At the beginning of each simulation (a 1st of April) a virtual population was created with individuals from each life stages to represent a standard population from the Scorff river (Caudal and Prévost, 2008): 1.2 emerging juvenile/m2 of riffle and rapid equivalent area (RA), 1.32 parr of 1 year/100 m2 RA, 3.61 smolt running to sea/100 m2 RA, 3.61 grilse/1000 m2 RA (1 sea-winter anadromous adult) and 0.6 multiple sea winter individuals/1000 m2 RA. Each individual’s state variable was initialized by drawing from probability distributions describing their variability (see Appendix A). The first simulation step was then a summer.