Development and validation of numerical habitat models for juveniles of Atlantic salmon (Salmo salar)

We evaluated the ability of numerical habitat models (NHM) to predict the d istribution of juveniles of Atlantic salmon (Salmo salar) in a river. NHMs comprise a hydrodynamic model (to predict water depth and current speed for any given flow) and a biological model (to predict habitat quality for fis h using water depth, current speed, and substrate composition). We implemen ted NHMs with a biological model based on (i) preference curves defined by the ratio of the use to the availability of physical conditions and (ii) a multivariate logistic regression that distinguished between the physical co nditions used and avoided by fish. Preference curves provided a habitat sui tability index (HSI) ranging from 0 to 1, and the logistic regression produ ced a habitat probabilistic index (HPI) representing the probability of obs erving a parr under given physical conditions. Pearson's correlation coeffi cients between HSI and local densities of parr ranged from 0.39 to 0.63 dep ending on flow. Corresponding values for HPI ranged from 0.81 to 0.98. We c oncluded that HPI may be a more powerful biological model than HSI for pred icting local variations in fish density, forecasting fish distribution patt erns, and performing summer habitat modelling for Atlantic salmon juveniles .