Maximum sustainable yield
At intermediate population densities, also represented by half the carrying capacity, individuals are able to breed to their maximum rate.
Above this point, density dependent factors increasingly limit breeding until the population reaches carrying capacity.
[2][3] This fraction differs among populations depending on the life history of the species and the age-specific selectivity of the fishing method.
The concept of MSY as a fisheries management strategy developed in Belmar, New Jersey, in the early 1930s.
The international MSY treaty that was eventually adopted in 1955 gave foreign fleets the right to fish off any coast.
[9] 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.
Even while the scientific community was beginning to question the appropriateness and effectiveness of MSY as a management goal,[10][11] 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.
[8] According to Walters and Maguire, an ‘‘institutional juggernaut had been set in motion’’, climaxing in the early 1990s with the collapse of northern cod.
[12] 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.
Equation 1.2 is the usual way in which logistic growth is represented mathematically and has several important features.
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.
With logistic growth, this point, called the maximum sustainable yield, is where the population size is half the carrying capacity (or
An important feature of the MSY model is how harvested populations respond to environmental fluctuations or illegal offtake.
A small decrease in the population can lead to a positive feedback loop and extinction if the harvesting regime (
[14][15] 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.
In such cases, cyclic harvest is optimal where the yield and resource fluctuate in size, through time.
Instead, the population will either stabilize at a new lower equilibrium size or, if the harvesting rate is too high, decline to zero.
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.
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.
[22] 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.
Estimation problems arise due to poor assumptions in some models and lack of reliability of the data.
[25] 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.
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.
[27] For a more contemporary description of MSY and its calculation see [28] 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).
[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.
