Tripping over spatial scales: a comment on Guay et al. (2000)
Guay et al. (2000.Can.J.Fish.Aquat.Sci.57:20652075) report a test of numerical habitat models (NHMs) that combine two-dimensional hydrodynamic modeling of the depth and velocity fields of a stream reach with simple biological models. The hydrodynamic model was tested with field data from randomly chosen points in the stream, and the overall models were tested with observations of fish in a "verification" section of the stream. This is a proper procedure, but the execution and interpretation of the test seem flawed, largely 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 125 m2; the hydrodynamic component of the NHMs needs to be tested at the same spatial scale.