Tripping over spatial scales: a comment on Guay et al. (2000)

Guay et al. (2000.Can.J.Fish.Aquat.Sci.57:2065-2075) report a test of numer ical habitat models (NHMs) that combine two-dimensional hydrodynamic modeli ng of the depth and velocity fields of a stream reach with simple biologica l models. The hydrodynamic model was tested with field data from randomly c hosen points in the stream, and the overall models were tested with observa tions of fish in a "verification" section of the stream. This is a proper p rocedure, but the execution and interpretation of the test seem flawed, lar gely by a misunderstanding of the spatial scales appropriate. The NHMs used by Guay et al. make predictions for patches or "tiles" at a scale of 1-25 m(2); the hydrodynamic component of the NHMs needs to be tested at the same spatial scale.