1. Introduction
Longitudinal connectivity plays an essential role in riverine ecosystems, and migration is an intrinsic need for the majority of aquatic organisms such as fish [
1]. Fish migrate to satisfy certain needs which are not met in the current habitat; these may include the urge to reach spawning, feeding or resting habitats [
2]. It is well-known that anthropogenic obstructions such as hydropower plants block instream migration routes, and the effects of habitat fragmentation are especially deleterious for potamodromous and diadromous fish species [
3]. In recent decades, research has focused on studying the upstream migration of fish, in particular on diadromous species [
4,
5]. Nowadays well-functioning technical and nature-like solutions to ensure upstream passage exist [
6]. Downstream migration, in contrast, has not been investigated to the same extent as upstream movements [
4,
5]. Downstream-migrating fish tend to follow the main flow [
7] and therefore may be attracted towards the turbines of hydropower plants [
8]. If no effective measures are taken to impede turbine entrainment, injuries can occur, eventually leading to high mortality rates due to turbine passage [
9,
10]. These mortality rates largely depend on biotic conditions (such as fish species, size or fitness) and abiotic/technical factors (such as turbine type and size, net head or rotational speed) [
11].
The basic features in fish-protection measures for downstream migration include (i) a system preventing entrainment, (ii) concentration of the fish in a certain area preferably close to the bypass entrance, and (iii) attracting or alternatively forcing them into a bypass structure [
12]. Regarding the first, the systems preventing entrainment can be divided into physical barriers, mechanical–behavioral and sensory–behavioral barriers [
13]. Rack-type guidance structures with narrow bar spacing act as physical barriers, selectively blocking fish with greater dimensions than the clear bar spacing (s
b) physically. For a physical barrier to be effective for fish protection, especially that of small-bodied or juvenile fish, s
b has to be greatly reduced. The German association DWA, e.g., suggests a maximum bar spacing of s
b = 20 mm, which provides efficient protection for most fish [
14]. The bars can be oriented vertically or horizontally and at a certain angle to the flow [
4]. Rectangular bars are the most common type but are replaced by curved or streamline shaped bars in some cases [
14,
15,
16]. The smaller the s
b, the higher not only the investments but also the hydraulic and thus economic losses—in addition to an aggravated maintenance of the facility (e.g., cleaning of the rack) [
16,
17]. In any case, these bar racks are only applicable at small-to-medium-scale hydropower plants [
18]. One of the largest hydropower plants currently equipped with such a fine rack is located in Rothenburg (River Saale, Germany), with an average discharge of 68 m³/s [
19].
The presence of a rigid structure impacts the hydraulics around the structure by creating eddies, turbulent flows and changed bed morphology—all of which can either attract or repel fish [
20]. Mechanical barriers utilize hydrodynamic cues in front of the barrier, such as velocity and pressure gradients, to induce avoidance behavior in fish [
15]. This avoidance may allow for an increased s
b compared to purely physical barriers in order to reach the same fish-protection rate.
Behavioral barriers use avoidance and escape responses to keep fish from dangerous areas such as turbine intakes. These sensory barriers can include visual, hydraulic, acoustic or electrical signals. To achieve an escape into the right direction, the flow velocity must not exceed the swimming capabilities of the target fish species [
14]. As a general rule, these systems are only applicable where the average flow velocities (v
A) do not exceed v
A = 0.5 m/s [
4] so the fish has enough time to react to the stimulus [
21] and is not drifted through the barrier [
3]. Sometimes, even lower values for a proper function of a behavioral barrier in downstream direction are recommended in the literature (0.3 m/s) [
14].
Physical barriers used in combination with behavior-influencing stimuli apart from the hydrodynamic cues of mechanical barriers are called hybrid barriers [
3,
22]. In this context, it is crucial that the fish have a strong sensory perception well above a threshold value of the barrier’s stimulus. In case of an electric field used for deterrence, the efficiency depends on the used current and pulse shape, conductivity of the water and of the fish [
22], attenuation of the electric field [
23], fish species or size and physical condition of the individual [
23,
24,
25]. Since most fish cannot perceive electric fields spatially, the expansion of the field must be great enough to ensure a gradual intensity increase as the fish swim into the field, thereby allowing for an escape reaction into the desired direction away from the barrier [
23,
26,
27]. The field intensity can be described by the voltage gradient (V/m) [
28]. When fish orient themselves parallel to the field lines, the voltage gradient is maximized and thus a maximum current is transferred to the fish [
23]. Therefore, the perceived body voltage (V) results from the body length (cm) multiplied with the voltage gradient (V/cm) and depends on the orientation of the fish to the field lines [
22,
29]. Hence, larger fish are more suceptible to electric fields than smaller ones [
23,
24,
25]. Classical electric deterrence systems use two (or more) rows of electrodes in the direction of flow, mainly oriented vertically [
3,
22]. Therefore, if a fish passes the first row of electrodes and approaches the second row of electrodes directly, it is oriented parallel to the field lines and thus experiences maximum body voltage [
23]. Immobilization of the fish, especially upstream of the electric field, must be prevented to avoid passive drift towards the undesired route [
20]. Factors such as voltage, pulse parameters and electrode distance have to be optimized for the current application and fish species/size to prevent taxis, narcosis or tetany, as described in the literature [
30,
31,
32]. A pulsed direct current (pDC) allows for more controlled escape reactions of fish when compared to direct current, in addition to showing a less harmful effect on deterred fish [
22,
33,
34]. To reduce fish injuries in case of a passage through the electric barrier, the electrodes can be placed in one plane perpendicular to the direction of flow.
Intake structures at hydropower plants are usually equipped with trash racks to protect the turbine from damage caused by materials transported in the water and for personal protection. Most of these rack structures have a s
b of several centimeters to filter drifting material of critical size while limiting hydraulic losses [
18]. However, such racks do not ensure proper fish protection due to the great bar spacing and missing behavioral effects.
The FishProtector technology enables the retrofitting of existing bar racks by mounting electrodes on the upstream side of the bars, inducing an electric field in the water and thus creating a hybrid barrier called the “Bar-Screen-FishProtector” [
35]. When a fish approaches the system, the mechanical barrier is perceived first and the fish orients itself in a rheotactically positive position (i.e., orientated upstream). Subsequently, it approaches cautiously and enters the electric field tail first. If the stimulus-triggering field strength is exceeded, an escape reaction in an upstream direction follows. This escape reaction is usually carried out by only a few centimeters to decimeters [
36]. Another cautious approach follows. Thus, with a horizontal angle of incidence of the system, a guiding effect towards a bypass route could be achieved, preventing fish passage through the powerhouse [
37]. Additionally, the system can be used as a barrier, purely preventing entrainment while oriented perpendicular to the flow without a guiding effect towards a bypass (pump-storage/cooling water intakes). The effective expansion of the electric field for fish protection varies with the clear spacing of the electrodes (s
e) with constant voltage, so that different spatial expansions can be generated as desired for certain site-specific conditions. Previous ethohydraulic experiments demonstrated the effectiveness of this hybrid fish-protection system [
37]. Fish injuries due to the electric field, as mentioned in the literature, mostly occur when the fish is oriented parallel to the field lines in between the electrodes [
9,
23,
24,
38]. If a fish passes the barrier investigated in this study, it will not be parallel to the field lines in case of a passage, since it passes parallel to the direction of the flow in between the electrodes, and the duration spent in the zone of high field intensities is also minimized. Studies investigating electrical barriers using pDC of voltages between 50 and 113 V to block or guide fish have found no injured or killed fish [
34,
39,
40].
In this study, etohydraulic experiments at an unscaled section model of a vertical bar rack (sb = 30 mm) retrofitted with the FishProtector technology were conducted to investigate the situation at a pilot site. In detail, the experimental fish-protection rate (EFPR) at five different scenarios was assessed, including a bar rack without electrodes (reference) and four different electrified setups with se = 80 mm, 120 mm, 160 mm and 200 mm. Overall, the present study aims to achieve the following objectives: (i) to evaluate the EFPR of a purely mechanical barrier (a bar rack), (ii) to estimate the enhancement of the EFPR of the hybrid system with complementary electrodes, (iii) to detect an optimal electrode distance in terms of EFPR, and (iv) to evaluate the effect of anodes and cathodes on the location of rack passages.