Upstream Migration Research Articles

Lake sturgeon seasonal movements in regulated and unregulated Missouri River tributaries

AbstractSpatio‐temporal movement patterns of aquatic organisms drive many ecological processes. However, dams block migrations and alter the hydrologic and thermal regimes influencing movement behaviour of freshwater fishes. In North America, many recovering southern Lake Sturgeon populations occur in rivers with hydroelectric dams, but few studies have examined the impact of hydrologic alteration on their seasonal movements. We conducted a 3‐year telemetry study of 96 adult and subadult Lake Sturgeon to compare their migratory responses to temperature and hydrology in adjacent regulated and unregulated tributaries of the Missouri River. Many other populations of Lake Sturgeon use tributaries primarily for spring spawning; however, in our study, Lake Sturgeon used Missouri River tributaries during 78% of the year. Differences in river size, hydrologic and thermal regimes in the regulated Osage River may have contributed to the greater year‐round residency, later initiation, more frequent directional changes and longer duration of spring migrations compared to the unregulated Gasconade River. Lake Sturgeon made spring upstream migrations at temperatures of 13–19°C and elevated discharges in both rivers. However, Osage River migrants responded less to changes in discharge or temperature during spring migrations, especially those that overwintered at upstream locations. Fall tributary migrations occurred in the Osage River at rising or high discharges but were uncommon in the Gasconade River. Our identification of the influences of abiotic variables on the timing, duration and extent of Lake Sturgeon seasonal migrations can help guide management of habitat and hydrology in regulated rivers to recover migratory fishes globally.

Coupling 3D hydraulic simulation and fish telemetry data to characterize the behaviour of migrating smolts approaching a bypass

Human-induced river fragmentation is a major threat to migratory fish species. Restoring river connectivity requires the construction of fish passage solutions, such as fishways for upstream and downstream migration. While many studies focussed on the upstream migration of diadromous fishes, and especially of adult Atlantic salmons (Salmo salar), we analyze juvenile behaviour under different hydraulic conditions at reservoir to improve the development of effective bypass systems for downstream passage of salmon smolt. Based on coupling three-dimensional (3D) computational fluid dynamics (CFD) simulations to smolt positions tracked by two-dimensional (2D) telemetry, the present study aimed to explore smolt behaviour in relation to hydraulic cues. More specifically, we explored how hydraulic conditions influence fish behaviour and how fish navigate depending on this behaviour. In 2017, 23 smolts were tracked in the reservoir of Poutès (Allier River, France), associated with different turbine discharge rates. 3D CFD simulations were performed and validated against field measurements in the reservoir upstream of the hydropower plant. The study of fish displacements in relation to flow conditions provided new insights with the use of thrust force, swimming orientation and direction as means to precisely characterize smolt behaviour, which can help in the design of downstream migration passage facilities. At Poutès dam, flow velocity, flow acceleration and turbulent kinetic energy are very low and therefore can lead to fish disorientation. However, results underlined that having a minimum flow velocity of 20 cm/s in reservoirs is sufficient to prevent delay and allow fish navigation.

Adhesive force and endurance of the pelvic sucker across different modes of waterfall-climbing in gobiid fishes: Contrasting climbing mechanisms share aspects of ontogenetic change

Waterfall-climbing gobiids from oceanic islands use a suction-based adhesive mechanism formed by fused pelvic fins (pelvic sucker) and exhibit rock-climbing behavior during upstream migration. Although adhesion is a common feature of locomotion in these fishes, two distinct climbing styles – powerburst climbing and inching – have evolved. We compared the performance of the pelvic sucker during climbing across a range of body sizes between two species that use these different styles, collecting new data from the powerburst climber Lentipes concolor, and comparing these to published data for the inching climber Sicyopterus japonicus. Suction force for adhesion generated during continuous climbing did not differ between the species, with similar mean safety factors of 2.5–3.0. However, L. concolor engaged its pelvic sucker for a significantly longer duration of time (approximately 34 % longer per climbing cycle) than S. japonicus during continuous climbing. During sustained adhesion, both species exhibited non-linear scaling of fatigue time, with intermediate-sized individuals (e.g., large juveniles to small adults) showing the greatest endurance. However, the two species exhibited strikingly different maxima and variability in the endurance of their pelvic suckers. Maximum time to fatigue in L. concolor was less than half that of S. japonicus, but L. concolor showed more than double the variability of S. japonicus in time to fatigue. Our comparisons of these species reveal that despite differences in several aspects of their adhesive performance, some features of sucker function remain similar across climbing styles, including several related to how performance changes through ontogeny.

Culverts delay upstream and downstream migrations of river herring (Alosa spp.)

AbstractAlewife (Alosa pseudoharengus) and blueback herring (Alosa aestivalis) are iteroparous anadromous fish found throughout the East Coast of North America. The phenology of anadromous fish migrations is important for fitness, and the duration of spawning migrations has been compressed in recent years in response to climate change. Anthropogenic barriers to movement, such as dams and culverts at road‐stream crossings, can further disrupt migration phenology by delaying movement and increasing predation risk. We used passive integrated transponder (PIT) telemetry to quantify upstream and downstream migratory delay at five road‐stream‐crossing culverts on the Herring River (MA, USA). Groundspeeds were reduced at all culverts in both directions, confirming that the culverts impede movement despite high passage proportions. The cumulative delay of the culverts on the upstream migration was sufficient to more than double the amount of time required to traverse the river if the culverts had been absent. Furthermore, the presence of snapping turtles (Chelydra serpentina) ambushing river herring within one of the culverts resulted in reduced passage rates beyond the reduction in movement caused by the physical structure itself. This highlights that physical barriers can create cascading ecological consequences and the importance of taking a holistic approach to understanding barrier effects.

Fate of translocated American eel (Anguilla rostrata) in the lower Ottawa River and passage behavior at a multichannel barrier

AbstractAmerican eel (Anguilla rostrata) is considered endangered under the IUCN's red list. Hydropower facilities are considered a significant threat to American eel, impacting both the outmigration of adults and upstream migration of juveniles. To overcome upstream passage issues, juvenile eels may be trapped and transported around barriers as a mitigation strategy, though few studies have evaluated the efficacy of this approach. To understand the fate of transported eels, we monitored posttranslocation movements in a 110‐km reach of the Ottawa River bounded by two hydropower facilities: Carillon Hydroelectric Generating Station (lower barrier) and Chaudière Falls Hydroelectric Facility (upper barrier). Additionally, we assessed the approach behavior of eels that reached the upper barrier, a multichannel facility, to assess potential fishway locations. To assess these objectives, 40 juvenile eels (440–640 mm) were implanted with acoustic transmitters and were transported and released either just upstream (~6 km) of the lower barrier or just downstream of the upper barrier (~2 km), approximately, 60 and 166 km from the capture site, respectively. Over the three‐month study period, 78% of tagged eels remained upstream of the lower barrier. Of the nine eels that returned downstream of the lower barrier, seven were from the downstream release site; however, the proportion of eels that returned downstream of the lower barrier did not differ significantly between release sites. One eel passed the upper barrier despite no existing fish‐passage structures. At the upper barrier, most eels visited just one of the five channels, suggesting that more than one passage structure may be necessary to allow eels entering different channels to pass upstream of the barrier. Findings from this work will help inform passage efforts for American eel, particularly in the Ottawa River where eel populations have declined severely from their historic abundance.

Relative importance of host‐dependent versus physical environmental characteristics affecting the distribution of an ectoparasitic copepod infecting the mouth cavity of stream salmonid

AbstractUnderstanding parasite distributional patterns is fundamental for elucidating host–parasite relationships. The genus Salmincola is an ectoparasitic copepod group specifically infecting freshwater salmonids. Considering their strong association with their hosts, we can predict that the distribution and prevalence (analogs to abundance) of Salmincola reflect host salmonids. An alternative hypothesis is that their distribution will be strongly affected by environmental factors like stream drift because they have a free‐living stage with low swimming ability. If this is the case, we predict a longitudinal gradient with higher occurrence or infection levels in downstream areas. To estimate the relative strength among factors affecting infection levels, we investigated the distribution pattern of Salmincola sp. on wild white‐spotted charr Salvelinus leucomaenis in a southern Hokkaido river system. Based on data from 19 sites across three seasons, we found that host density and flow velocity affected the prevalence of Salmincola. On the other hand, no longitudinal gradient was observed and the prevalence was extremely low in some fragmented habitats (i.e., above dams and waterfalls). This indicates some compensation mechanisms against unidirectional downstream dispersal. We found that parasite prevalence and intensity were much higher in large migratory (anadromous) fish and, therefore, hypothesize that long‐distance upstream migration helps the redistribution and population persistence of parasites in upstream areas.

Freshwater migratory movements in a widely distributed New Zealand amphidromous fish Cheimarrichthys fosteri

ABSTRACT Amphidromous fishes engage in migration to overcome the spatial mismatch between adult and larval habitat. The torrentfish (Cheimarrichthys fosteri), an endemic New Zealand amphidromous fish and only member of the Family Cheimarrichthyidae, display distinct spatial sexual segregation, but the migration assumed to overcome this segregation for reproduction is undocumented. We investigated the instream distribution of torrentfish, gaining insight into how barriers to reproduction are overcome, and how sex shapes migratory choices for the individual. We surveyed torrentfish at two spatial scales, examining the effects of sex and sexual maturity on distribution. We found that female torrentfish engage in prolonged upstream migrations, up to 100 km inland, with male torrentfish migrating upstream to a lesser extent. Both sexes return downstream to spawn, with males returning at the onset of maturity, while females may return and migrate back upstream, potentially making multiple downstream spawning migrations. Juvenile upstream migration likely results in competitive release and access to resources, but sets up a conflict between reproduction and transport of larvae into the nursery habitat in a timely manner. Adult downstream migration in torrentfish resolves this conflict and is similar to documented downstream spawning migrations in catadromous species.

The effect of rainbow trout (Oncorhynchus mykiss) invasions on native fish communities in the subtropical Blyde River, Mpumalanga province, South Africa

This study investigated the effects of rainbow trout (Oncorhynchus mykiss) invasion on native fish communities in the upper Blyde River catchment. A fish survey was undertaken between September 2017 and October 2018 from 11 sites in the Blyde and Treur Rivers using electrofishing and fyke nets. Differences in species composition, relative abundance, and community structure among sites were tested using multivariate analysis. A total of ten fish species were captured. There were significant differences in composition and abundance between fish communities that could be attributed to O. mykiss invasion and variation in habitats. Populations of native species that historically occurred throughout the upper catchment, such as Enteromius treurensis, were greatly reduced and fragmented in the presence of O. mykiss. However, instream migration barriers such as waterfalls have prevented upstream migration of O. mykiss, and these invasion-free areas have remnant populations of native fishes that appear to be largely intact. This finding is consistent with other studies that have shown that introduced alien predatory fish can have a significant effect on fish communities and highlighted the need to prevent human-facilitated introductions in biodiversity sensitive areas, such as mountain headwater streams, that are inhabited by endemic and range-restricted minnows.

Migration and survival of Okanagan River Sockeye Salmon Oncorhynchus nerka, 2012\u20132019

BackgroundOkanagan River Sockeye Salmon Oncorhynchus nerka (Okanagan Sockeye) are one of two remaining self-sustaining Sockeye Salmon populations in the Columbia River Basin. We used detection histories of smolts implanted with passive integrated transponder (PIT) tags between 2012 and 2019 to estimate survival and behavioral metrics during reintroduction efforts and changing environmental conditions over the monitoring period.ResultsSmolts migrating to McNary Dam, whose route includes 130 km of the Okanagan River and 388 km of the Columbia River, generally had high survival (mean of 87.0% per 100 km) and fast migration speeds (up to 50 km/day) relative to other salmonids in the region. Smolt-to-adult returns (SARs) ranged from 0.4 to 6.1% and were greater for fish originating from Skaha Lake compared to cohorts tagged in Osoyoos Lake. Most adults returned after 2 years in the ocean (69%), followed by jacks (27%), and adults that spent 3 years at sea (4%), though Skaha Lake adults had a significantly younger age structure than cohorts from Osoyoos Lake. Survival of adults from Bonneville Dam (rkm 235) upstream to Wells Dam (rkm 830) was generally high (80–92%), and migration speed decreased in upstream reaches. Survival from Wells Dam to the Okanagan River was only estimable in 2018, where 64% of adults survived to the spawning grounds. The upstream migration of adult Okanagan Sockeye was significantly compromised during the drought of 2015 when less than 5% of Okanagan Sockeye that returned to the Columbia River reached spawning grounds.ConclusionsOur results indicate that Okanagan Sockeye have exceptional survival and migratory ability relative to other salmonids, though poor ocean conditions combined with warming water temperatures in freshwater habitats in recent years have the potential to devastate the population. The success of reintroduction efforts to increase spatial structure and diversity of Okanagan Sockeye is, therefore, critical to maintaining the population in years to come.

Technical fishway passage structures provide high passage efficiency and effective passage for adult Pacific salmonids at eight large dams

Fishways have been widely used for upstream passage around human-built structures, but ‘success’ has varied dramatically. Evaluation of fishway success has typically been conducted at local scales using metrics such as fish passage efficiency and passage time, but evaluations are increasingly used in broader assessments of whether passage facilities meet population-specific conservation and management objectives. Over 15 years, we monitored passage effectiveness at eight dams on the Columbia and Snake rivers for 26,886 radio-tagged spring-summer and fall Chinook Salmon O. tshwaytscha, Sockeye Salmon O. nerka, and summer steelhead O. mykiss during their migrations to spawning sites. Almost all fish that entered dam tailraces eventually approached and entered fishways. Tailrace-to-forebay passage efficiency estimates at individual dams were consistently high, averaging 0.966 (SD = 0.035) across 245 run×year×dam combinations. These estimates are among the highest recorded for any migratory species, which we attribute to the scale of evaluation, salmonid life history traits (e.g., philopatry), and a sustained adaptive management approach to fishway design, maintenance, and improvement. Full-dam fish passage times were considerably more variable, with run×year×dam medians ranging from 5–65 h. Evaluation at larger scales provided evidence that fishways were biologically effective, e.g., we observed rapid migration rates (medians = 28–40 km/d) through river reaches with multiple dams and estimated fisheries-adjusted upstream migration survival of 67–69%. However, there were substantive uncertainties regarding effectiveness. Uncertainty about natal origins confounded estimation of population-specific survival and interpretation of apparent dam passage ‘failure’, while lack of post-migration reproductive data precluded analyses of delayed or cumulative effects of passing the impounded system on fish fitness. Although the technical fishways are effective for salmonids in the Columbia-Snake River system, other co-migrating species have lower passage rates, highlighting the need for species-specific design and evaluation wherever passage facilities impact fish management and conservation goals.

Population demographics of golden perch (

An understanding of population demographics and life history processes is integral to the rehabilitation of fish populations. In Australia’s highly modified Murray–Darling Basin, native fish are imperilled and fish deaths in the Darling River in 2018–19 highlighted their vulnerability. Golden perch (Macquaria ambigua) is a long-lived percichthyid that was conspicuous in the fish kills. To guide population rehabilitation in the Darling River, pre-fish kill age structure, provenance and movement of golden perch were explored using otolith microstructure and chemistry (87Sr/86Sr). Across the Lower and Mid-Darling River, recruitment was episodic, with dominant cohorts associated with years characterised by elevated discharge. There was substantial variability in age structure, recruitment source and movement patterns between the Lower and Mid-Darling River. In the Mid-Darling River, tributaries were an important recruitment source, whereas in the Lower Darling fish predominantly originated in the Darling River itself. Downstream movement of juveniles, upstream migration of adults and return movements to natal locations were important drivers of population structure. Restoring resilient golden perch populations in the Darling River will be reliant on mitigating barriers to movement, promoting a connected mosaic of recruitment sources and reinstating the hydrological and hydraulic factors associated with spawning, recruitment and dispersal. Globally, increasing water resource development and climate change will necessitate such integrated approaches to the management of long-lived migratory riverine fishes.

Influence of Steelhead Trout Stocking Size and Location on Emigration and Adult Returns

AbstractEffective stocking programs are an essential part of maintaining most high‐quality tributary steelhead Oncorhynchus mykiss fisheries in the Great Lakes, requiring sufficient numbers of stocked fish to survive, imprint, and return as adults. New York’s Lake Erie tributaries have long supported popular and economically important recreational steelhead fisheries, attracting anglers from across the country. Recent research has illustrated New York’s dependence on “stray” steelhead from other jurisdictions, making New York fisheries vulnerable to other agencies’ management decisions. We hypothesized that the limited contribution by New York‐stocked steelhead was attributable to poor poststocking survival and imprinting due to the smaller stocking size relative to other Lake Erie jurisdictions. We took an experimental approach to describe how the combination of stocking size and within‐stream stocking location interact to influence emigration and adult returns and identify mechanisms that may be driving these relationships, with the goal of identifying stocking strategies that improve adult returns. Stocked steelhead >150 mm total length emigrated from the stream by the end of May; small‐size steelhead tended to remain in the stream and exhibited upstream migration. The residual population of steelhead lost body condition and likely experienced high mortality. Adult returns from large‐size fish that were stocked 6.4 km upstream were 4.1 times greater than small‐size fish stocked upstream and six times greater than fish stocked near the lake confluence (0.8 km upstream). We conclude that many of the steelhead stocked by New York are too small to imprint and emigrate, instead remaining as stream residents that experience poor survival and ultimately make minimal contributions to the adult fishery. Increases in the stocking size of New York steelhead coupled with stocking well upstream of the lake confluence would result in increased adult returns of New York‐stocked fish to their streams of origin and improved fishery performance and resiliency.

Identification of Environmental Factors that Influence the Upstream Migration of Sea Trout Salmo trutta in Shetland Waters

Migrating sea trout Salmo trutta L. in the waters of the Shetland Islands (UK) were once found in abundant numbers but they have been in decline since the mid-1980s. Therefore, it is becoming ever more critical to investigate sea trout behaviour with respect to the prevailing environmental conditions in the unique Shetland habitat so that we can determine how this prized and important species might be re-established to more sustainable levels. Here, during a period of four months (July to October) in 2015, environmental and fish catch rate data were collected for a small sample size of sea trout caught at six Shetland locations. We then looked for trends and correlations in the data that might help account for the migratory behaviour of the fish with respect to the environmental conditions at the time of capture. Our data showed that wind speed and pH correlated to the catch rate. However, no significant correlation was found between the water temperature, air temperature, barometric pressure, or humidity, and catch rate. Multivariate regression showed that wind speed and pH explained 26.3% of the variance in the catch rate. The sex ratio of the sea trout caught was skewed towards females at a ratio of 4.7 : 1.0 and the largest trout caught were all female. Recording biological data and understanding the migration patterns and behaviours of the remaining wild populations of sea trout are important for optimizing management plans to support current efforts to increase the numbers of this valuable anadromous species in Shetland waters.

Development of an environmental DNA assay and field validation for the detection of invasive pink salmon Oncorhynchus gorbuscha

Abstract Pink salmon Oncorhynchus gorbuscha was introduced from its native range in the Pacific to Northwest Russia several times since the 1950s. While this species has been regularly observed in rivers in northern Norway since then, there has been an upsurge in the numbers of odd‐year O. gorbuscha individuals observed in rivers, particularly in northern Norway in recent years, and particularly in 2017 and 2019. In the present pilot study, an assay was developed to detect O. gorbuscha from eDNA water samples. Positive control water samples were taken at two locations of the River Signaldalselva in northern Norway during the summer of 2019, when adults were spawning in the river. Samples showed positive detection of this species in the river, while negative control samples collected upstream migration barriers in central and southern Norway confirmed the absence of the target species. These findings reveal that eDNA‐based methods can be used to track the ongoing O. gorbuscha invasion of northern Europe and other regions where it might be or become invasive.

Evaluation of By-Pass Fishway Operation for Attraction Efficiency Based on GPS Drifter Field Experiments

The attraction efficiency of a by-pass fishway installed at Gangjeong-Goryeong Weir on the Nakdong River in South Korea was evaluated according to flow rate variation of the main channel. Optimal flow rate that achieved the maximum value of attraction efficiency at the fishway entrance was also determined. The weir can adjust the flow rate of the main channel and the fishway by operating sluice gates. The weighted usable area (WUA) calculated on the basis of habitat suitability criteria (HSC) for target species using River2D (a two-dimensional physical habitat model) was regarded as an indicator of attraction efficiency. The simulated velocity field by River2D was validated by virtue of measured data acquired from GPS drifter field experiments. Additionally, monthly fish monitoring data obtained with a fish trap served as supporting data to confirm the validity of the estimated attraction efficiency. The monitoring results revealed that the fishway attraction efficiency was the highest during the spawning season (from April to June). The target fish used the fishway most frequently in April. However, many other fish species used the fishway in June. Simulation results revealed that the flow rate of the main channel at the weir should be maintained at 190 m3/s in order to most effectively attract the target fish species into the fishway entrance. Managing the optimal flow rate by operating the sluice gate is especially important during the spawning season of the target fish to facilitate upstream and downstream migration.

Biofilm viscoelasticity and nutrient source location control biofilm growth rate, migration rate, and morphology in shear flow

We present a numerical model to simulate the growth and deformation of a viscoelastic biofilm in shear flow under different nutrient conditions. The mechanical interaction between the biofilm and the fluid is computed using the Immersed Boundary Method with viscoelastic parameters determined a priori from measurements reported in the literature. Biofilm growth occurs at the biofilm-fluid interface by a stochastic rule that depends on the local nutrient concentration. We compare the growth, migration, and morphology of viscoelastic biofilms with a common relaxation time of 18 min over the range of elastic moduli 10–1000 Pa in different nearby nutrient source configurations. Simulations with shear flow and an upstream or a downstream nutrient source indicate that soft biofilms grow more if nutrients are downstream and stiff biofilms grow more if nutrients are upstream. Also, soft biofilms migrate faster than stiff biofilms toward a downstream nutrient source, and although stiff biofilms migrate toward an upstream nutrient source, soft biofilms do not. Simulations without nutrients show that on the time scale of several hours, soft biofilms develop irregular structures at the biofilm-fluid interface, but stiff biofilms deform little. Our results agree with the biophysical principle that biofilms can adapt to their mechanical and chemical environment by modulating their viscoelastic properties. We also compare the behavior of a purely elastic biofilm to a viscoelastic biofilm with the same elastic modulus of 50 Pa. We find that the elastic biofilm underestimates growth rates and downstream migration rates if the nutrient source is downstream, and it overestimates growth rates and upstream migration rates if the nutrient source is upstream. Future modeling can use our comparison to identify errors that can occur by simulating biofilms as purely elastic structures.

Geomorphic effects of recurrent outburst superfloods in the Yigong River on the southeastern margin of Tibet

Landslide dam outburst floods have a significant impact on landform evolution in high mountainous areas. Historic landslide dams on the Yigong River, southeastern Tibet, generated two outburst superfloods > 105 m3/s in 1902 and 2000 AD. One of the slackwater deposits, which was newly found immediately downstream of the historic dams, has been dated to 7 ka BP. The one-dimensional backwater stepwise method gives an estimate of 225,000 m3/s for the peak flow related to the paleo-stage indicator of 7 ka BP. The recurrence of at least three large landslide dam impoundments and super-outburst floods at the exit of Yigong Lake during the Holocene greatly changed the morphology of the Yigong River. More than 0.26 billion m3 of sediment has been aggraded in the dammed lake while the landslide sediment doubles the channel slope behind the dam. Repeated landslide damming may be a persistent source of outburst floods and impede the upstream migration of river knickpoints in the southeastern margin of Tibet.

Pregnant salmon: the real Olympians

Once in their lifetime, coho salmon undergo an arduous upstream migration to reach the same spawning ground they left as larvae years earlier. Transitioning from sea to freshwater, swimming against the current for hundreds of kilometres, navigating rapids and avoiding predators, salmon migrations are perilous journeys that require elite athletic performances to complete. Recent increases in river temperatures are further challenging the stamina of salmon, meaning that fewer fish may survive the migration and reproduce, which worries conservationists, fishers and indigenous peoples. The mortality is often higher in female salmon, which is perhaps unsurprising, as they must complete the migration while hauling a large mass of eggs up the river – the fish equivalent to running an up-hill marathon, in the summer heat, while pregnant. Just like you don't see many pregnant Olympians, female salmon may also be at a disadvantage when it comes to exercise, but whether pregnancy is to blame entirely is still unclear.Krista Kraskura at the University of California Santa Barbara, USA, and her international team of collaborators set out to test whether mature female salmon struggle with exercise and whether higher water temperatures affect them more than the males. In a tremendous sampling effort, the team intercepted migrating salmon in the rivers of British Columbia, Canada, and transported them to the lab. There, they measured the athletic performance of females and males in a swim tunnel, a fish-treadmill of sorts, at three different temperatures: present low (9°C) and high (14°C) river temperatures, and a climate change scenario (18°C) that the fish do not yet see in the wild. The results were a surprise.Across all of the temperatures, female salmon, despite carrying their heavy egg loads, matched the performance of the leaner males in all tests: swimming to exhaustion, escaping a simulated predator and recovering from bursts of activity. During exercise, female and male salmon needed similar amounts of oxygen, indicating that the larger female gonads did not impair performance as had been predicted. On the contrary, at rest and during recovery from exercise, female salmon actually required less oxygen than males, indicating that females may be more conservative with their energy stores, which is critical as salmon stop feeding during their journey. But once the temperatures were raised, both female and male salmon struggled in some way.In warm waters that mimic predicted river temperatures due to climate change, salmon of both sexes were able to maintain their swimming performance surprisingly well. However, at the higher temperatures, all salmon needed longer to recover from the effort. This is a critical finding, as migrating salmon must repeatedly overcome rapids and other obstacles that require exhaustive bouts of exercise. A longer recovery period would delay their upriver journey and offset their timing, which is key when the next generation depends on everyone meeting at the same place, at the same time. Combined, these findings stoke the fears of conservationists that fewer salmon will be able to reproduce in a warmer future, putting populations at risk.However, the main question of the study still remains a mystery: why do female salmon experience higher migration mortality rates? Kraskura and the team showed that elevated river temperatures affect both sexes and, if anything, female salmon are more efficient at metering out their limited energy. All things considered, female salmon are the real Olympians, which not only match the athleticism of males but also raise the stakes by carrying the future salmon brood up the rivers.

Cumulative effects of low‐height barriers on distributions of catadromous Japanese eels in Japan

AbstractFish distributions in river systems are known to be affected by large dams and other artificial cross‐river structures such as weirs that prevent their movements and migrations. However, little is known about the cumulative effect of low‐height barriers on fish distributions. Although fishes like eels and lampreys, which include endangered species, have a unique type of ability to climb or attach to obstacles, the strength of their anguilliform swimming locomotion style is generally lower than in other fishes. To investigate the cumulative effects of small barriers irrespective of their distances from the river mouth on riverine distributions of endangered eels, the individual density of Japanese eels and habitat characteristics of five model river systems having differences in numbers, size and positional patterns of small barriers were surveyed and analysed using a generalized linear mixed model approach. The results clearly showed that a continuum of small barriers and the distance from the river mouth affected the riverine distributions of Japanese eels, and that these effects were eel size‐ or stage‐dependent. The density of small eels decreased as the cumulative height of barriers downstream of each station increased, while that of large eels decreased as the distance from the river mouth increased. Small individuals dispersed in a wide range of habitats before the end of their upstream migration phase, suggesting that small barriers have an impact on their ideal distributions, life‐history patterns (becoming residents or nomads shifting growth habitats to estuaries) and sex differentiation. This study and existing information indicate that conservation/restoration actions are needed to maintain or improve riverine connectivity for migratory species, especially those having lower swimming ability and a benthic growth phase in freshwater ecosystems that are at risk even when migration barriers are not very high.

Gravitational Circulation as Driver of Upstream Migration of Estuarine Sand Dunes

AbstractSand dunes are large‐scale rhythmic bed patterns commonly occurring in sandy shelf seas, estuaries and rivers. Field observations of sand dunes in the Gironde estuary (France) have suggested that the gravitational circulation may cause seasonal reversal of asymmetry, which is linked to migration direction. In this study, we aim to unravel the influence of gravitational circulation on the migration of estuarine sand dunes. To this end, we present an idealized process‐based sand dune model with tide and river forcing, and the addition of a longitudinal salinity gradient (imposed diagnostically) that induces gravitational circulation. Linear stability analysis shows the emergence of dunes from a flat bed and confirms that reversal of dune migration may occur, provided the gravitational circulation is sufficiently strong compared to the river flow velocity. Lastly, we show that the migration direction of estuarine sand dunes is not necessarily the same as the direction of net sediment transport.