Optimisation of adjusted barrier management to improve glass eel migration at an estuarine barrier

Estuarine barriers may significantly reduce the upstream migration of diadromous fish species like the European eel (Anguilla anguilla L.). Previous research showed that limited barrier opening during tidal rise was a cost-efficient and effective mitigation option to improve upstream glass eel migration, without significant intrusion of sea water. This paper aims to optimise this adjusted barrier management to improve eel passage at a tidal barrier complex at the mouth of the River Yser, Flanders, Belgium, one of the most important migration routes for glass eel in Flanders. Specifically, three hypotheses were tested. The first hypothesis analysed the impact of the number of barriers opened on the upstream glass eel migration. The second hypothesis evaluated the relation between the size of the barrier opening and glass eel migration. Finally, we tested whether the suggested adjusted barrier management may lead to a significant increase in conductivity in the River Yser. Increased opening of one barrier appeared more efficient than opening several barriers slightly. Conductivity increased during periods of extreme drought and at base flow, but decreased within 24 h after the first peak flow. This indicates that adjusted barrier management does not entail salt intrusion in the Yser basin, as long as this management is not applied in extremely dry periods. Since the adjusted barrier management is easily implemented and could be applied on numerous tidal barriers, the presented results may contribute to restoration of eel populations worldwide and be of interest to a wide range of river managers and stakeholders.     

Evident but context-dependent mortality of fish passing hydroelectric turbines

Globally, policies aiming for conservation of species, free-flowing rivers, and promotion of hydroelectricity as renewable energy and as a means to decarbonize energy systems generate trade-offs between protecting freshwater fauna and development of hydropower. Hydroelectric turbines put fish at risk of severe injury during passage. Therefore, comprehensive, reliable analyses of turbine-induced fish mortality are pivotal to support an informed debate on the sustainability of hydropower (i.e., how much a society is willing to pay in terms of costs incurred on rivers and their biota). We compiled and examined a comprehensive, global data set of turbine fish-mortality assessments involving >275,000 individual fish of 75 species to estimate mortality across turbine types and fish species. Average fish mortality from hydroelectric turbines was 22.3% (95% CI 17.5–26.7%) when accounting for common uncertainties related to empirical estimates (e.g., handling- or catch-related effects). Mortality estimates were highly variable among and within different turbine types, study methods, and taxa. Technical configurations of hydroelectric turbines that successfully reduce fish mortality and fish-protective hydropower operation as a global standard could balance the need for renewable energy with protection of fish biodiversity.     

Propulsion efficiency and cost of transport for copepods: a hydromechanical model of crustacean swimming

In the absence of direct measurement, costs of locomotion to small swimming Crustacea (<10 mm) have been derived exclusively through application of the fluid dynamic theory. Results indicate very low swimming costs, and contradict experimental data on larger Crustacea (15 to 100 mm) that suggest a three-fold increase in metabolic rate with increasing swimming speed. This paper introduces a swimming model that analyzes the hydrodynamic forces acting on a crustacean swimming at non-steady velocity. The model treats separately the hydrodynamic forces acting on the body and the swimming appendages, approximating the simultaneous solution of equations quantifying the drag and added-mass forces on each by stepwise integration. Input to the model is a time-series of instantaneous swimming-appendage velocities. The model output predicts a corresponding time-series of body velocities as well as the mechanical energy required to move the swimming appendages, dissipated kinetic energy, and metabolic cost of swimming. Swimming of the calanoid copepod Pleuromamma xiphias (Calanoida) was analyzed by extrapolating model parameters from data available in the literature. The model predictions agree well with empirical observations reported for larger crustaceans, in that swimming for copepods is relatively costly. The ratio of active to standard metabolism for P. xiphias was >3. Net cost of transport was intermediate to the values found experimentally for fish and larger crustaceans. This was a consequence of the predicted mechanical efficiency (34%) of the copepod's paddle propulsion, and of increased parasitic resistance resulting from non-steady velocity swimming.     

Feeding under predation hazard: response of the guppy and Hart's rivulus from sites with contrasting predation hazard

Summary Populations of guppies, Poecilia reticulata and Hart's rivulus, Rivulus harti, in Trinidad experience different levels of predation hazard from piscivorous fish. Those from the larger rivers (downstream sites) experience chronically high predation hazard, while those from headwater streams (upstream sites) have few predators. Guppies and Hart's rivulus, collected from downstream and upstream sites, were assayed for their feeding rate in the presence and absence of predators. We defined tenacity as the ratio of the feeding rates in the presence and absence of a predator stimulus. Thus, tenacity expresses the degree to which the forager maintains its feeding rate when a predator stimulus is present. Previous work by Seghers (1973) showed that non-feeding guppies from downstream sites responded more strongly to predators than did guppies from upstream sites. Based on this, we initially hypothesized that fish from downstream sites would show lower tenacities than fish from upstream sites. However, we found the opposite in every case. When confronted with a predator stimulus, guppies and Hart's rivulus from downstream sites fed at consistently greater rates and displayed greater tenacities than did those from upstream sites. These differences were found in experiments using both live and model predators. The results suggest that upstream fish readily trade off feeding for hiding and avoiding predation hazard, a likely response when predators appear infrequently, while downstream fish appear to be selected for boldness and tenacity while foraging under chronically high hazard.     

Efforts to enhance anadromous brown trout on Funen,Denmark

The access for anadromous brown trout (Salmo trutta) to watercourses on Funen was poor due to migratory barriers for upstream moving fish. Since 1990 fish passages have been established at 192 of these obstructions. We investigated the effect of the fish passages on parr abundance (age-0) at 24 obstructions between 1999 and 2008. Parr density increased significantly in stream sections upstream of the fish passage with a typical increase of 200 to 300 % with a maximum of 500 %. Mean parr density increased from 44 to 85 individuals 100 m² bottom area during the study period in the streams where the fish passages were established. Parr density was unchanged in streams where the migratory barriers remained. Thus, anadromous brown trout populations can be enhanced by the establishment of fish passages in rivers with migratory obstacles, and trout production increased quickly after access to new spawning and nursery areas were opened.     

Predicting barrier passage and habitat suitability for migratory fish species

Fish migrate to spawn, feed, seek refuge from predators, and escape harmful environmental conditions. The success of upstream migration is limited by the presence of barriers that can impede the passage of fish. We used a spatially explicit modeling strategy to examine the effects of barriers on passage for 21 native and non-native migratory fish species and the amount of suitable habitat blocked for each species. Spatially derived physical parameter estimates and literature based fish capabilities and tolerances were used to predict fish passage success and habitat suitability. Both the fish passage and the habitat suitability models accurately predicted fish presence above barriers for most common, non-stocked species. The fish passage model predicted that barriers greater than or equal to 6 m block all migratory species. Chinook salmon (Oncorhynchus tshawytscha) was expected to be blocked the least. The habitat suitability model predicted that low gradient streams with intact habitat quality were likely to support the highest number of fish species. The fish passage and habitat suitability models were intended to be used by environmental managers as strategy development tools to prioritize candidate dams for field assessment and make decisions regarding the management of migratory fish populations.     

Dietary effects on multi-element composition of European eel (<Emphasis Type="Italic">Anguilla anguilla</Emphasis>) otoliths

Otolith microchemistry is widely used as a tool to track individual migration pathways of diadromous fish under the assumption that the elemental composition of fish otoliths is directly influenced by the physicochemical properties of the surrounding water. Nevertheless, several endogenous factors are reported to affect element incorporation into fish otoliths and might lead to misinterpretations of migration studies. This study experimentally examined the influence of eight different diets on the microchemical composition of European eel (Anguilla anguilla) otoliths using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Seven natural prey types and one artificial diet were fed during 8 weeks in freshwater circuits. Results show for the first time that food has no significant influence on the incorporation of Na, Sr, Ba, Mg, Mn, Cu and Y into European eel otoliths. This indicates that the incorporation of elements usually chosen for migration studies is not affected by diet and that individual feeding behaviour of A. anguilla will not lead to any misinterpretation of migration pathways.     

Density-dependent migratory waves in the marine pelagic ecosystem

The migration of large aggregations of animals that sweep through the landscape is a phenomenon with large consequences in many ecosystems. It has been suggested that such migrations are mediated by resource depletion. Under this hypothesis it has been shown that simple foraging rules may generate density-dependent migratory waves (DDMW) in which the speed and amplitude increase with animal abundance. We tested these predictions on a 32-year data set of the spatial distribution of the two youngest age groups of a small pelagic schooling fish, the capelin (Mallotus villosus), by the end of their annual feeding migration in the Barents Sea. Our data suggest that the two age groups divided the Barents Sea by forming migratory waves that moved in opposite directions. The aggregation and spatial displacement of these waves increased with increasing age-specific abundance. However, possibly through social interactions, migratory pattern was modified by the abundance of the other age group.     

Mechanistic basis of individual mortality in Pacific salmon during spawning migrations

Reproductive-based migration is a challenging period for many animals, but particularly for Pacific salmonids, which must navigate from the high seas to freshwater natal streams. For the first time, we attempt to answer the question as to why some migratory adult Pacific salmon die en route to spawning grounds. Summer-run sockeye salmon (Oncorhynchus nerka) were used as a model, and the migration behavior of 301 fish was followed by intercepting them in the ocean about 215 km from the mouth of the Fraser River, British Columbia, Canada, and implanting a gastric radio transmitter. Before release, telemetered fish were also bio-sampled, which included drawing a blood sample, collecting a gill biopsy, and quantifying energetic status with a microwave energy meter. We tested the predictions that the fish that died prematurely would be characterized by low energy reserves, advanced reproductive development, elevated indicators of stress, and low osmoregulatory preparedness compared with fish that completed their river migration. Just over half (52.3%) of the sockeye tagged were subsequently detected in the Fraser River. Salmon that failed to enter the river had exhibited indicators of stress (e.g., elevated plasma lactate, glucose, and cortisol). Contrary to our prediction, fish that failed to enter the river tended to have higher gross somatic energy and be larger at the time of sampling in the ocean than fish that successfully entered the river. Of the fish that were detected in the river (i.e., 134 fish excluding fishery removals), 9.7% did not migrate beyond the lower reaches (approximately 250 km from ocean), and a further 14.2% reached the upper reaches but failed to reach natal sub-watersheds, whereas the remainder (76.1%) reached natal sub-watersheds. Of these, fish unsuccessful in the lower reaches tended to have a high plasma osmolality in the ocean, whereas fish failing in the upper reaches had lower levels of reproductive hormones in the ocean.     

Improved formulae of velocity distributions along the vertical and transverse directions in natural rivers with the sidewall effect

The distribution of flow velocity is a basis for the research into the transport of flow and sediment in natural rivers. Characteristics of velocity distribution in narrow-deep natural rivers are different from those in wide-shallow open channels, and the effect of sidewalls on the distribution of flow velocity is considerable, which leads to a large transverse gradient of the depth-averaged velocity, with the maximum velocity occurring below the water surface. Based on the Reynolds equation of turbulence flow and the analysis of the features of velocity distribution in natural rivers, improved formulae with two empirical parameters α and β have been proposed for the velocity distributions along the vertical and transverse directions, with the effect of sidewalls being considered, through solving the definite solution problem by the method of variable separation. The proposed formulae were validated fully through comparisons between the calculated and measured velocity profiles and depth-averaged velocities at several sections in the Yangtze and Baitarani Rivers, with close agreement between them being obtained. The formula of velocity distribution along the transverse direction in natural rivers with the sidewall effect was also compared with previous studies, and the calculation accuracy of this formula at a section with a narrow-deep geometry was higher than the accuracy of the previous equations. It is confirmed that the proposed formulae can reproduce well the distribution characteristics of flow velocity along the vertical and transverse directions in narrow-deep natural rivers, with a more wide application in practice.     

Floodplain Rehabilitation as a Hedge against Hydroclimatic Uncertainty in a Migration Corridor of Threatened Steelhead

A strategy for recovering endangered species during climate change is to restore ecosystem processes that moderate effects of climate shifts. In mid‐latitudes, storm patterns may shift their intensity, duration, and frequency. These shifts threaten flooding in human communities and reduce migration windows (conditions suitable for migration after a storm) for fish. Rehabilitation of historic floodplains can in principle reduce these threats via transient storage of storm water, but no one has quantified the benefit of floodplain rehabilitation for migrating fish, a widespread biota with conservation and economic value. We used simple models to quantify migration opportunity for a threatened migratory fish, steelhead (Oncorhynchus mykiss), in an episodic rain‐fed river system, the Pajaro River in central California. We combined flow models, bioenergetic models, and existing climate projections to estimate the sensitivity of migration windows to altered storm patterns under alternate scenarios of floodplain rehabilitation. Generally, migration opportunities were insensitive to warming, weakly sensitive to duration or intensity of storms, and proportionately sensitive to frequency of storms. The rehabilitation strategy expanded migration windows by 16–28% regardless of climate outcomes. Warmer conditions raised the energy cost of migrating, but not enough to matter biologically. Novel findings were that fewer storms appeared to pose a bigger threat to migrating steelhead than warmer or smaller storms and that floodplain rehabilitation lessened the risk from fewer or smaller storms across all plausible hydroclimatic outcomes. It follows that statistical downscaling methods may mischaracterize risk, depending on how they resolve overall precipitation shifts into changes of storm frequency as opposed to storm size. Moreover, anticipating effects of climate shifts that are irreducibly uncertain (here, rainfall) may be more important than anticipating effects of relatively predictable changes such as warming. This highlights a need to credibly identify strategies of ecosystem rehabilitation that are robust to uncertainty. Rehabilitación de Planicies Inundables como Cerco contra la Incertidumbre Hidroclimática en un Corredor Migratorio de Oncorhynchus mykiss, Especie Amenazada     

Comparison of sinking velocity,swimming velocity,rotation and path characteristics among six marine dinoflagellate species

Six marine dinoflagellate species representing a range of equivalent spherical diameters between 12 and 36 m were examined for several characteristics that influence their translation velocity. Sinking velocities estimated by three independent techniques and applied to swimming and narcotized cells generally agreed, and followed the cell-size relationships previously reported for diatoms. Dinokont sinking and swimming velocities both decreased with increasing surface area: volume ratio, but a small desmokont deviated from the dinokont relationships. Sinking velocities influenced the relative ascent/descent capabilities of a species. The swim:sink ratio decreased as equivalent spherical diameter increased to 25 m and then remained constant at 7.6, despite further increases in cell size. This relationship suggests a minimum required swimming capability relative to cell size. The swim:sink ratio increased with increasing surface area:volume ratio for all the surveyed species. Out observations of decreasing cell rotation:translation ratio and increasing cell drag with increasing cell size supported the hypothesis that the dinoflagellate flagellar apparatus generates maximum swimming velocity at intermediate cell sizes. However, an alternate analysis supported the hypothesis that swimming velocity increases with cell size and that variations among genera are due to subtle differences in the basic dinoflagellate propulsion system. A three-dimensional helical path index provided a more realistic estimate of the actual translation velocity (along the helix axis) during diel vertical migration when applied as a correction factor to the more typically measured helix velocity (along the helix) of a given dinoflagellate.     

Seasonal migrations of plaice (Pleuronectes platessa) through the Dover Strait

Analysis of paired-haul mid-water trawling experiments conducted in the five months November to March during the years 1985 and 1988 revealed the seasonal pattern of migration of plaice (Pleuronectes platessa L.) moving through the Dover Strait between the Southern Bight of the North Sea and the English Channel. Mid-water catches of maturing fish were highest on south-going tidesprior to peak spawning (late January), while catches of spent fish, were highest on north-going tidesafter peak spawning. Detailed analysis of the catch data provided evidence that males complete their pre-spawning migration earlier, and their post-spawning migration later, than female fish and consequently spend more time on the spawning ground. Catches of immature female fish indicated a migration south following that of the mature fish. This dummy run may be a method by which immature plaice use the presence of high densities of mature and spawning fish to learn the location of the spawning ground. The analysis also indicated that differences in catch rates between day and night are unlikely to be the result of gear avoidance by day. The results confirm that selective tidal-stream transport is a key migratory mechanism for plaice in areas of fast tidal streams on the European continental shelf.     

Relationship of oxygen consumption to swimming speed in Euphausia pacifica

Oxygen consumption rate was measured as a function of swimming velocity for the vertically migrating euphausiid Euphausia pacifica at two temperatures (8° and 12°C) and pressures (1 and 40 atm) typical of its bathymetric distribution. Increased swimming speed (x; mh^-1) required increased oxygen consumption (y; μl O₂ mg dry weight^-1 h^-1), described by the equation y = 0.012x + 0.64 at 8°C, and by y = 0.020x + 0.85 at 12° C. The current concept of low swimming costs of zooplankton, based on determinations of dead drag in copepods, is contradicted by our measurements. Temperature had a more profound effect on metabolism at higher swimming speeds (112 m h^-1; Q₁₀=2.8) than on standard metabolism (O m h^-1; Q₁₀=2.0), indicating that activity is more costly at higher temperatures. Pressure caused a small but significant (P>0.05) rise in the relationship of respiratory rate to swimming speed at both temperatures. The energy cost of vertical migration for E. pacifica was estimated by applying our data on oxygen consumption vs swimming speed to published observations on sonic scattering layer movement and the day-night distribution pattern of this species. Results indicate that the cost of a diel migration of 254 m, through a temperature change of 4 °C (8° to 12° C), would cancel any energetic benefit gained by time spent at the lower temperature typical of daytime depth. The act of vertical migration is energetically expensive; its cost should be thoroughly considered in attempts to describe the energetics of vertically migrating species.     

Seasonal variations of transport pathways and potential sources of PM2.5 in Chengdu,China (2012–2013)

PM_2.5 in Chengdu showed clear seasonal and diurnal variation. 5, 5, 5 and 3 mean clusters are generated in spring, summer, autumn, and winter. Short-distance air masses are important pathways in Chengdu. Emissions within the Sichuan Basin contribute significantly to PM_2.5 pollution. Long-range transport from Southern Xinjiang is a dust invasion path to Chengdu. Seasonal pattern of transport pathways and potential sources of PM_2.5 in Chengdu during 2012–2013 were investigated based on hourly PM_2.5 data, backward trajectories, clustering analysis, potential source contribution function (PSCF), and concentration-weighted trajectory (CWT) method. The annual hourly mean PM_2.5 concentration in Chengdu was 97.4 mg·m^–3. 5, 5, 5 and 3 mean clusters were generated in four seasons, respectively. Short-distance air masses, which travelled within the Sichuan Basin with no specific source direction and relatively high PM_2.5 loadings (>80 mg·m^–3) appeared as important pathways in all seasons. These short pathways indicated that emissions from both local and surrounding regions of Chengdu contributed significantly to PM_2.5 pollution. The cities in southern Chengdu were major potential sources with PSCF>0.6 and CWT>90 mg·m^–3. The northeastern pathway prevailed throughout the year with higher frequency in autumn and winter and lower frequency in spring and summer. In spring, long-range transport from southern Xinjiang was a representative dust invasion path to Chengdu, and the CWT values along the path were 30-60 mg·m^–3. Long-range transport was also observed in autumn from southeastern Xinjiang along a northwesterly pathway, and in winter from the Tibetan Plateau along a westerly pathway. In summer, the potential source regions of Chengdu were smaller than those in other seasons, and no long-range transport pathway was observed. Results of PSCF and CWT indicated that regions in Qinghai and Tibet contributed to PM_2.5 pollution in Chengdu as well, and their CWT values increased to above 30 mg·m^-3 in winter.     

Behaviour of stream-dwelling brown trout towards odours present in home stream water

Summary Preference behaviour patterns of wild brown trout (Salmo trutta L.) towards odours in their home-stream water were measured in laboratory experiments. Stream-dwelling, sexually mature trout were captured by electrofishing in different sections of a stream, transferred to a hatchery, and exposed to water sampled from various stream sections and to neutral water, scented by fish captured in the same stream sections. The fish preferred stream water originating from their home stream over that from a neighboring stream. Among water samples from their home stream, they preferred water from the home sections over water from distant sections both upstream and downstream. In most cases they also preferred neutral water scented by other mature fish captured in their home section over water scented by fish from the distant sections. Trout from two neighboring stream sections showed indifferent responses towards water and fish from the adjacent section. The attractive properties of stream water sampled from home sections coincided with those obtained with neutral water scented by fish from the same sites. Accordingly, the attractive components in stream water may be intraspecific odours derived from separate spawning demes of fish present in local areas of the stream.     

Prevalence of malformed frogs in Kaoping and Tungkang river basins of southern Taiwan

In this study we found many amphibians with bizarre appearances, known as malformations in Pingtung County southern Taiwan. For this investigation we collected frogs inhabiting the Kaoping and Tungkang river watersheds between February 2006 and June 2007. Among the total number of 10,909 normal frogs (i.e., anurans) collected during the investigation period, the Indian rice frogs (Rana limnocharis) account for the greatest number next is the Chinese bullfrog (Rana rugulosa). Of all the 244 captured malformed frogs, the Indian rice frog account for the greatest proportion. These malformed frogs have their main distribution in upstream areas of these two rivers. Our result indicates that the appearance rate of malformed frogs is 1.8% in the upstream reaches of the Kaoping River and 2.6%, and 0.8%, respectively in the upstream and midstream reaches of the Tungkang river. The most-commonly-found malformation is the lack of palms, followed by the lack of appendages, exostosis, and a malformed appendicular. It is, therefore, reasonable to speculate that the causes for the malformation may be related to the increased organic pollutants and agricultural chemicals used in the upstream reaches of these two rivers.     

Hydrodynamic patterns in lotic intertidal sands and their bioclimatological implications

Water movement is probably the most important factor influencing life in the interstices of sandy beaches. In order to better understand this ecosystem, e.g. the distribution and migration of interstitial fauna and overall energy transfer, measurements of water flow through high energy beaches were made on the Atlantic coast of the USA. Special hot thermistor-probes and associated electronic circuits were built, capable of measuring water-flow velocity down to less than 10 m/sec. These were buried in the sand at various depths and locations in the intertidal zone and the flow velocity recorded continuously for one or more tidal cycles. In addition, an orientation sensor was constructed and used together with the flow probe so that a picture of the flow patterns in the intertidal zone of a beach and their changes during a tidal pattern could be developed and defined quantitatively.     

Integrated modeling of flood flows and tidal hydrodynamics over a coastal floodplain

The interactions of physical processes between estuaries and upstream river floodplains are of great importance to the fish habitats and ecosystems in coastal regions. Traditionally, a hydraulic analysis of floodplains has used one- or two-dimensional models. While this approach may be sufficient for planning the engineering design for flood protection, it is inadequate when floodwaters inundate the floodplain in a complex manner. Similarly, typical estuarine and coastal modeling studies do not consider the effect of upstream river floodplains because of the technical challenge of modeling wetting and drying processes in floodplains and higher bottom elevations in the upstream river domain. While various multi-scale model frameworks have been proposed for modeling the coastal oceans, estuaries, and rivers with a combination of different models, this paper presents a modeling approach for simulating the hydrodynamics in the estuary and river floodplains, which provides a smooth transition between the two regimes using an unstructured-grid, coastal ocean model. This approach was applied to the Skagit River estuary and its upstream river floodplain of Puget Sound along the northwest coast of North America. The model was calibrated with observed data for water levels and velocities under low-flow and high-flood conditions. This study successfully demonstrated that a three-dimensional estuarine and coastal ocean model with an unstructured-grid framework and wetting-drying capability can be extended much further upstream to simulate the inundation processes and the dynamic interactions between the estuarine and river floodplain regimes.     

Risk of predation and the feeding behavior of juvenile coho salmon (Oncorhynchus kisutch)

Summary During their first 1–2 years of life, juvenile coho salmon (Oncorhynchus kisutch) are stream-dwelling, and feed upon drifting invertebrates. They move upstream from a holding position to intercept individual prey items; the distance moved (attack distance) is an increasing, but decelerating, function of prey size. Since the fish are presumably more visible to predators during such feeding excursions, prey size and risk are associated variables.The effect on attack distance of the presentation of a model predator (a photograph of a rainbow trout) was examined in the laboratory. Attack distances are shortened following presentation of a predator; this is particularly true when the prey are large (Fig. 1). The extent of the reduction of attack distance is directly related to predator presentation frequency, although there appears to be a minimum level to which it will decline (Fig. 2). Hungry fish and fish in the presence of a competitor (simulated by a mirror) are less responsive to the predator, suggesting a trade-off of energetic requirements and risk (Fig. 3 and Table 3). The effect of predation risk should be to reduce the relative proportion of large prey in a juvenile coho's diet, and its net rate of energy intake.