Sex‐Specific Atlantic Salmon Upstream Passage and Fallback at a Natural Cascade After Dam Removal

Migration is critical for life-cycle completion in diadromous fish species. River connectivity is vital in facilitating these large-scale movement events, but the extent of present-day river fragmentation can interfere with these migrations. Fish passage solutions (FPSs) are commonly implemented with the aim of improving river connectivity. In our study, we investigated the performance of two types of FPSs, spill regimes and complete dam removal, on Atlantic salmon (Salmo salar) smolt migrations. We used acoustic telemetry to monitor migration behavior and passage success of 120 wild smolts released in three different groups/sites: one group with two dams to pass to reach the river mouth, a second group with one dam to pass, and a control group without any barriers to pass (upstream of a recently removed hydroelectric dam). Smolt passage probabilities were similar for the two studied dams (87% and 86%) but showed variation in path choice, delay times, and loss rates. Passage success was influenced by several factors, such as body size, diel period, and water temperature, but not flow. Cumulative passage success to the river mouth was 61%, with most individuals being lost within lentic river stretches, either in the forebays of hydroelectric power stations or in naturally wide river stretches. Within the recently rehabilitated river sections (post dam removal), passage speeds were significantly faster than all other sections of the river (post-rehabilitation x¯ = 56.1 km/day) with significantly faster speeds compared to pre-rehabilitation (pre-x¯ = 28.0 km/day). Our findings provide valuable information on the benefits of dam removal and highlight the need for further rehabilitation measures in upriver reaches where barriers still affect downstream passage.

Acoustic telemetry was used to assess riverine behavior and passage success for prespawn male adult Atlantic salmon Salmo salar in the lower Penobscot River, Maine, in 2005 (n = 10) and 2006 (n = 25). Only 3 of 10 (30%) and 2 of 25 (8%) tagged Atlantic salmon successfully passed all three dams between the head of tide and presumed spawning habitat in 2005 and 2006, respectively. Migrants that failed to pass the second upstream dam frequently fell back into the estuary (3 of 4 in 2005; 17 of 23 in 2006), and few successfully reascended Veazie Dam at the head of tide. Fallback behavior was associated with temperatures exceeding 22°C and may reflect a strategy for coping with thermal stress and migratory delays. Atlantic salmon were also observed to actively seek out thermal refuge near one of the dams. Passage data were compared with results from previous telemetry studies that used Carlin tags and radio telemetry from 1987–1990 and 1992, and passive integrated transponder tags from 2002–2004. For all 10 years of study combined, median passage success was 64, 72, and 93% for the three dams. While 2006 may represent an uncommonly poor year for upstream passage at these dams, median cumulative passage past two of these dams was only 71% and ranged from 8% to 87% among years. Study results indicate that poor upstream passage severely limits migratory success in this system, particularly during periods of high discharge. Planned removal of two of these lower river dams is expected to improve migratory success for adult Atlantic salmon in the Penobscot River system.

Threatened Fishes of the World: Oncorhynchus masou formosanus (Jordan & Oshima, 1919) (Salmonidae)

The return of Eurasian beaver (Castor fiber) to large areas of Europe represents a conservation success with the current population estimated to be around 1.2 million individuals. Their reintroduction to many areas, including Great Britain, has in some cases been controversial. Despite numerous documented benefits to biodiversity, concerns relate to localised flooding, adverse impacts on land use and engineered structures (e.g. culvert blockage), disease transfer, and the influence of beaver habitat modifications on fisheries, particularly in relation to salmonids. This study investigated the impacts of a series of four beaver dams on the upstream movement of brown trout during the spawning period (October-December) at a field site in Scotland. The study site comprised two streams entering a common loch, one modified by a series of four beaver dams, the other remaining unaltered during the Study Period. Trout were captured using electric fishing, fyke nets and rod and line and were tagged with Passive Integrated Transponders (PIT) before release. PIT telemetry antennas were installed below and above each dam to establish successful passage of trout during the monitoring period that included trout spawning movements in 2015 (high flows) and 2016 (low flow). There was a distinct difference in passage success between years, with high flows (using prior rainfall as a proxy measure) and larger fish size being important positive predictors of upstream passage success. A combination of environmental (prior rainfall and water temperature) and biotic (fish size) factors influenced passage success with high flows being a significant covariate at all four dams in two models used to define trout passage dynamics (Weibull and exponential base models), providing the best explanatory variable for fish passage at two of the four dams. Survival analysis and associated modelling indicated that migratory delay was inversely related to previous passage success, whilst motivation was also a determinant of success, with greatest passage in highly motivated trout. Our findings indicate that given the right environmental and biotic factors, brown trout are adept at passing beaver dams, although under certain conditions, beaver dams can impede the movement of brown trout and the magnitude of impact is influenced by these factors. In particular, the barrier effects of beaver dams are exacerbated under low flow conditions, and this may become a greater challenge in the future due to shifting climatic conditions if periods of warmer and drier weather persist and coincide with peak migratory movements of fish.

ABSTRACTDemolition and removal of aging dam structures is increasingly common as a part of river restoration efforts, with anticipated benefits for re‐establishing connectivity among river biota and habitats. We studied fish movement behavior in the vicinity of a run‐of‐river, low‐head dam before and after its removal. Radio telemetry was used to track the movement of smallmouth bass and determine if movement past the dam site increased following removal. We also conducted a translocation experiment to confirm that habitat structure and flow conditions at the restored site did not impede passage. Movement of radio‐tagged smallmouth past the dam site was moderately different following dam removal, with a minor improvement in upstream passage. Results of translocation experiments indicated that the dam remnant site did not impede passage, as the return rate of fish translocated past the dam was comparable to homing behavior observed in previous studies. We hypothesize that technological constraints combined with ontogenetic patterns in smallmouth bass spatial ecology reduced our likelihood of observing individual dispersal. In systems lacking migratory (e.g., diadromous) species, research to document the effects of dam removal on fish movement and connectivity must consider how ontogenetic variation in movement behavior may limit our ability to observe and document ecological outcomes. A combination of approaches is likely to be most effective for studying non‐migratory species responses to dam removal. Experimental translocation can be effective for confirming short‐term improvements in fish passage, though other methods are more appropriate for documenting ecological benefits to fish populations and communities over longer timescales.

Immediate changes in stream channel geomorphology, aquatic habitat, and fish assemblages following dam removal in a small upland catchment

Demolition and removal of aging dam structures is increasingly common as a part of river restoration efforts, with anticipated benefits for re-establishing connectivity among river biota and habitats. We studied fish movement behavior in the vicinity of a run-of-river, low head dam before and after its removal. Radio telemetry was used to track movement of smallmouth bass and determine if movement past the dam site increased following removal. We also conducted a translocation experiment to confirm that habitat structure and flow condition at the restored site did not impede passage. Movement of radio-tagged smallmouth past the dam site was moderately different following dam removal, with a minor improvement in upstream passage. Results of translocation experiments indicated that the dam remnant site did not impede passage, as return rate of fish translocated past the dam was comparable to homing behavior observed in previous studies. We hypothesize that technological constraints combined with ontogenetic patterns in Smallmouth Bass spatial ecology reduced our likelihood of observing individual dispersal. In systems lacking migratory (e.g. diadromous) species, research to document effects of dam removal on fish movement and connectivity must consider how ontogenetic variation in movement behavior may limit our ability to observe and document ecological outcomes. A combination of approaches is likely to be most effective for studying non-migratory species responses to dam removal. Experimental translocation can be effective for confirming short-term improvements in fish passage, though other methods are more appropriate for documenting ecological benefits to fish populations and communities over longer timescales.

Effect of dam removal on habitat use by spawning Atlantic salmon

Maintaining connectivity in aquatic ecosystems is important to ensure adequate ecological functioning. A large dam removal project in the Sélune River (Normandy, France) would reconnect 827 km2 of catchment area to the sea. Only the downstream section of the Selune is currently available to diadromous fish, which migrate between freshwater and the marine environment. In particular, managers focus on the future potential abundance of Atlantic salmon, Salmo salar, for conservation and fishery purposes. As in stream channel habitat drives carrying capacity of juvenile salmon, salmon abundance is usually inferred from intensive and linear habitat surveys. However, this approach is neither cost-effective for large-scale surveys nor feasible for riverbed sections with low accessibility for measurement with traditional methods, e.g. dam lakes. We used well-defined relationships between gradient, hydrology and channel habitat structure to construct a simple model to estimate potential suitable habitat for juvenile salmon. Using fine-scale habitat data from nearby rivers, we parameterized a linear mixed model to estimate the area of suitable habitat based on simple physical descriptors of river characteristics. We compared our predictions to fine-scale habitat surveys on the upper Sélune. Using only slope and width, our model was able to explain 80% of the variance in suitable habitat. Estimates indicated that dam removal on the Sélune River would generate a threefold increase in suitable habitat for juveniles. This could increase the mean number of adult salmon returning to the river by 1420.9 (s.e. = 1015.5). More generally, this model provides an alternative and cost-effective tool to help better manage salmon populations in rivers impacted by dams.

Mining sites are frequently equipped with dams and reservoirs for water supply. At the time of closure, the dam decommissioning represents an important component of reclamation programmes. Several small dams in North America are reaching the end of their lifespan and the removal process is becoming an increasingly popular technique in the restoration of fish habitats. During the reclamation process at the Heath Steele Mine (NB, Canada), approximately 90 km2 of drainage basin area has been reopened to fish migration in the North Branch of the Miramichi River system through dam removals and modifications to existing obstructions. It was difficult to attribute any changes in the water temperature regime to the dam removal because data were only available after the dam was removed. Nonetheless, no major impact on water temperature was detected as a result of the dam removal. An increase in water temperature was observed in only one branch of the newly formed stream (draining within the old reservoir). Water temperature in other branch remained cold (and similar to its upstream site), suggesting the presence of groundwater seepage. The mean monthly water temperature increased by a maximum of 2.2oC at the site located downstream of the decommissioned dam. However, water temperature at the mouth of the Little South Branch Tomogonops River (10.5 km downstream of the decommissioned dam) returned to values similar to those recorded at the inlet of the old reservoir. This decrease in water temperature can be attributed to the forest canopy (stream shading), groundwater seepage or colder tributaries flowing into the main stem in this section of the river. Brook trout abundance in the newly formed stream within the reservoir section was low (<0.04 individual/m2) compared to the downstream site (<1.2 individual/m2) but was within densities frequently observed in other streams of the province of New Brunswick. Another dam removal project was conducted in White Rapids Brook (Miramichi River) in 2006. Although sediment transport was present at some sites following the removal, the impacts were low, localised, and of short duration. Atlantic salmon recolonised the newly accessible habitat within one year of the dam removal. The planned work sequence and proper mitigative techniques were key components in protecting fish habitat in these projects. Moreover, results showed that when proper techniques were implemented, negative impacts were minor and localised compared to the long-term benefits. These projects represented a unique opportunity for researchers and habitat managers to study the recovery of fish habitat following dam removals, and to better understand river restoration processes. Such studies dealing with river restoration will permit the development of procedures, methods and guidelines for decision-making in future dam removal projects.

Millions of dams impair watershed connectivity across the globe and have severely affected migratory fish populations. Fishways offer upstream passage opportunities, but artificial selection may be imposed by these structures. Using juvenile American eel Anguilla rostrata as a model species, we consider whether individual differences in behaviour (i.e. personality) and fish size can predict passage success. We evaluated the expression of bold and exploratory behaviours using open field and emergence assays in the laboratory. Then we assessed the propensity for individuals to volitionally climb through an experimental fishway to understand if personality and fish size could predict climbing success. We demonstrate personality in juvenile eels, and swimming speed in the open field was negatively associated with climbing propensity. Slower swimmers were up to 60% more likely to use the passage device suggesting that more exploratory eels incurred greater passage success. For successful climbers, climbing time was negatively associated with fish length. Synthesis and applications. Our results suggest fish may segregate at barriers based on personality and size. Preventing a subset of individuals from accessing upstream habitat is likely to have negative consequences for fish populations and aquatic ecosystems. Selection may be alleviated by increasing passage opportunities, maximizing fishway attraction and avoiding inefficient passage solutions.

Sedgeunkedunk Stream, a third‐order tributary to the Penobscot River, Maine, historically supported several anadromous fishes, including the Atlantic Salmon Salmo salar, Alewife Alosa pseudoharengus, and Sea Lamprey Petromyzon marinus. However, two small dams constructed in the 1800s reduced or eliminated spawning runs entirely. In 2009, efforts to restore marine–freshwater connectivity in the system culminated with removal of the lowermost dam, thus providing access to an additional 4.6 km of lotic habitat. Because Sea Lampreys utilized accessible habitat prior to dam removal, they were chosen as a focal species with which to quantify recolonization. During spawning runs of 2008–2011 (before and after dam removal), individuals were marked with PIT tags and their activity was tracked with daily recapture surveys. Open‐population mark–recapture models indicated a fourfold increase in the annual abundance of spawning‐phase Sea Lampreys, with estimates rising from 59±4 () before dam removal (2008) to 223±18 and 242±16 after dam removal (2010 and 2011, respectively). Accompanying the marked increase in annual abundance was a greater than fourfold increase in nesting sites: the number of nests increased from 31 in 2008 to 128 and 131 in 2010 and 2011, respectively. During the initial recolonization event (i.e., in 2010), Sea Lampreys took 6 d to move past the former dam site and 9 d to expand into the furthest upstream reaches. Conversely, during the 2011 spawning run, Sea Lampreys took only 3 d to penetrate into the upstream reaches, thus suggesting a potential positive feedback in which larval recruitment into the system may have attracted adult spawners via conspecific pheromone cues. Although more research is needed to verify the migratory pheromone hypothesis, our study clearly demonstrates that small‐stream dam removal in coastal river systems has the potential to enhance recovery of declining anadromous fish populations.

We measured the upstream passage success of juvenile coho salmon Oncorhynchus kisutch in relation to select experimental factors in a culvert test bed at a salmon rearing facility on the Skookumchuk River in western Washington State. Passage success, the term used for the response variable, was defined as the number of fish in the headwater tank at the end of the test divided by the number of fish released in the tailwater pool at the beginning of the test (2‐ to 17‐h test periods). Passage success was higher for large (139 mm fork length [FL]) than small (55 mm FL) fish, was higher at night than during the day, was not affected by shading, decreased as tailwater pool depth increased (22.9–53.3 cm), and did not differ significantly among the fish densities tested in the tailwater pool (35–141 fish/m3). There was a clear, negative exponential trend in the response relationship between transformed (arcsine of the square root) passage success and culvert discharge (0.028–0.099 m3/s; mean velocities, 0.59–0.98 m/s). The horizontal distribution of fish moving upstream that successfully exited the culvert into the headwater tank was skewed to the right side of the inlet where the reduced‐velocity zone (RVZ) was located. This observation supported the RVZ hypothesis about upstream movement of juvenile salmon in a culvert—fish accomplish upstream movements in roughened culverts via pathways in the low‐velocity, low‐turbulence boundary layer. To facilitate salmonid passage at road crossings, the preferred resource management alternatives are bridges or stream simulation, but in situations where these approaches are constrained by cost or logistics, the hydraulic design of culverts may be appropriate. The findings from these experiments in the culvert test bed are applicable to hydraulic designs of culverts where upstream passage of juvenile salmon is a concern.

Troubled waters: Riparian ecosystem services and community opposition to the largest dam removal project in Europe, Vezins Dam, France

Spawning success of Atlantic Salmon Salmo salar is challenged when migratory routes to natal streams are obstructed by hydropower generation stations and reservoirs that lack directional cues, potentially causing migratory delay. This study used 74 acoustic-tagged adult Atlantic Salmon during their spawning migrations to quantify migratory success, rates, and delay through the Mactaquac Reservoir in the Saint John River, New Brunswick, during three migration seasons in 2014–2016. Tag loss or mortality was considerable, reducing the effective sample size to 34 successfully tracked adults. Of these, 41% experienced fallback over the dam, 12% were unsuccessful in exiting the reservoir, and 47% were successful in exiting the reservoir on the way to the spawning grounds. Migration rates were significantly slower in the reservoir (median ± SE = 9.3 ± 1.9 km/d) than upriver (39.0 ± 4.1 km/d), and the tagged Atlantic Salmon spent 31–53% of their time reversing direction and thus travelled longer distances (73 ± 58 km) than the minimum 37-km midline route through the reservoir. Traveling the distance of the reservoir at the upriver migration rate could have shortened their journey by a median of 3.8 d. Sensor tags indicated that individual Atlantic Salmon experienced temperatures of 10–20°C (median ± SE = 16.0 ± 0.03°C) and migrated at depths of 5–35 m (23.4 ± 0.1 m) within the reservoir. Given that some of the energy needed for reproductive development and activities was likely appropriated by migratory delay and that a moderate proportion (47%) of adults emigrated from the reservoir, volitional passage may not be a successful management strategy in the studied reservoir. However, a substantially reduced sample size negates conclusive remarks about which management strategy would maximize survival and spawning success, and further study is needed.