Economic variables are often included in the biological model that suggests the relationship between sustainable revenue, fishing cost and fishing effort. For more than 50 years, it has been generally accepted that the fishing sector stood to gain from managing fisheries at the effort level producing maximum economic yield rather than maximum sustainable yield, which occurs at a higher effort level. Surplus production models are also called as Holistic models. This model deals with total stock biomass along with fishing effort and yield. To operate this model, catch and effort data are needed as input data. These models could be computed with less input data unlike analytic models. This model does not take into account age and growth. Hence, it could be safely applied to tropical stocks, where calculation of age of tropical fish is more cumbersome.

1. 1
NOAKHALI SCIENCE AND TECHNOLOGY UNIVERSITY
DEPARTMENT OF FISHERIES AND MARINE SCIENCE
Assignment On:
Separation Surplus yield model (MSY, Fmsy, MEY, Fmsy).
Course Tittle: Fish Population Dynamics and Stock Assessment
Course Code: FIMS-3211
YEAR-3, TERM-2
Session: 2017-2018
Prepared by
Degonto Islam
Department of fisheries and marine science
Noakhali science and Technology University

2. 2
Index
Surplus Yield Model ( MSY, Fmsy, MEY, Fmey )
Serial Content Page Number
01 Introduction 03
02 Surplus yield model 03
03 MSY 03
04 Why MSY is a target? 03
05 Fmsy 04
06 Fmey 04
07 Surplus Production Model 04
08 How can overfishing be stopped using surplus model 05
09 Advantages 05
10 Disadvantages 05
11 Conclusion 05
12 Reference 05

3. 3
Introduction
Economic variables are often included in the biological model that suggests the relationship between
sustainable revenue, fishing cost and fishing effort.
For more than 50 years, it has been generally accepted that the fishing sector stood to gain from
managing fisheries at the effort level producing maximum economic yield rather than maximum
sustainable yield, which occurs at a higher effort level.
Surplus production models are also called as Holistic models. This model deals with total stock
biomass along with fishing effort and yield. To operate this model, catch and effort data are needed
as input data. These models could be computed with less input data unlike analytic models. This
model does not take into account age and growth. Hence, it could be safely applied to tropical stocks,
where calculation of age of tropical fish is more cumbersome.
Surplus Yield Models are based on – “estimates of biomass”,- so are also known as biomass
dynamic models. Surplus production models are used to assess the biomass and exploitation level of
marine populations in data limited situations where age and size information are unavailable. Data
which are required in these program.
 Time series of index of relative abundance
o Survey index
o Catch per unit of effort from the commercial fisheries
 Time series of catch
MSY (Maximum Sustainable Yield)
The concept of MSY is 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, 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.
Why MEY as a target?
Maximum Economic yield (MEY), is the value of largest positive difference between total revenues
from and total costs of fishing with all input valued at their opportunity costs when relating total
resources from fishing to total fishing effort in surplus production model. It is also called as Economic
Optimal Catch.
• Maximizes fishery profits, regardless of changes in the price of fish or cost of fishing (MSY, by
contrast, is independent of profits).
 Improves international competitiveness.

4. 4
 Provides resilience to economic shocks.
• Prevents over-capitalization, falling incomes/asset values and expensive adjustment programs.
In most cases stock size is larger than that associated with MSY, generating a ‘win-win’ outcome.
Provides added resilience and other environmental benefits Proper resource allocation (when
combined with the right instrument, e.g., ITQs).
MSY, BMSY and FMSY, as well as MEY, BMEY and, FMEY are reference points which are expected to
remain fixed unless the environment changes or better data become available. Conversely B, Y and
F may change every year, and in the EU context are also corrected retroactively (backwards in time)
by the International Council for the Exploration of the Seas (ICES).
FMSY
FMSY is the effort level corresponding to MSY. FMSY refers to fishing mortality rate at which the
average long term catches from the stock is the highest. In maximum Economic Yield, the catch at
this level will be less than open access catch on MSY by the economic rent to the industry i.e. surplus
production will be at its maximum.
FMEY
FMEY is the fishing mortality (the proportion of fish caught and removed by fishing) resulting in
MEY. FMEY is typically 10-20% smaller than FMSY.
Surplus Production Model:
Surplus Production Models are used to assess stock status and exploitation in data-limited areas
where reliable information on age and length structure and natural mortality are not available. They
are applied not only to stocks with available commercial catch data and some index of exploitation
biomass, such as catch per unit of effort (CPUE) derived from scientific surveys, but also to migratory
stocks and crustaceans that are difficult age. They assume that sustainable catch is a simple function
of population biomass, regardless of the size and age composition of that biomass.
The most widely used surplus production model is the one developed by Schaefer:
Bt+1 = Bt + rmaxBt(1-Bt/K) - Ct
Where Bt is the biomass of the stock at time t and t+1, rmax is the maximum intrinsic rate of population
increase, K is a parameter which corresponds to the unfinished equilibrium stock size or carrying
capacity, and C is the catch per unit of time (usually a year).
Surplus production or yield (Y) is calculated as:
Y = rmaxBt(1-Bt/K)
How can overfishing be stopped using surplus model
It is simple to stop overfishing, but not necessarily easy for a fish stock that is already at or above
BMSY, fishing pressure should not exceed FMSY. To stop overfishing of an overfished stock, the same
application

5. 5
Figure: Surplus model in relation between MSY, MEY, Fmey, Fmsy, Bmsy.
However, to bring the stock back to BMSY or above, in a defined timeframe, fishing pressure needs
to be reduced even further below FMSY. It is simple to do this as fishing pressure is a human
intervention and as such manageable.
Advantages:
The surplus yield model has been widely used in managing fisheries because it is based on only on-
“catch and effort data”, which are relatively easy to obtain. such as,
calculate MSY and Fopt without catchability
requires only catch and effort data
don’t need to know age structure
Disadvantages:
The main dis-advantages is that the model ignores the biological process (growth, recruitment and
mortality) affecting stock biomass.
does not incorporate environmental factors
excludes trophic linkages
assumes stock has stabilized at current rate of fishing
doesn’t tell us much about the mechanisms affecting the population dynamic
Conclusion
Economic variables and sustainable yields are often included in the biological model that suggests
the relationship between sustainable revenue, fishing cost and fishing effort in surplus model. When
the fishing rate is above FMSY, The biomass of the stock will then diminish. Overfishing can occur
whether a stock is above BMSY or not.

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Reference
William, W, Jr,. (09 Jan 2011), “An Exponential Surplus-Yield Model for Optimizing Exploited Fish
Populations”, tandfonline, retrieved from,
https://www.tandfonline.com/doi/abs/10.1577/15488659(1970)99%3C80%3AAESMFO%3E
2.0.CO%3B2?fbclid=IwAR1V6oX3pMR60MHhaplQ_9RaTN2Qq2og4gQ48UQIlTYw1lGY
ssOz7JNKx0&
E.K.S., (2 April),” Surplus production models”, unit 6, ecoursesonline, retrieved from,
http://ecoursesonline.iasri.res.in/course/view.php?id=586&fbclid=IwAR3q-
s0w9tKQ8QheAZtWHYd689fBmTwo869SbDNUyclY28He9As_pyWv9Yk
E.K.S., (2 April),” Maximum Sustainable Yield & Maximum Economic Yield”, Unit 6, chapter-2,
ecoursesonline, retrieved from,
http://ecoursesonline.iasri.res.in/course/view.php?id=586&fbclid=IwAR3q-
s0w9tKQ8QheAZtWHYd689fBmTwo869SbDNUyclY28He9As_pyWv9Yk