Abstract
Monitoring of fish passage at hydropower plants largely relies on stow-fyke-net captures installed downstream of turbine outlets, yet little is known about which fish behavior contributes to reduced catch efficiency. We studied fish-net interactions as well as biological and physical factors potentially influencing behavior in three experiments: (i) fall-through experiment, to measure the general physical ability of a fish to fit through a certain mesh size; (ii) net-perception experiment, where fish were filmed while being exposed to different mesh sizes, flow and lure conditions in a controlled arena setup; and (iii) stow-fyke-net experiment, where fish behavior was recorded using 20 cameras simultaneously inside a stow net during regular hydropower fish monitoring. In total, we analyzed 382 h of video recordings. The material revealed that fish interacted with the net on a high rate, independent of flow conditions, and tried to swim through the mesh regardless of whether their body fits through. Under field conditions, the fish showed three specific behavioral patterns, “sneaking,” “dwelling” and “commuting,” which led to a reduced recapture rate in the catch unit of the stow-fyke net. This study highlights the importance of considering fish behavior in future fish monitoring programs to improve the accuracy of turbine-effect assessments on fish.
Highlights
In light of the controversy about the pros and cons of hydropower, a variety of monitoring programs have been initiated to examine the effects of conventional and innovative hydropower technologies on fish passage
Findings of this study confirm that fish of a size between 3 and 23 cm swim through the meshes of standardized stow nets to either enter or leave the net during regular the meshes of standardized stow nets to either enter or leave the net during regular fish fish monitoring of hydropower plants
Our experiment suggests an influence of daytime with an increase in the catch efficiency from morning hours to noon and evening, which may be explained by the diurnal activity patterns of fish that are well-known from other studies on fish passage [1]
Summary
In light of the controversy about the pros and cons of hydropower, a variety of monitoring programs have been initiated to examine the effects of conventional and innovative hydropower technologies on fish passage. Most scientific studies on catch efficiency focus on commercial fish catching methods and include analyses of mesh size selectivity of cod ends in trawl gear, and in gillnets. Individual fish were observed escaping and entering through the larger meshes of the net (“sneaker fish”) or dwelling at a certain spot of the net that is not the catch unit (“dwellers”) This behavior remained unconsidered in fish-monitoring practices at hydropower plants to date. The opposite, an underestimation of turbine effects in the total catch, can occur in the case of healthy fish entering the net from outside These examples illustrate the importance of understanding fish behavior in stow nets and its role in catch efficiency and turbine related fish injury estimations. It was hypothesized that: (i) brown trout interact with the net on a voluntarily basis by trying to swim through; (ii) larger brown trout differ in their behavior from smaller brown trout corresponding to their greater ability for sustained and burst swimming; and (iii) catch efficiency is reduced when individuals show specific behavioral patterns, which prevent the fish from getting trapped in the fyke net (e.g., sneaking)
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