Factors influencing stranding of wild juvenile brown trout (<i>Salmo trutta</i>) during rapid and frequent flow decreases in an artificial stream

Abstract

Abstract To provide environmental guidelines for operation of peaking hydropower plants, stranding experiments with juvenile brown trout ( Salmo trutta ) were conducted in a 3.8 m wide and 19.2 m long artificial stream. We found a significant decrease in stranding of trout fry by reducing the dewatering speed from &gt;60 cm h −1 to &lt;10 cm h −1 . At water temperatures around 11°C we found that less than 8% of the trout stranded during night experiments (on average for all subsets at dewatering rate &gt;60 cm h −1 ). However, the highest stranding rate occurred at water temperatures around 7°C during rapid dewatering at night (mean=22% stranding). Significantly more fry were stranded in the first versus the second to fifth dewatering episode (mean=22% versus 10% stranding) at rapid daytime dewatering in 11°C water. Stranding of larger fry (&gt;0+) was negligible at water temperatures around 11°C, except at rapid dewatering during daylight. Further, twice as many 0+trout as larger fry (&gt;70 mm in June, &gt;60 mm in late summer) stranded during rapid dewatering during daytime, and almost three times as many 0+stranded during rapid dewatering when they were mixed with trout parr compared to homogeneous 0+experiments. Our study showed a tendency towards an increased stranding of fry with long habituation time (&lt;30 hours versus &gt;30 hours with stable flow). We recommend dewatering in darkness at all times of year to reduce stranding of salmonids, and to use slow ramping rates &lt;10 cm h −1 . After longer periods with stable flows, a gentle drop in discharge is recommended, which might also reduce stress and possible sub‐lethal effects. Copyright © 2003 John Wiley &amp; Sons, Ltd.