We present a mathematical model of a fishery on several sites with a variable price. The
model takes into account the evolution during the time of the resource, fishes and boats
movements between the different sites, fishing effort and price that varies with respect
to supply and demand. We suppose that boats and fishes movements as well as prices
variations occur at a fast time scale. We use methods of aggregation of variables in order
to reduce the number of variables and we derive a reduced model governing two global
variables, respectively the biomass of the resource and the fishing effort of the whole
fishery. We look for the existence of equilibria of the aggregated model. We show that the
aggregated model can have 1, 2 or 3 non trivial equilibria. We show that a variation of
the total number of sites can induce a switch from over-exploitation to sustainable
fisheries.