
The GordonSchaefer Model
The Gordon–Schaefer model is a bioeconomic comparative static fishery
model based on logistic biological growth, constant harvest price,
constant unit cost of effort, and harvest linear in stock biomass and
fishing effort. TR denotes total revenue, TC total cost, AR average revenue (TR/E), MR marginal revenue, and MC marginal cost. 

Surplus Production in Logistic Growth
The Verhulst growth function (red curve) consists of the logistic
growth function (lower blue curve) and its time derivative. 

QuasiRent in Open Access Fisheries
A phase plot (on the left) of an open access fishery dynamic system
includes biological dynamics (blue isocline) and economic dynamics (red
isocline). The first represents biological growth by a logistic growth
equation while growth in fishing effort is proportional to obtained
rent. Corresponding developments in effort and rent (rent adjustment,
labeled quasirent by Alfred Marshall) as functions of time are shown on
the right. The equilibrium solution gains no rent, while positive or
negative rent is obtained outside equilibrium. Positive and negative
quasirent are shown as functions of time in the lower panel on the
right as green or red areas, respectively. The discounted sum
constitutes the total quasirent presented. 

Cellular Automata Model of an MPA Fishery
This model uses a continuous cellular automata technique to model the
distribution of a fish stock along a coastline. The distribution of
fishing activities is related to previously obtained rent from the
fishery and initial fishing effort is controlled by a slider. Stock
biomass growth depends on local density; local stock collapses occur
when the stock biomass passes the environmental saturation level. All
cells (localities) have the same growth properties. Biomass as a
function of time is shown in the two panels on the left; the upper panel
shows biomass gradients distributed over time and area (coastline),
while the lower panel shows the total biomass as a function of time, the
dashed line indicating the average natural stock biomass level. Total
rent over the period is discounted and shown on the right as the
quasirent of an open access fishery, only restricted by economic
parameters and an MPA (marine protected area) regulation. The MPA
regulation is a fixed fraction of the total coast length, controlled by a
slider. The closed (protected) area is indicated by the red bar on the
right in the upperleft panel, while the open area is indicated by the
green bar. 

Bioeconomics of a Discrete Ricker Model with Delayed Recruitment
The discrete Ricker population model with delayed recruitment is given by for a fish stock population. denotes the stock biomass at time and is the intrinsic growth rate. An open access fishery is constrained by the biological properties of the stock, the cost/price ratio (cost of producing fishing mortality and unit price of harvest), and the rate of fleet entry/exit to the fishery. The entry/exit dynamics are linear to the profitability of the fishery, with increasing fishing mortality when economic rent (resource rent) is positive and decreasing fishing mortality when rent is negative. Stable equilibria are found as limit cycles or stationary equilibrium; chaotic behavior may also exist. While an increasing growth rate destabilizes the system, increasing fishing mortality tends to stabilize it. The natural equilibrium or stable focus (when fishing mortality is zero) is shown by the intersection of the two dashed lines. In the case of positive fishing mortality, corresponding equilibria are found southwest of the intersection between the dashed lines. 

Surplus Production Models and Equilibrium Harvest
Textbook bioeconomics (here considering fisheries) assumes logistic
population growth and short term harvest production to be linear in
stock biomass and fishing effort. The first assumption is expressed by
the growth equation F(X)=rX(1X/K) (red curve), X being the stock biomass and the parameters r and K the intrinsic growth rate and the
environmental saturation level, respectively. F(X) is the per period of time surplus production.
X=K is the natural equilibrium in the absence of
fishing. The second assumption is the bilinear harvest equation h(E,X)=qEX (green line), E being the fishing effort, while the parameter
q is known as the catchability coefficient. The
fishing activity disturbs the natural equilibrium (X=K) and each level of fishing effort includes
new equilibria determined by F(X)=h(E,X). This Demonstration illustrates how the
collection of all existing equilibria found by varying values of E describes the equilibrium catch h(E), (blue curve). Depensation (decrease in
marginal growth by decreasing population biomass) may give rise to two
equilibrium points, one stable and the other unstable. The dashed blue
curve indicates where unstable equilibrium points are found. Positive
critical depensation levels are found when depensation causes negative
biomass growth. The term depensation refers to a situation where decline
in biomass is not compensated by increased per unit of biomass
production, as in the logistic growth equation, where F(X)/X=r(1X/K) is a downsloping line for . 

NonRenewable Resource Economics
The
capitaltheoretic approach to nonrenewable resource economics—as
described by Harold Hotelling—is to exhaust the resource over time while
maximizing the present value of the resource (Hotelling's rule). This
Demonstration illustrates the case of producer market power, indicated
by the blue demand curve declining by increasing quantity (note that the
quantity axis points to the left).
The total resource stock is represented by the
shaded area in the lowerleft panel enveloped by the green curve, which
is the extraction rate. The shadowed area is a finite area, limited by
the extraction start (initial extraction value given by the green curve
at time zero) and the point of completed stock extraction (the green
curve intersects the time axis where the extraction quantity reach
zero). Depending on the chosen values, all parts of the finite area
defined by the green curve may not be visible within the range of the
axes.
The red curve is
the optimal price of the resource owner according to the Hotelling’s
rule, , where is the unit price of the resource and is the resource owner’s discount rate. In
this Demonstration there are no extraction costs.
The shape of the green curve is
determined by the stock size and discount rate while the red curve is
determined by the discount rate and the demand for the extracted
resource. The blue demand curve is in principle given exogenously,
influencing the optimal exploitation path of the resource owner.
Drag the locator connecting the
three curves to study the relation between them. The interconnection
between the red, green and blue curves represents a point in time of
optimal exploitation according to the given demand, stock size, and
discount rate. The shaded area below the dashed line representing the
point in time of optimal extraction is the remaining stock size at that
very point in time. The capitaltheoretic conclusion is that the optimal
extraction path is found when the marginal growth in the value of the
resource at every point in time equals the discount rate, as stated in
the Hotelling's rule.


Offspring of Adam and Eve
This Demonstration
starts with two people (Adam and Eve) at year 0. After some years they
get their first child. Each year after that, they may get a new child or
not, depending on the predefined number of children.
Observe the effect of using birth
control (reducing the number of children) compared with increasing the
age of first birth. You may be surprised!


Beverton and Holt's Yield per Recruit Model
In 1957 R. J. H. Beverton and S. J. Holt published their seminal work on
the dynamics of exploited fish populations. The work includes
individual growth models of von Bertalanffy, mortality models of
Baranov, and a number of other methods to describe population dynamics.
The most famous model presented in the work of Beverton and Holt is the
simple yield per recruit model, presenting the sustainable yield of a
fish population as a function of age of first catch tc, assuming knifesharp selection and rate of
fishing mortality F. The weightlengthrelation b is assumed to be close to 3 (cubic relation)
and the natural mortality rate M is in most cases close to the natural
individual growth rate k. 

Origami Bases
The art of origami involves some basic forms referred to as origami
bases. The origami bases are certain sets of creases usually named after
the most common final figure. This Demonstration presents 13 of the
most common origami bases and how they should be folded. 

Optimal Queue of Cars
This Demonstration considers the problem of finding the optimal speed in
a queue of cars. It allows a sufficient distance between cars that
depends on the road quality (due to friction) and the reaction time of
the drivers and cars. Short, medium, or long cars carry 1, 1.5, or 2
people per meter of car. 

A Nonlinear StageStructured Cannibalism Model
A cannibalism model published in [1] is described
by a system of difference equations:
,
where and are the immature and mature parts of the
population at time , respectively, (density independent) is the fecundity
(number of newborns per adult), and is the fraction (density independent) of the
immature population that survives and enters the mature stage one unit
time later (). The parameters and may be interpreted as the natural death rates
(not the fishing mortality) caused by factors other than cannibalism.
The nonlinear interactions in the model are of the Ricker type and the
corresponding parameters will be referred to as the cannibalism
parameters. The model has a large parameter region with stable
equilibrium points. It may transition from stable to unstable through a
(supercritical) Hopf bifurcation, which means that beyond the
instability threshold the dynamics are a quasiperiodic orbit restricted
to an invariant curve that surrounds the unstable equilibrium and ).


Continuous and Discrete Time Discounting
The wellknown concept of discounting may be implemented as a discrete
or continuous process in time, the first representing the common
approach in financial institutions. The discrete time discounting term
is , where is the discount rate and is the time variable. The expression may be
regarded as the present value of one unit of value at time . For , the expression decreases over time. The
corresponding continuous time expression is . Note that . The integral is shown as the PV (red) area
(the present value of receiving one unit of value each unit of time
eternally), while the PV (blue) area represents the sum. You can see the
discrete time discounting as the light blue bars and/or a connecting
blue line. 

Word Memory Game
Word Memory is a
simple game where the aim is to find a fiveletter hidden word. For each
try the player gets to know how many of the proposed letters are in the
hidden word. If for example the hidden word is "faced" and your
suggestion is "place", it gives 3 points (for the letters "a", "c" and
"e"), independent of the position of letters. Only valid words (found in
the dictionary) are accepted and a word may only be used once. If the
same letter is found twice in the word (as the "d" in "dread") and you
suggest another word with that letter also repeated, you receive two
points.
In the game
you challenge your computer and start the game by deciding the hidden
word for the computer, before suggesting your own words. A table on the
righthand side lists all suggested words of the game, the red being
your proposals and the blue the computer's. Average points of each
player are displayed below the table.


Ecosystem Dynamics
Consider an
ecosystem consisting of three trophic levels, 1 being the lowest and 3
the top predator level. Let the system be described by a set of
differential equations, each representing the biomass dynamics of one of
the three levels. The model is within the basic framework introduced by
May and coworkers [1]; represents biomasses of the trophic level.
,
,
.
A
biologically consistent system is obtained with nonnegative parameter
values. The corner solution reduces the system to just the lowest trophic
level (1), while and , , , and positive defines a system of three distinct
levels, where levels 1 and 3 only interact through level 2.
Initial biomass levels are
indicated in the graph by a red point, while corresponding terminal
values are found at the green point. The connecting blue curve is the
time path of the biomass development within the three trophic levels
between these two points. The three bars give a graphical representation
of the terminal biomasses of the three levels.


Guitar Chord Maker
Guitar chords are sets of tones created by pressing
the strings against the frets on the guitar fingerboard. This
Demonstration identifies different possible finger positions that create
different kind of chords. A large variety of different chords are found
in the advanced version.
Guitars
may be tuned in different ways; this is determined here by the base
tune of the sixth (thickest) string and the intervals between the
strings.

Other demonstrations not yet published by Wolfram Research (flash previews not available):
webMathematica
webMathematica adds computations and visualisation to web sites. This are examples hosted by Norwegian College of Fishery Science, University of Tromsø:
