Fish stranding in relation to river bar morphology and baseflow magnitude: Combining field surveys and hydrodynamic–numerical modelling

Abstract

Hydropeaking mitigation based on down‐ramping thresholds is crucial to avoid stranding of fish and ensure sustainable hydropower production. However, the risk of stranding is influenced by several biotic and abiotic factors that interact with the ramping rate, including river morphology and baseflow magnitude. Also, at the river bar scale, the lateral or horizontal down‐ramping velocity may be of even greater importance than the vertical one as the prior integrates bar morphology. This study investigates the connection between ecologically relevant hydraulic parameters (vertical and lateral ramping velocity, dewatered river bar width) and the stranding of young‐of‐the‐year European grayling (Thymallus thymallus) due to down‐ramping, based on field observations of stranded fish at the hydropeaked Drava River in Austria combined with hydrodynamic–numerical modelling. We also examine the interplay between baseflow conditions and river topography in the dewatered areas. Stranding observations in spring and summer found a median rate of 90 individuals per 100 m, ranging from 50 to 500 fish per 100 m shoreline. Stranding rates were three times higher at night than during the day. Our data revealed a positive relationship between stranded fish and down‐ramping hydraulics, with higher intensity resulting in more stranded fish. This association was stronger for the lateral than the vertical ramping velocity, presumably because the former accounts for bar morphology, suggesting that the lateral dewatering velocity is a better indicator to assess fish stranding at the bar scale than the vertical velocity. We also found a negative relationship between the extent of the dewatered area and baseflow magnitude, indicating that the stranding risk is higher during down‐ramping at low flow conditions.

Characterizing the effects of morphological microstructures and hydropeaks on fish stranding in rivers

“…However, their disconnection and drying out, due to down-ramping, probably trap fish in these habitats. Hayes et al (2024) observed, at the start of the downramping, that fish began to seek out deeper sections, where they remained temporarily, before no longer being able to escape when the flow receded. At low flow range, this behavior may have a strong impact on trout fry, whose preference for shallow and slow-flowing habitats makes them use habitats along the bank that are subject to stranding; grayling fry may be less affected due to their preference for higher velocities, especially during the day (Sempeski & Gaudin, 1995).…”

“…Stranding may be linked to the season: when fish grow in size, they become less susceptible to being stranded (Führer et al, 2022;Hayes et al, 2024;Saltveit et al, 2001). The time of day may also be important: it has been shown that more trout fry became stranded in the day (Halleraker et al, 2003), that grayling fry tended to be more stranded at night (Hayes et al, 2024), and that cyprinid stranding preferentially occurred at night, regardless of the down-ramping rates (Führer et al, 2022). In our case, we sampled only during the day for all species.…”