No support for using brown trout as an indicator species for ecological impacts of low flow in Swedish rivers

Abstract

The impact of low flows on riverine ecology in Sweden is not fully understood. Recent summer droughts, along with their regionally projected increase, together demonstrate the need for a more solid foundation guiding ecologically responsible planning. Impact assessments can be made via indicator species sensitive to low flow, if their response to the flow conditions can be clearly understood, using for example historical data on species abundance and flow. In Swedish rivers, there are extensive electrofishing data with a special focus on salmonids, predominantly brown trout Salmo trutta, which are previously reported to be sensitive to low flow. There are also available national data on river flows, largely based on simulations. We processed and used these data sets, along with information on additional environmental factors. We tested if sites had less than their median trout abundance during the year of the minimum winter or summer low-flow. Adverse impacts of low flow could be shown only for young-of-the-year trout. The impact was small with at most 57 % of remaining sites having lower than median trout abundance (compared to 44 % overall) during the year of the lowest winter flow. The insubstantial effect means that using trout as a low-flow indicator species in Sweden cannot be supported by the currently available data. We believe the main causes of the small effects are limitations in the time resolution of trout data and spatial resolution of the flow data, followed by the ability of trout to escape low flows by seeking deeper habitats.

Keywords

• Ecological quality indicator
• Low flow
• Salmo trutta
• Salmonid
• Sweden
• Water framework directive