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For example, individual fish were observed escaping but also 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”). These provide evidence that fish display a diverse set of behaviors that lead to them not being caught. While selection models only allow conclusions on the probability of fish swimming through the net mesh, different kinds of fish behavior have been observed during several stow-net experiments (e.g., ).
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The fish are thought to be disorientated after turbine passage and get quickly carried away by the current to net zones with smaller mesh sizes where they no longer fit through. However, the risk of fish swimming through the meshes is typically ignored in studies on fish passage monitoring. Hence, one would assume that fish would more likely escape or enter the net in the guiding unit, which is characterized by visible fiber and larger mesh sizes compared to the catch unit and thus easier to access by the fish. The largest mesh sizes are located at the entrance, i.e., the front of the stow net and then gradually become smaller towards the tail, with the fyke net having the smallest mesh size. Hence, the net is separated into different sections with different mesh sizes. Although small mesh sizes provide a higher catch efficiency, they increase the risk of net damage during high loads of debris or under unfavorable hydraulic conditions. It is important to differentiate between the selectivity of the guiding unit and the actual catch unit in a stow-fyke net. Understanding the reasons of lower than expected catch rates in relation to fish behavior is an important prerequisite in interpreting data from such monitoring, yet remains largely unconsidered. More specifically, catch efficiency of the species Salmo trutta was 55.2% after 1 h and 26.2% after 12 h exposure time. This study revealed that some stow-fyke nets had a catch efficiency of only 73%. studied catch efficiency and fish damage in stow nets combined with different catch units. Both fish behavior as well as net performance are most likely influenced by size, shape and material of the catch device, the amount and composition of floating debris, fish biomass, fish species and size, as well as exposure time. Additionally, fish behavior may play a major role in catch efficiency. Besides the extreme hydraulic conditions at turbine outlets, which challenge the technical installation of stow-fyke nets, the catch efficiency of those can be highly dependent on unique onsite conditions, which determine the technical constraints for installation of the net. Information on catch efficiency in stow-fyke nets used for hydropower monitoring are scarce. This study highlights the importance of considering fish behavior in future fish monitoring programs to improve the accuracy of turbine-effect assessments on fish. 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. 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. In total, we analyzed 382 h of video recordings. 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. 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.