The third volume in a series of reports describing the methodology and results of a joint research venture between RAND and the Netherlands Rijkswaterstaat. The POLANO project was founded to help the Dutch government analyze the different alternatives (open, closed, and storm-surge barrier cases) for protecting the Oosterschelde region from North Sea floods. Volume III describes how the abundances of the Oosterschelde's different species would change in the long run with variations in the alternatives and certain assumptions; the variations included different apertures for the storm-surge barrier, different sizes for the salt-water basin, and different rates for fishing and detritus import. The report also discusses in detail the ecology model that was developed using mathematical concepts new to ecology. Results of an attempt to validate the model are given, using data from the Grevelingen, an estuary adjacent to and similar to the Oosterschelde.