Abstract Studies on active and sedated fish passing through turbines and pumps show different mortality and injury rates for both cases. Consequently, fish behavior appears to play a substantial role in these outcomes. However, direct behavioral observations in hydraulic machines using quantitative parameters to draw conclusions about the underlying mechanisms are hardly possible and remain understudied. In this study, we examined the behavior of adult brown trout ( Salmo trutta ) in an experimental flume under hydraulic conditions characterized by strong flow acceleration and high velocities typical of turbine and pump intakes. Fish movement behavior was analyzed based on a quantitative approach to enable the analysis of swimming behavior even in flow velocities exceeding the sprint swimming speed of fish. The application of Hidden Markov Models (HMM) to analyze activity states and movement modes of fish from video tracking data demonstrated significant effects of the spatial velocity gradient (SVG) and flow velocity on fish behavior. Notably, SVG emerged as the primary trigger for avoidance reactions when exceeding a threshold of . Fish exhibited distinct movement patterns under dark and daylight conditions, with more avoidance reactions in darkness. Whereas a considerable proportion of fish in daylight increased their swimming activity in the zone were flow velocity exceeded sprint swimming speed, in dark conditions no activity peak occurred in the same zone. The results illustrate how hydraulic conditions and lighting influence fish behavior. Integrating the behavioral rules identified in this study into numerical mortality-risk models could substantially improve their predictive accuracy. Thus, the findings allow for the development of less fish harming engineering solutions for hydropower facilities and pumping stations.

Riverine barriers are threatening freshwater fish migration, with major impacts on fish populations. Effective management requires understanding of fish movement and behaviour as they approach a barrier and fish pass, which can inform optimal mitigation options and barrier management. Here, the movements of upstream migrating barbel Barbus barbus and grayling Thymallus thymallus near a barrier were analysed and results used to develop predictive models. Fish were tracked via 2D acoustic telemetry. Hidden Markov models were used to distinguish behavioural states and step selection functions were applied to determine habitat selection by the fish in each state. Model results were explored to assess the benefits of including behavioural state and understand state-specific habitat preferences, then cross-validated and used to develop an individual based model to predict fish spatial usage. Little difference existed in habitat selection between states and individual variation was high, limiting general trends that could be described. Overall, barbel preferred deeper or faster water while for grayling, few trends could be described. Under the tested flow conditions, high spatial usage was predicted in the area directly downstream of the barrier. In addition, barbel usage was high in the area by and downstream of the fish pass entrance but not for grayling, which may indicate a need to improve pass attractiveness for grayling. The predictive model produced directed upstream movements of fish similar to those expected for upstream migrating freshwater fish, highlighting model potential for fish passage applications in future iterations. The high individual variability in fish behaviour drives the need for individual-based approaches for predicting fish movement.

Recent developments in fine-scale acoustic telemetry have resulted in large datasets containing highly detailed information on fish movement. A common tool in movement ecology is the application of hidden Markov models (HMMs) to uncover hidden behavioural states from telemetry data. Currently, the data collection can take place at a finer temporal scale than is typically used for HMMs. Although HMMs can still provide valuable insights into fish behaviour, the current fine-scale data can introduce some conceptual and practical challenges in model development. In this paper, we look at the potential of straightness index. This index retains fine-scale movement data while smoothing movement data, allowing for the development of HMMs. Using such an approach can be essential in finding behavioural responses of fish to the ecohydraulic environment, and might, in turn, inform fishway design.

Rivers worldwide are becoming increasingly changed by human activities. Barriers, such as dams, are a frequent feature in rivers and have many impacts, from altering river hydraulics to impeding fish migration. Hydraulic conditions around a barrier and pass may be very different from natural systems and confuse approaching fish as they attempt to pass. Advancing technologies such as fine-scale 2D acoustic telemetry and computational fluid dynamic models have led to a wealth of data on fish movement and corresponding hydraulics in rivers. Such data can be analysed to determine fish habitat preference-whether fish avoid or prefer certain environmental conditions. Two approaches for analysing habitat preference are resource selection functions and step selection functions. Both methods statistically analyse habitat usage patterns of animals. In this paper, we provide an overview of the approaches and a brief review of their limited applications so far in riverine environments. With increasing data accumulation on fine-scale habitat usage of riverine fish, habitat preference analysis is a promising tool for identifying hydraulic preferences of migrating fish.

Multiple anthropogenic forces have pushed river ecosystems into undesirable states with no clear understanding of how they should be best managed. The advancement of riverine fish habitat models intended to provide management insights has slowed. Investigations into theoretical and empirical gaps to define habitat more comprehensively across different scales and ecological organizations are crucial in managing the freshwater biodiversity crisis. We introduce the concept of novel riverscapes to reconcile anthropogenic forcing, fish habitat, limitations of current fish habitat models, and opportunities for new models. We outline three priority data-driven opportunities that incorporate the novel riverscape concept: fish movement, river behavior, and drivers of novelty that all are integrated into a scale-based framework to guide the development of new models. Last, we present a case study showing how researchers, model developers, and practitioners can work collaboratively to implement the novel riverscape concept.

Migration is a vital element of the life cycle of many freshwater fish species but is increasingly hampered globally by riverine barriers. Fish passes are a common approach to enable migration past barriers but are often ineffective. More knowledge is required on fish behaviour as they approach barriers such as habitat preferences. We evaluate the habitat selection of two upstream migrating fish species, barbel Barbus barbus and grayling Thymallus thymallus, at a hydropower plant in southern Germany, considering individual variation and population trends. Fish were tracked via fine-scale 2D acoustic telemetry in 2018 during their spawning migration. Step selection functions were used to evaluate selection of hydraulic parameters by the fish for a time step of 20 s. Exploratory models were built via model selection for each individual fish, to evaluate the extent of individual variation in model structure. A population model was developed for each species by averaging coefficients from individual models to describe general trends. The extent of individual variation was determined and confidence intervals for the population model coefficients were calculated. Fish varied greatly in individual model structure though common terms were apparent in both species, such as depth, flow velocity, the angular difference between fish and velocity, and the logarithm of the step length. Final population models for barbel included several parameters describing habitat selection and displacement. Barbel selected for faster flows, deeper water, and higher spatial velocity gradients. In addition, they selected to move more with the flow than against. Interactions were also present between habitat parameters, suggesting selection is context dependent. Barbel movement speed also changed with depth, flow velocity and spatial velocity gradient. With grayling, terms often had contrasting effects among individuals and thus general trends could not be distinguished for most terms. Our findings demonstrate habitat selection by upstream migrating fish approaching a fish pass and differences in individual selection which may have an impact on barrier management. Step selection functions are a promising approach and can provide useful insight into habitat selection and movement by migrating freshwater fish in an altered river system.

Many fish species depend on migration for various parts of their life cycle. Well-known examples include diadromous fish such as salmon and eels that need both fresh water and salt water to complete their life cycle. Migration also occurs within species that depend only on fresh water. In recent decades, anthropogenic pressures on freshwater systems have increased greatly, and have resulted, among other effects, in drastic habitat fragmentation. Fishways have been developed to mitigate the resulting habitat fragmentation, but these are not always effective. To improve fishway efficiency, the variety of navigation cues used by fish must be better understood: fish use a multitude of sensory inputs ranging from flow variables to olfactory cues. The reaction of a fish is highly dependent on the intensity of the cue, the fish species involved, and individual traits. Recently developed monitoring technologies allow us to gain insights into different combinations of environmental and physiological conditions. By combining fish behavioural models with environmental models, interactions among these components can be investigated. Several methods can be used to analyse fish migration, with state-space models, hidden Markov models, and individual-based models potentially being the most relevant since they can use individual data and can tie them to explicit spatial locations within the considered system. The aim of this review is to analyse the navigational cues used by fish and the models that can be applied to gather knowledge on these processes. Such knowledge could greatly improve the design and operation of fishways for a wider range of fish species and conditions.

River fragmentation is an increasing issue for water managers and conservationists. Barriers such as dams interfere with freshwater fish migration, leading to drastic population declines. While there are a range of widely implemented mitigation approaches, e.g. fish passes, such measures are often inefficient due to suboptimal operation and design. There is increasing need to be able to assess mitigation options prior to implementation. Individual based models (IBMs) are a promising option. IBMs can simulate the fine-scale movement of individual fish within a population as they attempt to find a fish pass, incorporating movement processes themselves. Moreover, IBMs have high transferability to other sites or conditions (e.g. changing mitigation, change in flow conditions), making them potentially valuable for freshwater fish conservation yet their application to the fine-scale movement of fish past barriers is still novel. Here, we present an overview of existing IBMs for fine-scale freshwater fish movement, with emphasis on study species and the parameters driving movement in the models. In this review, we focus on IBMs suitable for the simulation of fish tracks as they approach or pass a single barrier. The selected IBMs for modelling fine-scale freshwater fish movement largely focus on salmonids and cyprinid species. IBMs have many applications in the context of fish passage, such as testing different mitigation options or understanding processes behind movement. Existing IBMs include movement processes such as attraction and rejection behaviours, as reported in literature. Yet some factors affecting fish movement e.g. biotic interactions are not covered by existing IBMs. As the technology available for fine scale data collection continues to advance, such as increasing data linking fish behaviour to hydraulics, IBMs could become a more common tool in the design and implementation of fish bypass structures.

BACKGROUND:Fish migration has severely been impacted by dam construction. Through the disruption of fish migration routes, freshwater fish communities have seen an incredible decline. Fishways, which have been constructed to mitigate the problem, have been shown to underperform. This is in part due to fish navigation still being largely misunderstood. Recent developments in tracking technology and modelling make it possible today to track (aquatic) animals at very fine spatial (down to one meter) and temporal (down to every second) scales. Hidden Markov models are appropriate models to analyse behavioural states at these fine scales. In this study we link fine-scale tracking data of barbel (Barbus barbus) and grayling (Thymallus thymallus) to a fine-scale hydrodynamic model. With a HMM we analyse the fish's behavioural switches to understand their movement and navigation behaviour near a barrier and fishway outflow in the Iller river in Southern Germany.METHODS:Fish were tracked with acoustic telemetry as they approached a hydropower facility and were presented with a fishway. Tracking resulted in fish tracks with variable intervals between subsequent fish positions. This variability stems from both a variable interval between tag emissions and missing detections within a track. After track regularisation hidden Markov models were fitted using different parameters. The tested parameters are step length, straightness index calculated over a 3-min moving window, and straightness index calculated over a 10-min window. The best performing model (based on a selection by AIC) was then expanded by allowing flow velocity and spatial velocity gradient to affect the transition matrix between behavioural states.RESULTS:In this study it was found that using step length to identify behavioural states with hidden Markov models underperformed when compared to models constructed using straightness index. Of the two different straightness indices assessed, the index calculated over a 10-min moving window performed better. Linking behavioural states to the ecohydraulic environment showed an effect of the spatial velocity gradient on behavioural switches. On the contrary, flow velocity did not show an effect on the behavioural transition matrix.CONCLUSIONS:We found that behavioural switches were affected by the spatial velocity gradient caused by the attraction flow coming from the fishway. Insight into fish navigation and fish reactions to the ecohydraulic environment can aid in the construction of fishways and improve overall fishway efficiencies, thereby helping to mitigate the effects migration barriers have on the aquatic ecosystem.

Wageningen Marine Research rapport C058/21 | 5 van 66 Samenvatting Marker Wadden is een eilandengroep die sinds 2016 aangelegd wordt in het Markermeer. In het kader van het Kennis- en Innovatieprogramma Marker Wadden (KIMA) is onderzocht hoe de nieuw gevormde habitats op en rond Marker Wadden worden benut als paaigebieden en opgroeigebieden voor jonge vis en op die manier bijdragen aan de visproductie en voedselbasis voor visetende watervogels. Dit rapport vormt een achtergronddocument voor het KIMA-syntheserapport 2022 (KIMA, concept 2021) dat het onderzoek in de eerste 5 jaar van het ontstaan van de archipel samenvat. Voor het onderzoek naar paai- en opgroeigebieden voor vis zijn intensieve visstandbemonsteringen uitgevoerd naar het voorkomen en de abundantie van vislarven en jonge vis in het voorjaar en de zomer (april/mei-september) van 2019 (eerste eiland,) en 2020 en 2021 (alle eilanden). In juni 2020 is tevens eenmaal een bemonstering uitgevoerd in de nieuwe habitats van Trintelzand. Waar mogelijk werd wadend in ondiepe oeverzones bemonsterd met respectievelijk een larvennet, RAVON-net en een broedzegen. Bij instabiele bodems (met name in de eerste periode na aanleg) werd ook vanuit Canadese kano’s gevist. In 2020 en 2021 zijn de bemonsteringen in samenwerking met onderzoeksprogramma Natuur in Productie (Nederlands Instituut voor Ecologie, Rijksuniversiteit Groningen en Radboud Universiteit) uitgevoerd. Determinatie van jonge vis gebeurde grotendeels in het veld. Kleine larven werden verzameld op formol voor nadere identificatie in het lab (microscoop). In aanvulling op het onderzoek naar paai- en opgroeigebieden rond de eilanden van Marker Wadden zijn in 2018 in het westelijk Markermeer verschillende bemonsteringsmethoden getest voor het vaststellen van soortsamenstelling van vis in en tussen fonteinkruidvelden, een type habitat dat zich op grote schaal ontwikkelt tussen de eilanden van Marker Wadden en Trintelzand. In augustus 2019 is daarnaast nog het voorkomen van vis in zandwinputten rond Marker Wadden onderzocht. Belangrijke habitats voor productie van jonge vis zijn ondiepe oeverzones, met name zones met een combinatie van oevervegetatie (riet en moerasandijvie) en onderwatervegetatie (fonteinkruid, aarvederkruid, kranswier), terwijl kale zand- en sliboevers vooralsnog relatief weinig te bieden hebben voor de meeste soorten jonge vis. Trintelzand en Marker Wadden hebben vergelijkbare functies doordat het beschutte gebieden zijn met geleidelijke land-water overgangen zoals die elders in het Markermeer weinig te vinden zijn. Dit heeft meerwaarde als paai- en opgroeigebied voor vele soorten vis. Binnen deze gebieden is er een grote variatie in habitats en ecologische waarde, ook tussen seizoenen en jaren, mede als gevolg van variatie in ontwikkeling van de watertemperatuur in het voorjaar en van wind die fluctuaties in waterpeil veroorzaakt (Marker Wadden zijn “windwadden”). De zandwinputten (met een diepte van 10-30 m) bieden een additioneel type habitat wat door zowel grote vis (met name snoekbaars) als kleine vis (met name spiering en jonge baars) gebruikt wordt. Het gebied bevindt zich nog in een pionierssituatie maar zal zich waarschijnlijk verder ontwikkelen richting een door waterplanten gedomineerde toestand, zowel oevers als de geleidelijke land- waterovergangen als de diepere delen van het meer in de luwte van de eilanden. In bemonsteringen met stortkuil en zegen werd daar een variatie aan jonge vis, inclusief (kleinere) roofvis als jonge snoekbaars en baars aangetroffen. De belangrijkste conclusie tot dusver is dat geleidelijke land-waterovergangen met flauwe taluds in de luwte van eilanden zorgen voor een mozaïek van vegetatierijke oeverzones voor paaien en ondiepe baaien met ondergedoken vegetatie die van belang zijn voor de productie van jonge vis. Op de schaal van het Markermeer werd de afgelopen jaren echter een zeer lage visstand waargenomen waarbij de aanleg van kale zandoevers op voorheen relatief visrijke stenen oevers, alsmede baggerwerkzaamheden, vermoedelijk een rol van betekenis hebben gespeeld. Het is belangrijk te noteren dat het tot dusver gaat om pionierssituaties die in ontwikkeling zijn. Dit zijn langjarige processen waarbij verschillende vormen van beheer mogelijk zijn die uiteindelijk zullen bepalen welke habitatontwikkeling en welke visproductie gerealiseerd kan worden.