Uncatchable

It's apparently a metaphor for something uncatchable, light of body and fleet of foot.

Sometimes there was only the width of the road between the imposing precipices on the right and the clear cool water on the left with its shoals of uncatchable fish skimming about through the bars of sun and shadow; and sometimes, in place of the precipices, the grassy land stretched away, in an apparently endless upward slant, and was dotted everywhere with snug little chalets, the peculiarly captivating cottage of Switzerland.