You can use a program called Stella (which costs money) but there is also a free program that does nearly the same thing called Vensim. These software packages allow you to build simple dynamic models using a GUI interface.
The materials (including salmon run scenario model files) for Andy Ford's "Modeling the Environment" class, which he taught for many years, are here:
with the "Smolts Migration Model" and "Salmon Harvest Simulator" prebuilt.
His text can be obtained online, the title is
"Modeling the Environment: An Introduction To System Dynamics Modeling Of Environmental Systems"
Andrew Ford is Professor of Environmental Science and Regional Planning at Washington State University. He teaches modeling with an emphasis on environmental problems in the west, and he is the author of the Island Press text on Modeling the Environment. He uses the system dynamics approach to modeling, and he is the recipient of the Jay W. Forrester Award for the outstanding contribution to the field of system dynamics.