#=======================================================
# Figure 8.6.R.
# R script to draw a scatterplot of sustainable yield vs.
# harvesting effort for shrimp data from the Gulf of Mexico,
# with parabolic yield-effort curve superimposed.
#=======================================================

#---------------------------------------------------------
# 1.  Read and plot the data.
#  (a data file named shrimp_yield_effort_data.txt is assumed
#  to be in the working directory of R)
#---------------------------------------------------------
df=read.table("shrimp_yield_effort_data.txt",header=TRUE)
attach(df)
plot(effort,yield,type="p",xlab="harvest effort",ylab="sustainable yield")
detach(df)

#---------------------------------------------------------
# 2.  Calculate parabolic yield-effort curve and overlay it on
#     scatterplot.
#---------------------------------------------------------
elo=100            # Low value of effort for calculating quadratic.
ehi=350            # High value of effort.
eff=elo+(0:100)*(ehi-elo)/100  # Range of effort values
g=0.002866
h=1.342
sy=-g*eff^2+h*eff   # Sustainable yield calculated for range of
                    #  effort values.
points(eff,sy,type="l")  #  Add the quadratic model to the plot.