Abstract
Structures for guiding fish around migration barriers are frequently used for maintaining connectivity in regulated riverine systems. However, for non-physical barriers, experimental studies providing direct and detailed observations of fish–barrier interactions in rivers are largely lacking. In this study, we quantify the efficiency of bubble barriers (alone or in combination with light stimuli, and in both daylight and darkness) for diverting downstream migrating Atlantic salmon (Salmo salar). Both a laboratory-based migration experiment and a large-scale field experiment in a regulated river were used to evaluate efficiency of bubble barriers. In the latter, we used acoustic telemetry to provide in situ measurements of how downstream migrating Atlantic salmon smolts interact with bubble barriers. We show that bubbles divert smolts with high efficiency in both a laboratory flume (95%) and in natural settings (90%). This latter efficiency is higher compared to an already present physical barrier (46%) covering the upper two meters of the water column in the large river. The bubble barrier did not affect flume migration in darkness, suggesting that visual cues are crucial for the observed repelling effect of bubbles. We conclude that bubble barriers can be effective, largely maintenance free and low-cost alternatives to physical structures currently used to divert salmon away from high-mortality passages.
Highlights
Over half of the world’s large river systems are regulated by dams (Nilsson et al, 2005)
Atlantic salmon smolts are known to primarily use the surface layers during downstream migration (Thorstad et al, 2012) and physical barriers covering only the upper meters of the water column are often used to overcome the hydraulic forces encountered in rivers (Calles et al, 2013; Scruton et al, 2003)
Our results seem to partly contrast this and those of Welton et al (2002) who showed that a barrier using both bubbles and sound was more effective in guiding Atlantic salmon smolt during night compared to daytime
Summary
Over half of the world’s large river systems are regulated by dams (Nilsson et al, 2005). Atlantic salmon smolts are known to primarily use the surface layers during downstream migration (Thorstad et al, 2012) and physical barriers covering only the upper meters of the water column are often used to overcome the hydraulic forces encountered in rivers (Calles et al, 2013; Scruton et al, 2003) The drawback of this approach, besides the large work effort needed to remove accumulated debris, is the inability of this structure to steer deeper migrants, which a migration mode that Atlantic salmon smolts can adopt (Davidsen et al, 2005; Hvidsten and Johnsen, 1997; Svendsen et al, 2007). Studies that have been conducted in full scale have used indirect approaches based on downstream fish detections (Welton et al, 2002; Zielinski and Sorensen, 2015) and catches further up- or downstream from the barrier (Ruebush, 2011), but see Perry et al (2014) for studies on Chinook salmon (Oncorhynchus tshawytscha)
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