addition, the composition of global catches has changed.[34] As fishers deplete larger, long-lived predatory fish species such as cod, tuna, shark, and snapper, they move down to the next level – to species that tend to be smaller, shorter-lived, and less valuable.[35]

Overfishing is a classic example of the tragedy of the commons.[32]

## Optimum sustainable yield

In population ecology and economics, optimum sustainable yield is the level of effort (LOE) that maximizes the difference between total revenue and total cost. Or, where marginal revenue equals marginal cost. This level of effort maximizes the economic profit, or rent, of the resource being utilized. It usually corresponds to an effort level lower than that of maximum sustainable yield. In environmental science, optimum sustainable yield is the largest economical yield of a renewable resource achievable over a long time period without decreasing the ability of the population or its environment to support the continuation of this level of yield.

## Notes

1. ^ a b Schaefer, Milner B. (1954), "Some aspects of the dynamics of populations important to the management of commercial marine fisheries", Bulletin of the Inter-American Tropical Tuna Commission (reprinted in Bulletin of Mathematical Biology, Vol. 53, No. 1/2, pp. 253-279, 1991 ed.), 1 (2): 27–56, doi:10.1007/BF02464432
2. ^ Thorpe 2015
3. ^ Larkin PA (1977) "An epitaph for the concept of maximum sustained yield" Transactions of the American Fisheries Society, 106: 1–11.
4. ^ Walters C and Maguire J (1996) "Lessons for stock assessment from the northern cod collapse", Reviews in Fish Biology and Fisheries, 6:125–137.
5. ^ a b Russell, E. S. (1931). "Some theoretical Considerations on the "Overfishing" Problem". ICES Journal of Marine Science. 6 (1): 3–20. doi:10.1093/icesjms/6.1.3. ISSN 1054-3139.
6. ^ Hjort et al 1933
7. ^ Graham, M. (1935). "Modern Theory of Exploiting a Fishery, and Application to North Sea Trawling". ICES Journal of Marine Science. 10 (3): 264–274. doi:10.1093/icesjms/10.3.264. ISSN 1054-3139.
8. ^ IATTC, Inter-American Tropical Tuna Commission
9. ^ a b c d Mace 2001
10. ^ a b Botsford et al 1997
11. ^ a b Larkin, P. A. (1977). "An epitaph for the concept of maximum sustained yield". Transactions of the American Fisheries Society. 106 (1): 1–11. doi:10.1577/1548-8659(1977)106<1:AEFTCO>2.0.CO;2. ISSN 0002-8487.
12. ^ Sissenwine 1978
13. ^ Walters and Maguire, 1996
14. Milner-Gulland and Mace 1998, pp. 14-17.
15. Jennings et al 2001
16. Milner-Gulland and Mace 1998.
17. ^ Bolden and Robinson 1999
18. ^ a b Reed, William J. (1980-01-01). "Optimum Age-Specific Harvesting in a Nonlinear Population Model". Biometrics. 36 (4): 579–593. doi:10.2307/2556112. JSTOR 2556112.
19. ^ Boucekkine, Raouf; Hritonenko, Natali; Yatsenko, Yuri (2013-05-13). Optimal Control of Age-structured Populations in Economy, Demography, and the Environment. Routledge. ISBN 978-1136920936.
20. ^ Getz, Wayne M. (1980-01-01). "The ultimate-sustainable-yield problem in nonlinear age-structured populations". Mathematical Biosciences. 48 (3–4): 279–292. doi:10.1016/0025-5564(80)90062-0. ISSN 0025-5564. Archived from the original on 2017-02-03. Retrieved 2017-01-28.
21. ^ "Optimal Harvesting of Age-structured Fish Populations". Marine Resource Economics. 2009. doi:10.5950/0738-1360-24.2.147.
22. ^ Holden, Matthew H.; Conrad, Jon M. (2015-11-01). "Optimal escapement in stage-structured fisheries with environmental stochasticity". Mathematical Biosciences. 269: 76–85. doi:10.1016/j.mbs.2015.08.021. PMID 26362229.
23. ^ National Research Council (NRC). 1998. Improving Fish Stock Assessments. National Academy Press, Washington, D.C.
24. ^ Clark 1990
25. ^ National Marine Fisheries Service (NMFS). 1996. OUr Living Oceans: Report on the Status of U.S. Living Marine Resources 1995. NOAA Technical Memorandum NMFS0F/SPO-19. NMFS, Silver Springs, Md.
26. ^ a b c Townsend et al 2008
27. ^ Reed, William J (1979-12-01). "Optimal escapement levels in stochastic and deterministic harvesting models". Journal of Environmental Economics and Management. 6 (4): 350–363. doi:10.1016/0095-0696(79)90014-7.
28. ^ Holden, Matthew H.; Ellner, Stephen P. (2016-07-01). "Human judgment vs. quantitative models for the management of ecological resources". Ecological Applications. 26 (5): 1553–1565. arXiv:1603.04518. doi:10.1890/15-1295. ISSN 1939-5582. PMID 27755756.
29. ^ Maunder 2008
30. ^ sciencemag.org Worm, Boris, et. a;. "Impacts of Biodiversity Loss on Ocean Ecosystem Services," Science, 3 November 2006.
31. ^ Christy and Scott 1965
32. ^ a b Clark 1973
33. ^ FAO, Review of the State of World Marine Fishery Resources, FAO Technical Paper 335 (1994).
34. ^ Roberts, 2007
35. ^ Pauly, D. (1998). "Fishing Down Marine Food Webs". Science. 279 (5352): 860–863. Bibcode:1998Sci...279..860P. doi:10.1126/science.279.5352.860. ISSN 0036-8075. PMID 9452385.

## References

Bacterivore

Bacterivores are free-living, generally heterotrophic organisms, exclusively microscopic, which obtain energy and nutrients primarily or entirely from the consumption of bacteria. Many species of amoeba are bacterivores, as well as other types of protozoans. Commonly, all species of bacteria will be prey, but spores of some species, such as Clostridium perfringens, will never be prey, because of their cellular attributes.

Dominance (ecology)

Ecological dominance is the degree to which a taxon is more numerous than its competitors in an ecological community, or makes up more of the biomass.

Most ecological communities are defined by their dominant species.

In many examples of wet woodland in western Europe, the dominant tree is alder (Alnus glutinosa).

In temperate bogs, the dominant vegetation is usually species of Sphagnum moss.

Tidal swamps in the tropics are usually dominated by species of mangrove (Rhizophoraceae)

Some sea floor communities are dominated by brittle stars.

Exposed rocky shorelines are dominated by sessile organisms such as barnacles and limpets.

Ecological resilience

In ecology, resilience is the capacity of an ecosystem to respond to a perturbation or disturbance by resisting damage and recovering quickly. Such perturbations and disturbances can include stochastic events such as fires, flooding, windstorms, insect population explosions, and human activities such as deforestation, fracking of the ground for oil extraction, pesticide sprayed in soil, and the introduction of exotic plant or animal species. Disturbances of sufficient magnitude or duration can profoundly affect an ecosystem and may force an ecosystem to reach a threshold beyond which a different regime of processes and structures predominates. Human activities that adversely affect ecosystem resilience such as reduction of biodiversity, exploitation of natural resources, pollution, land use, and anthropogenic climate change are increasingly causing regime shifts in ecosystems, often to less desirable and degraded conditions. Interdisciplinary discourse on resilience now includes consideration of the interactions of humans and ecosystems via socio-ecological systems, and the need for shift from the maximum sustainable yield paradigm to environmental resource management which aims to build ecological resilience through "resilience analysis, adaptive resource management, and adaptive governance".

Ecopath

Ecopath with Ecosim (EwE) is a free and open source ecosystem modelling software suite, initially started at NOAA by Jeffrey Polovina, but has since primarily been developed at the formerly UBC Fisheries Centre of the University of British Columbia. In 2007, it was named as one of the ten biggest scientific breakthroughs in NOAA's 200-year history. The NOAA citation states that Ecopath "revolutionized scientists' ability worldwide to understand complex marine ecosystems". Behind this lie more than two decades of development work in association with Villy Christensen, Carl Walters, Daniel Pauly, and other fisheries scientists, followed with the provision of user support, training and co-development collaborations. In 2013, development efforts were centralized under Ecopath International Initiative, Spain. Per January 2019 there are an estimated 8000+ users across academia, non-government organizations, industry and governments in 150+ countries.

Environmental space

The concept of environmental space is the amount of any particular resource that can be consumed by a country without threatening the continued availability of that resource (sustainability), assuming that everyone in the world is entitled to an equal share.

The weakness of the concept is that it requires calculating the maximum sustainable consumption rate of each different resource globally. This rate could be set by either the maximum sustainable

yield (say for forests or fisheries) or the assimilative capacity of the environment (e.g. for CO2 or chlorine), but both quantities are very difficult to determine, so the 'environmental space' has rather large error limits and is therefore hard to defend in policy discussions. A further difficulty is that there is a different 'environmental space' for each kind of resource, and as they each necessarily have different units of measure, they cannot be added to get an overall environmental space for all the resources consumed by a country. The idea of environmental space was promoted quite strongly by Friends of the Earth Europe in the mid-1990s, but it is rarely used now, because of the aforementioned difficulties, and has essentially been superseded by 'ecological footprint'. The advantage of the ecological footprint is that every kind of resource use is converted to a land area basis, so that they can be added to produce an overall figure for a country, allowing comparisons to be made.

Feeding frenzy

In ecology, a feeding frenzy occurs when predators are overwhelmed by the amount of prey available. For example, a large school of fish can cause nearby sharks, such as the lemon shark, to enter into a feeding frenzy. This can cause the sharks to go wild, biting anything that moves, including each other or anything else within biting range. Another functional explanation for feeding frenzy is competition amongst predators. This term is most often used when referring to sharks or piranhas. It has also been used as a term within journalism.

Fish measurement

Fish measurement is the measuring of the length of individual fish and of various parts of their anatomy. These data are used in many areas of ichthyology, including taxonomy and fisheries biology.

Fish mortality

Fish mortality is a parameter used in fisheries population dynamics to account for the loss of fish in a fish stock through death. The mortality can be divided into two types:

Natural mortality: the removal of fish from the stock due to causes not associated with fishing. Such causes can include disease, competition, cannibalism, old age, predation, pollution or any other natural factor that causes the death of fish. In fisheries models natural mortality is denoted by (M).

Fishing mortality: the removal of fish from the stock due to fishing activities using any fishing gear. It is denoted by (F) in fisheries models.(M) and (F) are additive instantaneous rates that sum up to (Z), the instantaneous total mortality coefficient; that is, Z=M+F. These rates are usually calculated on an annual basis. Estimates of fish mortality rates are often included in mathematical yield models to predict yield levels obtained under various exploitation scenarios. These are used as resource management indices or in bioeconomic studies of fisheries.

Gordon-Schaefer model

The Gordon-Schaefer model is a bioeconomic model applied in the fishing industry. It may be used to compute the maximum sustainable yield. It takes account of biological growth rates, carrying capacity, and total and marginal costs and revenues.This model can be applied in three primary scenarios: Monopoly; Maximum Sustainable Yield (biological optimum); and Open Access.

Guimaras Strait

Guimaras Strait is a strait in the Western Visayas region of the Philippines, connecting the Visayan Sea with the Panay Gulf and Sulu Sea beyond. To the north and west are Panay and Guimaras Islands, while Negros Island is to the south and east. Bacolod City is a major seaport on the strait, which also provides access to the Port of Iloilo City via the Iloilo Strait.

The Guimaras Strait is an important fishing ground in the Philippines, having an average of annual fish production of 50,000 metric tons. The northern part is particularly known for blue crab, a top export commodity. Other species present in the strait include barracuda, cavalla, clams, cockles, crevalle, dolphin, flounders, flying fish, fusilies, glassfish, goatfish, goby, grouper, jacks, jelly fish, lizard fish, marine turtle, milkffish, mojarra, moonfish, moray, mullets, mussels, scallops, oysters, perchlet, scads, sea bass, sea catfish, sea cucumber, sea perches, sea urchins, seaweeds, shads, sickle fish, siganids, sillago, slipmouth, snappers, spade fish, sponges, surgeon fish, threadfin, and wrasses.The Guimaras oil spill, that occurred in the Panay Gulf on August 11, 2006, has severely affected the fishing industry. During this spill, considered Philippines' worst, the oil tanker M/T Solar 1 sank during a violent storm, spilling some 500,000 litres (110,000 imp gal; 130,000 US gal) of oil which formed an oil slick that drifted through the strait. This spill followed another one in December 2005, when a passenger ship ran aground in the strait. It spilled 360,000 litres (79,000 imp gal; 95,000 US gal) of fuel oil, polluting some 40 kilometres (25 mi) of coastline and 230 hectares (570 acres) of virgin mangrove forests.Besides these oil spills, the ecosystems of the Guimaras Strait also suffer from rapid coastal development and overfishing. Increased urban populations and transportation are causing anthropogenic pressures, while gillnet fishing has exceeded the maximum sustainable yield since 1999.

MSY

MSY or msy can mean:

Maximum sustainable yield

IATA airport code for Louis Armstrong New Orleans International Airport

The Mount School, York, United Kingdom

Motor sailing yacht, a ship prefix for a yacht that can use a motor for propulsion and also sail

Macao Sam Yuk Middle School

Optimum sustainable yield

In population ecology and economics, optimum sustainable yield is the level of effort (LOE) that maximizes the difference between total revenue and total cost. Or, where marginal revenue equals marginal cost. This level of effort maximizes the economic profit, or rent, of the resource being utilized. It usually corresponds to an effort level lower than that of maximum sustainable yield.

In environmental science, optimum sustainable yield is the largest economical yield of a renewable resource achievable over a long time period without decreasing the ability of the population or its environment to support the continuation of this level of yield, and enables an ecosystem to have a high aesthetic value. This concept is widely used specifically in the management of fisheries, where surplus fish are removed so the population stays at its carrying capacity. This allows the most fish to be harvested while still maintaining maximum population growth.

Orange roughy

The orange roughy (Hoplostethus atlanticus), also known as the red roughy, slimehead and deep sea perch, is a relatively large deep-sea fish belonging to the slimehead family (Trachichthyidae). The UK Marine Conservation Society has categorized orange roughy as "vulnerable to exploitation". It is found in 3 to 9 °C (37 to 48 °F), deep (bathypelagic, 180-to-1,800-metre (590 to 5,910 ft)) waters of the Western Pacific Ocean, eastern Atlantic Ocean (from Iceland to Morocco; and from Walvis Bay, Namibia, to off Durban, South Africa), Indo-Pacific (off New Zealand and Australia), and in the eastern Pacific off Chile.

The orange roughy is notable for its extraordinary lifespan, living for up to 149 years. It is important to commercial deep-trawl fisheries. The fish is a bright, brick-red color; fading to a yellowish-orange after death.

Like other slimeheads, orange roughy is slow-growing and late to mature, resulting in a very low resilience which makes them extremely susceptible to overfishing. Many stocks (especially those off New Zealand and Australia, which were first exploited in the late 1970s) became severely depleted within 3–20 years, but several have subsequently recovered (see Fisheries below).

Organotroph

An organotroph is an organism that obtains hydrogen or electrons from organic substrates. This term is used in microbiology to classify and describe organisms based on how they obtain electrons for their respiration processes. Some organotrophs such as animals and many bacteria, are also heterotrophs. Organotrophs can be either anaerobic or aerobic.

Population dynamics of fisheries

A fishery is an area with an associated fish or aquatic population which is harvested for its commercial or recreational value. Fisheries can be wild or farmed. Population dynamics describes the ways in which a given population grows and shrinks over time, as controlled by birth, death, and migration. It is the basis for understanding changing fishery patterns and issues such as habitat destruction, predation and optimal harvesting rates. The population dynamics of fisheries is used by fisheries scientists to determine sustainable yields.The basic accounting relation for population dynamics is the BIDE (Birth, Immigration, Death, Emigration) model, shown as:

N1 = N0 + B − D + I − Ewhere N1 is the number of individuals at time 1, N0 is the number of individuals at time 0, B is the number of individuals born, D the number that died, I the number that immigrated, and E the number that emigrated between time 0 and time 1. While immigration and emigration can be present in wild fisheries, they are usually not measured.

A fishery population is affected by three dynamic rate functions:

Birth rate or recruitment. Recruitment means reaching a certain size or reproductive stage. With fisheries, recruitment usually refers to the age a fish can be caught and counted in nets.

Growth rate. This measures the growth of individuals in size and length. This is important in fisheries where the population is often measured in terms of biomass.

Mortality. This includes harvest mortality and natural mortality. Natural mortality includes non-human predation, disease and old age.If these rates are measured over different time intervals, the harvestable surplus of a fishery can be determined. The harvestable surplus is the number of individuals that can be harvested from the population without affecting long term stability (average population size). The harvest within the harvestable surplus is called compensatory mortality, where the harvest deaths are substituting for the deaths that would otherwise occur naturally. Harvest beyond that is additive mortality, harvest in addition to all the animals that would have died naturally.

Care is needed when applying population dynamics to real world fisheries. Over-simplistic modelling of fisheries has resulted in the collapse of key stocks.

Recruitment (biology)

In biology, especially marine biology, recruitment occurs when a juvenile organism joins a population, whether by birth or immigration, usually at a stage whereby the organisms are settled and able to be detected by an observer.There are two types of recruitment: closed and open.In the study of fisheries, recruitment is "the number of fish surviving to enter the fishery or to some life history stage such as settlement or maturity".

Stock assessment

Stock assessments provide fisheries managers with the information that is used in the regulation of a fish stock. Biological and fisheries data are collected in a stock assessment.

A wide array of biological data may be collected for an assessment. These include details on the age structure of the stock, age at first spawning, fecundity, ratio of males to females in the stock, natural mortality (M), fishing mortality (F), growth rate of the fish, spawning behavior, critical habitats, migratory habits, food preferences, and an estimate of either the total population or total biomass of the stock.

The following data regarding fisheries activities is collected: the kinds of fisherman in the fishery, commercial versus recreational, and the gear that is used (longline, rod and reel, nets, etc.), pounds of fish caught by each type of fisherman, the fishing effort each kind of fisherman expends, the age structure of the fish harvested by each group of fisherman, the ratio of males to females that are captured, how the fish are marketed, the value of the fish to the different fisherman groups, and the time and geographic location of the best catches. Also in the assessment, geographical boundaries of different stocks or populations are defined. From the combined biological and fisheries data, the current status and condition of the stock is defined and managers use this assessment to predict how in the future, stocks will respond to varying levels of fishing pressure. Ultimately managers want to reduce the level of overfishing that occurs and restore stocks that have been overfished.

Sustainable yield

The sustainable yield of natural capital is the ecological yield that can be extracted without reducing the base of capital itself, i.e. the surplus required to maintain ecosystem services at the same or increasing level over time. This yield usually varies over time with the needs of the ecosystem to maintain itself, e.g. a forest that has recently suffered a blight or flooding or fire will require more of its own ecological yield to sustain and re-establish a mature forest. While doing so, the sustainable yield may be much less.

In forestry terms it is the largest amount of harvest activity that can occur without degrading the productivity of the stock.

This concept is important in fishery management, in which sustainable yield is defined as the number of fish that can be extracted without reducing the base of fish stock, and the maximum sustainable yield is defined as the amount of fish that can be extracted under given environmental conditions. In fisheries, the basic natural capital or virgin population, must decrease with extraction. At the same time productivity increases. Hence, sustainable yield would be within the range in which the natural capital together with its production are able to provide satisfactory yield. It may be very difficult to quantify sustainable yield, because every dynamic ecological conditions and other factors not related to harvesting induce changes and fluctuations in both, the natural capital and its productivity.

In the case of groundwater there is a safe yield of water extraction per unit time, beyond which the aquifer risks the state of overdrafting or even depletion.

Sustainable yield in fisheries

The sustainable yield of natural capital is the ecological yield that can be extracted without reducing the base of capital itself, i.e. the surplus required to maintain ecosystem services at the same or increasing level over time. This yield usually varies over time with the needs of the ecosystem to maintain itself, e.g. a forest that has recently suffered a blight or flooding or fire will require more of its own ecological yield to sustain and re-establish a mature forest. While doing so, the sustainable yield may be much less.

In fisheries, the basic natural capital, or virgin population, must decrease with extraction. At the same time productivity increases. Hence, sustainable yield would be within the range in which the natural capital together with its production are able to provide satisfactory yield. It may be very difficult to quantify sustainable yield, because dynamic ecological conditions and other factors not related to harvesting induce changes and fluctuations in both the natural capital and its productivity.

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