Résumé
The objective of this paper is to quantify the impact of human activitieson the abundance of fresh water fish populations. In the case of populationsof salmon juvenile on the Scorff river (Brittany, France), two punctualpollutions are caused by the discharges of two fish farms. Two perturbedsections have been identified on the main course of the river where salmonjuvenile abundance is reduced. Assuming the pollution does not involve anychange of density of juveniles salmons in the unperturbed sections,geostatistical simulations have been used to quantify the impact of the fishfarms on the abundance of juvenile salmon. To this end, spatial structures ofdensities are analysed and modelled for two different cases : one using allthe data to describe the "actual structure"; one using only the unperturbeddata to estimate the "possible structure" of juvenile salmon in the absence ofpollution. In each case, the structural model is used to simulate what couldbe the distribution of juveniles in areas perturbed by the fish farms. Thedifference between simulations conditioned by all the data or only by theunperturbed data, allows us to quantify the impact of the fish farms in termsof numbers of juveniles lost.