Fish‐Net: Probabilistic models for fishway planning,design and monitoring to support environmentally sustainable hydropower

The construction of fishways for upstream and downstream connectivity is the preferred mitigation measure for hydropower dams and other riverine barriers. Yet empirical evidence for effective design criteria for many species is missing. We therefore assembled a group of international fishway designers and combined their knowledge with available empirical data using a formal expert elicitation protocol and Bayesian networks. The expert elicitation method we use minimizes biases typically associated with such approaches. Demonstrating our application with a case‐study on the temperate Southern Hemisphere, we use the resulting probabilistic models to predict the following, given alternative design parameters: (i) the effectiveness of technical fishways for upstream movement of migratory fish; (ii) habitat quality in nature‐like bypasses for resident fish; and (iii) rates of mortality during downstream passage of all fish through turbines and spillways. The Fish Passage Network (Fish‐Net) predicts that fishways for native species could be near 0% or near 100% efficient depending on their design, suggesting great scope for adequate mitigation. Sensitivity analyses revealed the most important parameters as follows: (i) design of attraction and entrance features of technical fishways for upstream migration; (ii) habitat preferences of resident fish in nature‐like bypasses; and (iii) susceptibility of fish to barotrauma and blade strike during turbine passage. Numerical modelling predicted that mortality rates of small‐bodied fish (50–100 mm TL) due to blade strike may be higher for Kaplan than Francis turbines. Our findings can be used to support environmentally sustainable decisions in the planning, design and monitoring stages of hydropower development.     

Survival and behaviour of Atlantic salmon smolts passing a run‐of‐river hydropower facility with a movable bulb turbine

Downstream migration of radio‐tagged Atlantic salmon smolts, Salmo salar L., was studied in the Kinzig, Germany, to examine effects of passing a run‐of‐river hydropower station with a movable bulb turbine. Immediate mortality for smolts passing the power station was low (3%–6%), probably facilitated by a curved rack in front of the turbine and the possibility to pass over it. Mortality in the impounded stretch above the power station was also low (1.5% extra mortality compared to a control stretch). The combined mortality due to hydropower was 5%–8%, excluding delayed effects. Most smolts followed the main flow passing through the turbine area (94%). Only few used a fishway (4%) or a nearby millstream (2%). Migration speed was slowed down at the power station, but the passage only caused a short delay (average/median 8.6/1.3 hr). However, even low mortality and short delays at several power stations and reservoirs may have considerable cumulative effects.     

Migratory delay leads to reduced passage success of Atlantic salmon smolts at a hydroelectric dam

Passage of fish through hydropower dams is associated with mortality, delay, increased energy expenditure and migratory failure for migrating fish and the need for remedial measures for both upstream and downstream migration is widely recognised. A functional fish passage must ensure safe and timely passage routes that a substantial portion of migrating fish will use. Passage solutions must address not only the number or percentage of fish that successfully pass a barrier, but also the time it takes to pass. Here, we used radiotelemetry to study the functionality of a fish bypass for downstream‐migrating wild‐caught and hatchery‐released Atlantic salmon smolts. We used time‐to‐event analysis to model the influence of fish characteristics and environmental variables on the rates of a series of events associated with dam passage. Among the modelled events were approach rate to the bypass entry zone, retention rates in both the forebay and the entry zone and passage rates. Despite repeated attempts, only 65% of the tagged fish present in the forebay passed the dam. Fish passed via the bypass (33%), via spill (18%) and via turbines (15%). Discharge was positively related to approach, passage and retention rates. We did not detect any differences between wild and hatchery fish. Even though individual fish visited the forebay and the entry zone on multiple occasions, most fish passed during the first exposures to these zones. This study underscores the importance of timeliness to passage success and the usefulness of time‐to‐event analysis for understanding factors governing passage performance.     

Fishway with two entrance branches: Understanding its performance for potamodromous Mediterranean barbels

Many fish passes have been built across the world in recent years. This study analyses the performance of a modified type of pool and weir fishway with two access branches, using passive integrated transponder telemetry. A circum‐Mediterranean barbel, Luciobarbus bocagei (Steindachner, 1864), was chosen as the target species. Both hydraulic values (flow and volumetric energy dissipation) and biological parameters (attraction, entrance and passage success) were measured in the fishway, these being related to the environmental variables affecting upstream fish movement. Flow discharge, water temperature and atmospheric pressure were important for fish entering the fishway. There was no preference between path routes, even though the main discharge was concentrated in the turbine channel. This type of fishway design could be an alternative for the conservation of fish populations where multiple fish approach options are possible.     

Evaluation of downstream migration mitigation actions for eel at an Archimedes screw pump pumping station

Eel, Anguilla anguilla (L.), mortality was studied in a Belgian lowland canal after downstream passage through a large and small de Wit‐adapted Archimedes screw pump over a 12‐month period. The hypothesis tested was the minimisation of fish injuries with the de Wit adaptation. Simultaneously, downstream migration through a Dutch pool and orifice fishway alongside the pumping station (PS) was monitored. Nets were mounted on the outflow of the pumps, and a cage was placed in the fishway. Based on the condition of the fish and injuries sustained, the assessed maximum mortality rates ranged from 19 ± 4% for the large de Wit Archimedes screw pump to 14 ± 8% for the small de Wit Archimedes screw pump. The screw adaptations did not substantially minimise grinding injuries and overall mortality, and the fishway did not mitigate downstream eel migration. To achieve escapement targets set in the eel management plans, fish‐friendly pump designs and effective PS bypass solutions are needed.     

A test of the cumulative effect of river weirs on downstream migration success,speed and mortality of Atlantic salmon (Salmo salar) smolts: An empirical study

This study investigated the cumulative impact of weirs on the downstream migration of wild Atlantic salmon (Salmo salar) smolts in the River Foyle, Northern Ireland. In spring of 2013 fish were released in two tributaries of similar length; one tributary (impacted) had seven low‐head weirs along the migration pathway and the other was devoid of such structures (un‐impacted). Salmon smolts fitted with acoustic transmitters were monitored via a passive acoustic telemetry array during downstream migration. In 2014 the study was repeated only in the impacted tributary. Overall freshwater survival rates were high (>94%). There was no significant difference in mortality, movement pattern, delay or travel speeds between rivers or between years at any phase of migration. Escapement of salmon smolts through Lough Foyle (a marine sea lough) to the open ocean was low, approximately 18% in each year. Escapement did not differ between impacted and un‐impacted rivers. This study showed no postpassage effects of weirs on mortality, migration speed or escapement of downstream migrating smolts. This suggests that the elevated mortality at low‐head obstacles described in other studies is not inevitable in all river systems. Migration through rivers with natural riffle‐pool migration may result in similar effects as those from low‐head weirs. Causes of apparent high mortality in the early part of marine migration in this study, are unknown; however similar studies have highlighted the impact of fish predators on smolts.     

High fluid shear strain causes injury in silver shark: Preliminary implications for Mekong hydropower turbine design

The Lower Mekong Basin is facing unprecedented threats to fish diversity from hydropower development. There is increasing pressure on developers and construction authorities to design solutions to improve fish survival through turbines, thus protecting the resources in regions being developed for hydropower. A hydraulic characteristic of hydropower turbines with known fisheries impacts is fluid shear stress. Elevated shear stress occurs where rapidly flowing water passes near spillways, screens and within turbine draft tubes. Shear stress can have adverse impacts on fish, but no work has assessed whether this holds true for Mekong River species. A flume was used to determine critical tolerances of silver shark, Balantiocheilos melanopterus (Bleeker), to shear stress rates at a high‐velocity jet which simulated a hydropower turbine. Fish were assessed for injury or mortality following exposure. Results were compared against a no‐shear control. Injury and mortality were greater at higher shear stress exposures. Injuries occurred at all shear exposure levels with mortality at shear levels higher than 600/s. This approach should help design future hydropower turbines if data on other species demonstrate similar results. If the likelihood of adverse impact is high, then shear stress will need to be considered in the design of future hydropower facilities.     

Stocking location and predation by marine fishes affect survival of hatchery‐reared Atlantic salmon smolts

Abstract Release strategies of hatchery‐reared Atlantic salmon, Salmo salar L., smolts were compared by studying survival and migration of smolts (n = 99) and their predators (Atlantic cod, Gadus morhua L., n = 8; and saithe, Pollachius virens (L.), n = 2) during the first 37 km of the marine migration using acoustic telemetry. Survivorship was higher in smolts released at the river mouth (30%) compared with smolts released in the river (12%). This was likely due to mortality or reduced migratory behaviour in fresh water. The marine mortality was 37% during the first 2 km after leaving the river (at least 25% mortality because of predation from marine fishes), and total marine mortality over 37 km was 68%. Detection‐depth data were useful for evaluating whether the tagged smolts were alive or predated; mortality during the first 2 km of outward migration would have been underestimated at 26% instead of 37% without the analysis of depth detection. Transmitters from consumed post‐smolts remained in predators for up to 47 days (average 29 days).     

The effect of physiological and environmental conditions on smolt migration in Atlantic salmon Salmo salar

Hydropower development has negatively influenced Atlantic salmon (Salmo salar) populations. Compensatory hatchery‐rearing programmes exist, but released fish suffer from high mortality that may be related to the lack of experience from natural environments in hatchery‐reared smolts and their large body size and high energetic state. Here, we used acoustic telemetry to test how body size, energetic state, and the environmental conditions of the river affect migration in hatchery‐reared smolts. The study was conducted in three consecutive years between 2011 and 2013 in the lower part of the River Umeälven, Sweden. For individual fish, there was no effect of body size but the energetic state of the fish had a negative effect on sea entry. The most important factor affecting sea entry rate was the water discharge in the old river bed that differed among years. Smolts were more likely to enter the sea in years when the discharge was high or when the discharge increased substantially shortly after release. Hatchery‐reared fish had higher migration speed at a slower flowing section compared with a faster flowing section, which was likely a result of large hesitation to enter the rapid section. The increase in water discharge led to an increase in fish migration speed disproportional to the increase in water velocity. Our results highlight the importance of water discharge for the smolts during smolt migration, and we argue that concern should be given to migrating fish when managing regulated rivers.     

Effects of release timing on migration behaviour and survival of hatchery‐reared Atlantic salmon smolts in a regulated river

In this study, 221 two‐year‐old hatchery‐reared salmon, Salmo salar, smolts were tagged with radio transmitters over a period of three consecutive years and released in the river in groups of 20–21 fish in various dates between late April and early June. Tagged smolts were tracked during their downstream migration in the lower 36‐km stretch of the regulated River Oulujoki, with the focus on the effects of release date, water temperature and river flow on migration behaviour and survival. The results indicate that release timing and river temperature have profound effects on the initiation of migration, swimming speed and survival of released S. salar smolts. Smolts released early in the spring in cold waters ceased migration after brief downstream movement and were vulnerable to predation, whereas the migration speed and survival rates increased markedly for smolts released later in the spring.     

Variation in wind and piscivorous predator fields affecting the survival of Atlantic salmon,Salmo salar,in the Gulf of Maine

Abstract Observations relevant to the North American stock complex of Atlantic salmon, Salmo salar L., suggest that marine mortality is influenced by variation in predation pressure affecting post‐smolts during the first months at sea. This hypothesis was tested for Gulf of Maine (GOM) stocks by examining wind pseudostress and the distribution of piscivorous predator fields potentially affecting post‐smolts. Marine survival has declined over recent decades with a change in the direction of spring winds, which is likely extending the migration of post‐smolts by favouring routes using the western GOM. In addition to changes in spring wind patterns, higher spring sea surface temperatures have been associated with shifting distributions of a range of fish species. The abundance of several pelagic piscivores, which based on their feeding habits may predate on salmon post‐smolts, has increased in the areas that serve as migration corridors for post‐smolts. In particular, populations of silver hake, Merluccius bilinearis (Mitchell), red hake, Urophycis chuss (Walbaum), and spiny dogfish, Squalus acanthias L., increased in size in the portion of the GOM used by post‐smolts. Climate variation and shifting predator distributions in the GOM are consistent with the predator hypothesis of recruitment control suggested for the stock complex.     

The effects of stocking Atlantic salmon, Salmo salar, in a Norwegian regulated river

Abstract  Stocking is undertaken in the River Suldalslågen, western Norway, to compensate for an estimated annual loss of 20 000 Atlantic salmon smolts, Salmo salar L., caused by regulating the river for hydropower production. The annual contribution to angling catches from stocked hatchery fish varied from 7 to 334 kg, or <15% of the total number caught. Between 160 000 and 250 000 one-summer old fish were stocked, but only between 6 and 10 (<0.005%) were recaptured as adults in the river. Recaptured stocked fish never exceeded 0.03% by number, despite smolts dominating the stocking material in recent years. It is not certain whether the slight increase in catches comes in addition to or at the expense of natural reproduction. In most years more adults were used as parent stock than were caught as offspring. The lack of positive response to stocking is possibly due to lesser age, smaller size and later migration of hatchery smolts, and that seawater tolerance of hatchery smolts is poorly developed, all factors increasing mortality at sea.     

Atlantic salmon smolts in the Irish Sea: First evidence of a northerly migration trajectory

Results from an acoustic telemetry study revealed for the first time a northerly migration route for Atlantic salmon (Salmo salar L.) smolts leaving the east coast of Ireland. Atlantic salmon smolts were tagged in spring 2019 in the Castletown and Boyne rivers. Three tagged smolts registered on disparate deep‐water offshore marine receivers as they travelled northwards out of the Irish Sea through the North Channel. One fish had migrated an estimated 250 km in a period of 32 days. The remaining two individuals were detected on receivers located off the Northern Ireland coast, further corroborating the northward migration of salmon smolts through the Irish Sea.     

Riverine and fjord migration of wild and hatchery‐reared Atlantic salmon smolts

Migration timing, speed, survival and effects of environmental parameters on migration, between wild and hatchery produced Atlantic salmon, Salmo salar L., smolts in the River Lærdalselva were studied. Hatchery‐reared (n = 40) and wild pre‐smolts (n = 40) were tagged with acoustic tags, and an array of receivers along the migration route was deployed. In all, 77 and 85% of the fish from the two groups, respectively, were recorded as migrating smolts, that is, predation rate and/or numbers of fish opting to remain in the river were low. Hatchery‐reared smolts showed a migration pattern, speed and migration route similar to wild smolts, even though the time period between river release and onset of migration was relatively short. Both groups of smolt showed high migration speed through both the river and the fjord compared with other studies.     

Development of an accurate model to predict the phenology of Atlantic salmon smolt spring migration

1. Changes in migration timing, resulting from the alteration in river continuity or the effect of climate change, can have major consequences on the population dynamics of diadromous fish. Forecasting the phenology of fish migration is thus critically important to implement management actions aimed at protecting fish during their migration.
2. In this study, an 11‐year monitoring survey of Atlantic salmon smolts (Salmo salar) from the Ourthe River, Belgium, was analysed within a European Special Area of Conservation to improve the understanding of environment‐induced spring migration. A logistic model was fitted to forecast smolt migration and to calculate phenological indicators for management, i.e. the onset, end, and duration of migration, while accounting for the influence of photoperiod, water temperature, and hydrological conditions.
3. The results indicated that the photo‐thermal units accumulated by smolts above a 7°C temperature threshold was a relevant proxy to reflect the synergistic effect between temperature and photoperiod on smolt migration. After integrating the effect of river flow pulses, the model accurately explained the inter‐annual changes in migration timing (R² = 0.95). The model predictions provide decisive management information to identify sensitive periods during which mitigation measures (e.g. hydropower turbine shutdown, river discharge management) should be conducted to promote smolt survival.
4. The model was used to predict phenological characteristics under future scenarios of climate change. The results suggest a joint effect of hydrological alterations and water warming. Temperature increases of 1–4°C were associated with earlier initiation of migration, 6–51 days earlier, and spring flood events greatly influenced the duration of the migration period. Accordingly, the combined effects of human‐induced modifications of the hydrological regimes and increasing temperatures could result in a mismatch between the smolt and favourable survival conditions in the marine environment.

     

Variations in migration behaviour and mortality of Atlantic salmon smolts in four different hydroelectric facilities

Migration behaviour, route selection and mortality of Atlantic salmon, Salmo salar L., smolts were studied at four different hydroelectric facilities in the River Mustionjoki, Finland, in May 2017. Radio-tagged smolts were released upstream of the power stations and tracked by stationary antenna-receiver systems and hand-held receivers. Tracking revealed a general tendency of smolts to move downstream with the main flow, but also variable behaviour and mortality related to diverse conditions characteristic of each power station. Average migration delay at the power stations ranged between 13.8 and 101.1 h (median: 1.7–61.5 h). Estimated mortality ranges were 0%–50% in the forebays, 4%–64% in the power stations and 2–30%/km during river migration after passage of the dam. This study provided essential information on behaviour and mortality in relation to local conditions at each power station required for successful application of fish bypass systems in a salmon restoration project.     

Effects of tagging on migration behaviour,survival and growth of hatchery‐reared Atlantic salmon smolts

The effects of four different tagging methods (PIT, anchor T‐tag, Carlin tag and dummy radio transmitter) on survival, behaviour and growth of Atlantic salmon smolts during their downstream migration were examined in semi‐natural circular channels during a natural migration period in spring. Survival of smolts was high and tagging wounds healed well in all tagging groups. Tag loss rates were generally low, being the highest (2.5%) in the dummy radio transmitter group. Total length and body mass of the tagged and untagged smolts did not differ at the end of the experiment. Migration activity of smolts generally showed similar patterns among the treatments. However, Carlin‐tagged smolts started their migration slightly later than the PIT‐tagged fish, and smolts tagged with Carlin tag or dummy radio transmitter showed less overall migration activity than fish with PIT tag.     

Reservoir entrapment and dam passage mortality of juvenile Chinook salmon in the Middle Fork Willamette River

Abstract – High‐head dams in Oregon’s Willamette River basin inhibit seaward migration and present significant mortality risks to ESA‐listed juvenile Chinook salmon (Oncorhynchus tshawytscha). Over 7 years, we passively collected 13,365 downstream‐migrating juvenile salmon in rivers above and below Willamette dams. Most salmon emigrated from upstream sites in February–June, but passed dams in November–February when reservoirs were drawn down near annual lows, and access to deep‐water passage routes improved. Samples collected above reservoirs were dominated by subyearlings, whereas below‐dam samples were a phenotypically diverse mix of subyearling, yearling and older salmon. The life history data indicated that Willamette reservoirs seasonally entrap many salmon and some sea‐ready smolts probably residualise. Annual dam‐passage mortality estimates were 8–59% (mean = 26%). Individual salmon mortality risk increased significantly with body length and varied with reservoir elevation and discharge. Operational changes that allow timely volitional emigration and development of less hazardous passage routes would benefit this threatened population.     

A quantitative assessment of fish passage efficiency

In an attempt to restore the connectivity of fragmented river habitats, a variety of passage facilities have been installed at river barriers. Despite the cost of building these structures, there has been no quantitative evaluation of their overall success at restoring fish passage. We reviewed articles from 1960 to 2011, extracted data from 65 papers on fish passage efficiency, size and species of fish, and fishway characteristics to determine the best predictors of fishway efficiency. Because data were scarce for fishes other than salmonids (order Salmoniformes), we combined data for all non‐salmonids for our analysis. On average, downstream passage efficiency was 68.5%, slightly higher than upstream passage efficiency of 41.7%, and neither differed across the geographical regions of study. Salmonids were more successful than non‐salmonids in passing upstream (61.7 vs. 21.1%) and downstream (74.6 vs. 39.6%) through fish passage facilities. Passage efficiency differed significantly between types of fishways; pool and weir, pool and slot and natural fishways had the highest efficiencies, whereas Denil and fish locks/elevators had the lowest. Upstream passage efficiency decreased significantly with fishway slope, but increased with fishway length, and water velocity. An information‐theoretic analysis indicated that the best predictors of fish passage efficiency were order of fish (i.e. salmonids > non‐salmonids), type of fishway and length of fishway. Overall, the low efficiency of passage facilities indicated that most need to be improved to sufficiently mitigate habitat fragmentation for the complete fish community across a range of environmental conditions.     

Simulation of juvenile sockeye salmon (Oncorhynchus nerka) migrations in the Strait of Georgia, British Columbia

Previous research has documented two main migratory routes of juvenile sockeye salmon (Oncorhynchus nerka) through the Strait of Georgia, British Columbia, Canada, and large interannual variability in marine survival rates of the Chilko Lake stock. Simulation models were used to explore the influence of surface currents on the migratory route of juvenile sockeye salmon (smolts) through the Strait of Georgia. We used a model of downstream migration to generate daily numbers of Chilko Lake sockeye salmon smolts entering the Strait of Georgia, based on daily counts of smolts leaving the rearing lake. A numerical hydrodynamic model (driven by surface wind, tide, and Fraser River discharge) hindcasted surface currents in the Strait of Georgia on a 2 km × 2 km grid. A smolt migration model simulated fish moving through the Strait with different compass-oriented migratory behaviours (i.e. swimming speed and directional orientation) within the time-varying surface advection field. Results showed that surface currents within the Strait of Georgia can affect the migratory route of sockeye salmon smolts in spite of their large size (8 cm). Wind is the forcing mechanism primarily responsible for determining which migratory route would be used. Under prevailing wind conditions (i.e. toward the north-west), most sockeye salmon smolts would use the eastern migratory route; however, relatively brief south-eastward wind events (lasting about 2 days) would force most smolts into the western migratory route. Given the heterogeneity of food for salmon within the Strait, we hypothesize that wind-driven variability in the annual proportion of smolts that use the western and eastern migratory routes in the Strait of Georgia affects early marine survival rates of Fraser River sockeye salmon.