The effects of low summer flow on wild salmon (Salmo salar), trout (Salmo trutta) and grayling (Thymallus thymallus) in a small stream

Abstract

Summary 1. The effects of an experimentally imposed low summer flow on habitat use, displacement and survival of wild populations of juvenile salmon (Salmo salar), trout (Salmo trutta) and grayling (Thymallus thymallus) were investigated in a chalk stream. The habitat use and mobility of the fish in response to reduced flow was determined using passive integrated transponder‐tag detector systems. 2. Habitat use was compared to that available under different flow regimes. These consisted of an initial control phase of normal summer flow, an abrupt step change to 21 days of low flow, followed by a second control phase when normal summer flow was reinstated. First year (0+) salmon showed little change in their preferred substratum during low flow, whilst 1+ salmon increased their use of gravel and reduced that of mud during the day. Both 0+ and 1+ salmon were found in relatively deep water by day under low flow, whilst 1+ salmon preferred relatively shallow water at night. First year trout increased their use of gravel and reduced their use of mud and submerged tree roots under low flow, using relatively deep and fast flowing water by day. Older trout increased their use of gravel and occupied relatively deep, slow flowing water by day and relatively fast and shallow water at night. Grayling showed little change in their preferred substratum, but occupied relatively shallow water following the introduction of low flow. 3. The range of movement of juvenile salmon increased at night under low flow, and was greater by day under normal flow. There was also an initial increase in the range of movement of 0+ trout following the introduction of low flow. Older trout initially moved less under low flow. With reduced flow, the range of movement by grayling increased significantly during the day. 4. There was no net downstream displacement of any species with reduced flow, but the mortality rate in 0+ salmon, trout and grayling increased. This may be related to their small size and increased vulnerability to predation under low flow, and due to the reduction in depth and loss of the stream margins that are normally the preferred habitat of 0+ trout and grayling. 5. The findings of this field study have implications for the management of braided, and highly regulated, chalk stream systems. In particular, they underline the importance of the stream margins as juvenile salmonid habitat, and suggest that a flow management strategy is required to mitigate for drought conditions. Such a strategy might include pre‐emptive controls on abstraction and the maintenance of river flow via a prioritised route, predetermined using fish or habitat surveys, to minimise the effects of drought conditions on the more vulnerable or valued fish groups.

Diel activity and foraging mode of juvenile Arctic charr in fluctuating water flow

“…Rapid fluctuations in water flow can also affect temporal aspects of stream fishes behaviour, for example, small and large-scale movements, relocation, and habitat use (Korman & Campana, 2009;Krimmer, Paul, Hontela, & Rasmussen, 2011;Riley, Maxwell, Pawson, & Ives, 2009;Scruton et al, 2005;Taylor et al, 2014). Other studies have suggested that salmonids also alter their diel feeding activity in response to hydropeaking (e.g., Rocaspana, Aparicio, Vinyoles, & Palau, 2016).…”

The influence of changes to abiotic parameters on the fish assemblage structure of a lowland stream

“…Acreman et al (2008) recommend, even for the least ecologically sensitive of rivers, maximum abstractions in the range of 15-35% of the natural fl ow. Long-term critically low discharges are connected with adult fi sh migrations to deeper pools (Fischer & Kummer, 2000) and also increased mortality among juvenile fi sh, namely due to increased vulnerability to predation and the loss of stream margins as preferred habitat under conditions of low fl ow (Riley et al, 2009). These conditions substantially disadvantage more demanding fi shes such as salmonids, common nase Chondrostoma nasus (L.) and barbel compared with the more widespread and tolerant chub, gudgeon and roach, as documented in the comparison of Dyjákovice and Tasovice (Table II).…”

“…It is widely acknowledged that alterations to the fl ow regime impact riverine ecosystems (Halls & Welcomme, 2004;Acreman et al, 2008;Riley et al, 2009). To achieve sustainability, the water framework directive (WFD) covering hydrological regimes was established.…”

Habitat Requirements for Juvenile Salmonids in Chalk Streams: How will Management Best Address Conflicting Interests?

“…The reduced flow was, however, accompanied by a decrease in water temperature of approximately 4 • C. Since drought conditions are unlikely to be associated with lower water temperatures (except near springs), the preliminary findings of only the 2005 experiment are presented here (Riley et al 2009a). 0+ salmon utilised relatively deeper water during the day under low flow conditions (a pattern that disappeared on return to normal flow) but showed little change to their substrate preferences (gravel and aquatic weed).…”

“…The impact of a severe low-flow event during August, the warmest month of the year, was investigated in the summers of 2004 and 2005 (Riley et al 2009a). The aim was to reduce the flow to <33% of 'normal' summer flow in 2004 and to <10% in 2005.…”