Assessing discharge use by spawning Atlantic salmon: A comparison of discharge electivity indices and PHABSIM simulations

Abstract

The locations used by spawning Atlantic salmon (Salmo salar L.) in a reach of the Girnock Burn, Scotland, were monitored over three successive years. Reach discharge was estimated for each spawning observation using a conversion factor applied to continuous flow data from a gauge located in the catchment. Data on the availability and use of different discharges were used to construct a discharge electivity index for the reach. The index was compared to output from a Physical HABitat SIMulation (PHABSIM) model of the reach.Spawning fish used relatively high discharges, with the highest electivity value being for a discharge approximately three times the reach median flow. The electivity index and the PHABSIM weighted usable area (WUA) versus discharge curve were similar across the low flow range and both suggested similar optimum discharges for spawning (1.1 and 1.4 m 3 s −1 respectively). However, electivity values suggested unsuitable conditions were reached at discharges greater than 1.4 m 3 s −1 whereas PHABSIM predicted relatively high WUA values at discharges up to 2 m 3 s −1 . Electivity indices provide an insight into discharge selection that is not dependent on hydraulic simulations or assumptions about microhabitat (depth, velocity, substrate) preferences. Moreover, they can be used to assess discharge suitability in hydraulically complex streams where the one-dimensional hydraulic models used by PHABSIM may be inappropriate. However, unlike PHABSIM, they cannot be used to predict the suitability of flows outside observed discharge ranges and so are limited in their application. Further work is required to refine the methodology and assess its transferability to other streams. Nonetheless, indices may represent a useful tool that can be used to complement other methods of assessing instream flow needs.

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

“…In the Girnock, 1+ fish grow most markedly in the MarchMay period, prior to the emergence of 0+ individuals, probably because of the high seasonal prey abundance and low basal metabolic costs associated with lower temperatures (Gibbins et al, 2002;Elliot and Hurley, 2003;Bacon et al, 2005). The later emergence of the 0+ fish relative to this same period may reflect the overall fitness benefits of avoiding emergence when growth potential is high but hydraulic conditions are typically unfavourable for foraging.…”

“…Langan et al, 2001;Gibbins et al, 2002;Malcolm et al, 2003;Moir et al, 2004;Hannah et al, 2004). The stream is characterised by high inter-and intra-annual variability in discharge dynamics and thermal regimes (Moir et al, 1998;Malcolm et al, 2002;Soulsby et al, 2005a); though the effects of these on different salmon life-stages are not well-understood.…”

“…Many Highland rivers are internationally important as spawning and rearing habitats for Atlantic salmon (Salmo salar): particularly the early "spring" running fish which are of great economic, cultural and conservation importance. Salmon is often cited as a key higher order species and an indicator of wider ecosystem health, thus it is commonly a target species for instream flow studies aimed at providing an ecological basis for river management (Gibbins et al, 2002;Webb et al, 2002;Gordon et al, 2004;Moir et al, 2005). A number of Scottish salmon rivers are used for hydropower production and have regulated flow regimes that are only crudely relevant to the environmental requirements of salmon; and adverse impacts of flow regulation on salmon populations have been reported (Gilvear, 1994).…”

“…Flodmark et al, 2004;Enders et al, 2005). In addition, discharge thresholds have been observed to trigger some life-stage specific activities, including upstream migration to spawning areas, spawning activity itself and the downstream migration of smolts (Youngson et al, 1983;Bunt et al, 1999;Gibbins et al, 2002). Likewise, temperature exerts a major influence on physiology reflected, for example, in the growth rate of stream-dwelling juveniles (e.g.…”

“…However, in the past decade a number of dry autumns have been noted to delay spawning entry to the stream and focus spawning in the lower reaches of the river. In such situations, uneven spawning distributions may result in inadequate use of juvenile habitat and compromise smolt production (Gibbins et al, 2002).…”