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Every fish migrating upstream through vertical slot fish passes must swim through slots, where the resistance force of flowing water could affect the passage success. We measured the hydraulic force acting on the body of preserved benthic fish in a vertical slot at different water discharge rates (80 and 130 L/s) to compare the hydraulic burden individual fish species (round goby Neogobius melanostomus Pallas, 1814, gudgeon Gobio gobio L. and bullhead Cottus gobio L.) must overcome. The forces measured in three spatial axes were then compared to acoustic Doppler velocity measurements and the passage probability of 39–45 live fish per species. Passage probability reduction of 28.26% for round goby and 39.29% for bullhead was observed at the higher water discharge. Gudgeon showed increased numbers of passages and approaches when larger hydraulic forces were experienced at 130 L/s compared to the lower water discharge. Gudgeon experienced significantly lower hydraulic forces (mean 0.27 N ± 0.12 standard deviation) compared to round goby (mean 0.32 N ± 0.12 SD) and bullhead (0.35 N ± 0.14 SD). Potentially, the increased hydraulic forces at the higher water discharge contributed to the reduction in passages in round goby and bullhead. That gudgeon behaved differently from the other species illustrates how fish species deal differently with flowing water and the hydraulic forces experienced. Our approach provides a species-oriented assessment of the flow field in ecologically relevant fish passes. These findings represent an important step towards the development of purposeful fish pass designs, which is essential for ecosystem-oriented river connectivity.

Efforts to restore river ecosystem connectivity focus predominantly on diadromous, economically important fish species, and less attention is given to nonmigratory, small‐bodied, benthic fish species. Data on benthic fish swimming performance and behaviour in comparison with ecologically similar native species are especially relevant for the study of one of the most successful invaders in the last decades: the Ponto‐Caspian gobiid species Neogobius melanostomus. To evaluate future measures against its further upstream dispersal, we conducted comparative swimming performance and behaviour experiments with round goby and two native species: the European bullhead (Cottus gobio) and the gudgeon (Gobio gobio). Experiments in a swim tunnel revealed a high variation in the swimming performance and behaviour within and among the three species. Gudgeon performed best in both U$_{crit}$ and U$_{sprint}$ experiments and displayed a rather continuous, subcarangiform swimming mode, whereas bullhead and round goby displayed a burst‐and‐hold swimming mode. Experiments in a vertical slot pass model, which contained a hydraulic barrier as a challenge to upstream movement, confirmed the high swimming performance of gudgeon. Gudgeon dispersed upstream even across the hydraulic barrier at the highest flow velocities. Round goby showed a higher capability to disperse upstream than bullhead, but failed to pass the hydraulic barrier. Our results on comparative swimming performance and behaviour can inform predictive distribution modelling and range expansion models, and also inform the design of selective barriers to prevent the round goby from dispersing farther upstream.

The majority of the world’s rivers is fragmented and fish passes are frequently installed to enable fish passage across river obstacles. Beside native, also invasive species, such as round goby, can use fish passes to disperse upstream in uninvaded ecosystems. The present study aimed at evaluating a prototype hydraulic barrier in a vertical slot fish pass that impedes passage of the invasive round goby and enables passage of native, comparable species, such as gudgeon and bullhead. In addition, the study was designed to provide basic insight in the differences between the hydraulic forces experienced by the fish species in the flow and their individual behavioural response to different flow conditions. The barrier was designed to homogenize the flow field over an extended distance to exceed the swimming capability of the round goby and impede resting by station holding due to the smooth bottom. The selective effect was created by the flow field that induced hydraulic burden, varying between species due to the individual body shapes of the fish. The performance of the hydraulic barrier was assessed using a three step-approach: Flow measurements (Step 1), measurements of the hydraulic forces experienced by preserved fish (Step 2) and live fish swimming behaviour observations in the same flow field (Step 3). This approach was applied in a flow channel study under homogenised flow conditions and in a nearly full-scale vertical slot fish pass model. The purpose of this approach was to evaluate the prototype hydraulic barrier (i), create basic evidence about how flow affects the fish species individually and whether the hydraulic burden experienced in the flow field differ between species (ii), and to assess how the hydraulic burden experienced by benthic fish affect the passage behaviour across the prototype hydraulic barrier and at an unaffected vertical slot (iii). The results showed: There was a species selective effect of the prototype hydraulic barrier. No round goby passed the barrier at the highest water discharge tested (130 L/s), while six gudgeon passed the barrier and four bullhead attempted passage but immediately returned. This passage behaviour agreed with the hydraulic forces measured as gudgeon experienced significantly lower forces at 130 L/s water discharge compared to the other species. Because the hydraulic forces differed between species and corresponded to the live fish swimming behaviour, and the live fish passage behaviour differed between water discharges, it is probable that the selective effect of the barrier was mediated by the created flow field. Beside this selective effect, the prototype barrier had a general impact on the fish migration behaviour. There were significantly less passages recorded at the prototype hydraulic barrier compared to the downstream untreated slot. The fish indeed experience hydraulic burden differing between species and these hydraulic burden correspond to the individual swimming styles of the corresponding species. The fish respond to these hydraulic burden individually, depending on their own species biology. Nevertheless, there was a general adaptation of the passage behaviour to increased water discharge: All species swam faster with more speed variation and with straighter paths upstream the barrier. With providing basic evidence about the individual hydraulic burden an invasive species experiences in comparison to two native, comparable species, this thesis is a further step towards species selective fragmentation of rivers for ecosystem conservation purposes. These findings open an avenue to fish pass design adapted to the fish community of specific ecosystems and will inform fish pass engineers, decision makers and researchers who work about the behavioural response of fish to flowing water.

Conservation of riverine fish often aims to improve access to spawning grounds and restore longitudinal connectivity by removing migration barriers, and involves substantial investments. However, these investments also enable non‐native predators to invade upstream into spawning areas and potentially adversely affect the recruitment of threatened freshwater fish through egg or fry predation. Detecting egg predation is often challenging. Visual inspections of fish gut contents may underestimate predation of soft materials such as eggs and fry, which limits the discovery of predators preying upon these life‐stages. DNA‐based detection assays may offer a more sensitive tool to assess predation of soft materials. A conservation issue was confirmed by developing and applying a species‐specific DNA‐based detection assay: invasive round goby (Neogobius melanostomus ) prey on the eggs or fry of the threatened common nase (Chondrostoma nasus ) in Switzerland. DNA‐based detection assays were also developed for five other valuable native fish species, including endangered salmonid and cyprinid river spawners. The applicability of the assays was confirmed in a series of laboratory and field feeding experiments involving eggs and fish tissue. In addition, this work provides a guiding framework for conservation managers regarding the use and applicability of different DNA‐based detection approaches for gut content analysis. The results of this study could inform local conservation measures – such as temporary reductions in the density of round goby at spawning sites prior to spawning – and demonstrate how targeted application of species‐specific molecular markers may advance freshwater fish management.