Variability in stream discharge and temperature: a preliminary assessment of the implications for juvenile and spawning Atlantic salmon

摘要

This study focuses on understanding the temporal variability in hydrologicaland thermal conditions in a small mountain stream and its potentialimplication for two life stages of Atlantic salmon (Salmo salar) – stream residentjuveniles and returning adult spawners. Stream discharge and temperature inthe Girnock Burn, NE Scotland, were characterised over ten hydrologicalyears (1994/1995–2003/2004). Attention was focussed on assessing variationsduring particular ecologically "sensitive" time periods when selectedlife-stages of salmon behaviour may be especially influenced by hydrologicaland thermal conditions.

Empirical discharge data were used to derive hydraulic parameters to predictthe Critical Displacement Velocity (CDV) of juvenile salmon. This is thevelocity above which fish may no longer be able to hold station in the watercolumn and thus can be used as an index of time periods where feedingbehaviour might be constrained. In the Girnock Burn, strong inter- andintra-annual variability in hydrological and thermal conditions may haveimportant implications for feeding opportunities for juvenile fish; bothduring important growth periods in late winter and early spring, and theemergence of fry in the late spring. Time periods when foraging behaviour ofjuvenile salmon may be constrained by hydraulic conditions were assessed asthe percentage time when CDV for 0+ and 1+ fish were exceeded by mean dailystream velocities. Clear seasonal patterns of CDV were apparent, with highersummer values driven by higher stream temperatures and fish length.Inter-annual variability in the time when mean stream velocity exceeded CDVfor 0+ fish ranged between 29.3% (1997/1998) and 44.7% (2000/2001). For1+ fish mean stream velocity exceeded CDV between 14.5% (1997/1998) and30.7% (2000/2001) of the time.

The movement of adult spawners into the Girnock Burn in preparation forautumn spawning (late October to mid-November) exhibited a complexrelationship with hydrological variability with marked inter-annualcontrasts. In years when discharge in the period prior to spawning was low,fish movement was increasingly triggered by suboptimal flow increases asspawning time approached. In contrast, wet years with numerous eventsallowed a much more even distribution of fish entry. Elucidating linksbetween discharge/temperature variability and foraging opportunities andupriver migration of adult Atlantic salmon have the potential to contributeto the improvement of conservation strategies in both regulated andunregulated rivers.