Linking channel geomorphic characteristics to spatial patterns of spawning activity and discharge use by Atlantic salmon (Salmo salar L.)

This paper relates spatial patterns of spawning by Atlantic salmon (Salmo salar L.) to channel geomorphic and hydrologic characteristics in two upland Scottish streams. The full length of each stream was surveyed and classified using a process-based typology. The precise locations of 845 spawning events were mapped over 3 successive years. These locations were classified by reach type, and a location-specific discharge was calculated for each spawning event. Fish used ‘response’ reach types (pool-riffle and transitional pool-riffle/plane bed) preferentially, avoiding ‘transport’ reach types (plane bed and step pool). The spatial distribution of reach types contrasted markedly between the streams, and as a consequence, the distribution of spawning activity differed. In each stream, fish utilised relatively high flows for spawning with almost all spawning occurring at discharges greater than location-specific median annual flows, while optimum discharges (defined as those where values of use most exceeded values of availability) were as high as two to three times greater than reach median flows. In each stream, spawning occurred over a greater flow range in response than transport reach types, primarily as a result of higher maximum utilised flows in the former. Because the channel classification procedure is process based (and therefore transferable to other geographic areas), the present study suggests that reach type may be used to predict the spatial distribution of Atlantic salmon spawning activity within a catchment. Since the data indicate that discharge use by spawning fish is reach-type specific, the results also suggest that channel morphology should be considered when identifying environmental flow requirements.