% PROGRAM salmon_dd
%   Uses parameters from Ratner et al. (Conservation Biology 
%   11:879-889 (1997)) to perform a purely determinstic, but 
%   density-dependent, simulation of salmon population growth

n0=[0.7667; 0.6163; 0.4945; 0.3524; 0.1309]; % initial pop. vector
s0=0.002267;   % maximum egg survival
beta=0.001;    % density-dependent egg survival parameter    
s=[0.8 0.8 0.8 0.8];    % survival probabilities
b=[0 0 0.112 0.532 1];  % probability of breeding at each age
f=[0 0 3185 3940 4336]; % eggs per breeding female
tmax=100;       % time horizon
eggsurv=1;  	% Set eggsurv=1 for Ricker egg survival, 
				%    eggsurv=2 for Beverton-Holt egg survival

n=n0;           % Starting at initial pop. vector,
for t=1:tmax    % for each year
    eggs=(b.*f)*n;  % compute total eggs produced from Eq. 8.20,
    if eggsurv==1                     % compute surviving eggs
        n(1)=eggs*s0*exp(-beta*eggs); %    using Ricker or
    elseif eggsurv==2
        n(1)=(eggs*s0)/(1+beta*eggs); %    Beverton-Holt 
    end;                              %    function, and 
    n(2:5)=( s.*( 1-b(1:4) ) )'.*n(1:4); % update older age
                                         %   classes.  
    spawners(t)=b*n;  % Observed spawners=breeders of all ages
end;

plot(spawners)  % plot no. of spawners vs. year
xlabel('Year');
ylabel('Spawners');