Assessing the Transferability of Hydraulic Habitat Models for Atlantic Salmon Fry

Abstract:

Hydraulic habitat models, which are logistically and technically challenging and expensive to produce, are frequently transferred between rivers without validation. Although this is known to be associated with problems, few studies have assessed the potential consequences for model predictions. This study investigated the local (within sub-catchment) transfer of hydraulic habitat models developed for Atlantic salmon (Salmo salar) fry. Detailed 2D hydraulic models were developed for two adjacent reaches, each containing pool, riffle, glide and run habitats where salmon fry were stocked at uniform saturated densities. Substrate and cover were characterised using transects. Generalised Additive Models (GAM’s) were fitted to seasonal fry abundance data, with Froude number, dominant substrate and cover included as predictor variables. Despite attempts to select reaches with similar characteristics, the spatial distribution of Froude, dominant substrate and cover differed, with substrate and cover exhibiting the greatest inter-reach differences. Froude was the most important individual predictor of fry abundance, with the highest densities observed at moderate Froude across all seasons. When transferred between reaches, models which contained multiple predictor variables and their interactions transferred less well than models containing Froude alone potentially reflecting inter-reach differences in the distribution of substrate and cover. This study suggests that (1) habitat models should be developed at sites offering maximum environmental complexity at a local level (2) scientists and managers should avoid transferring models between locations with different environmental characteristics, especially in the absence of model validation (3) complex models should be avoided (4) the transferability of Froude only models should be further investigated, if predictions of habitat quality are to be made at new sites.