Downstream Displacement Research Articles

N processing within geomorphic structures in urban streams

Stream water often diverges from the main channel into sediments below the stream surface, gravel bars next to the stream, or organic debris dams in the middle of the stream. These geomorphic structures have the potential to support processes that produce or consume inorganic N (NH4 1 ,N O 3 2 ) and thus affect streamwater quality. We measured production (potential net mineral- ization and nitrification) and consumption (denitrification potential, net immobilization) of inorganic N, respiration, and organic-matter content in sediments from geomorphic structures in 4 streams in and around Baltimore, Maryland, USA. We sampled sediments from stream pools, riffles, gravel bars (vegetated and nonvegetated), and organic debris dams in forested reference and suburban catch- ments, and also sampled degraded (incised channel) and restored reaches of one stream. Denitrifi- cation potential was highest in organic debris dams and organic-rich gravel bars—structures with high organic matter content. Organic debris dams in suburban streams had higher denitrification than debris dams in the forested reference stream, likely because of higher NO3 2 concentrations in suburban streams. These results suggest that denitrification in debris dams increases in response to high NO3 2 levels and that denitrification may be an important sink for NO3 2 in urban or suburban streams. However, such denitrifying structures as organic debris dams may be difficult to maintain in urban streams because of high storm flows and downstream displacement. Geomorphic structures in N-rich streams also supported higher rates of nitrification than structures in a forested reference stream, suggesting that these structures can become sources of NO3 2 . The ultimate effect of different structures on NO3 2 concentrations in urban streams will depend on the balance of these production and consumption processes, which is a complex function of a stream's ability to retain organic matter and resist hydrologic changes associated with urbanization and elevated NO3 2 levels.

Macrofaunal distributions and habitat change following winter–spring releases of freshwater into the Breton Sound estuary, Louisiana (USA)

We examined the effect of freshwater inflows on the aquatic environment and macrofauna in the intermediate and brackish zones of the Breton Sound estuary. Following water releases from the Caernarvon Freshwater Diversion Structure in winter 2000 and spring 2001, we compared environmental conditions and the abundance and distribution of nekton in May 2001 between the inflow area, which receives freshwater directly from the structure, and a nearby reference area. We used these data and stable isotope analyses for C, N, and S in brown shrimp Farfantepenaeus aztecus and two species of grass shrimp ( Palaemonetes paludosus and Palaemonetes intermedius) to test four null hypotheses: (1) water quality and SAV (submerged aquatic vegetation) coverage were similar between the inflow and reference areas, (2) macrofaunal abundance and biomass were similar between the two areas, (3) stable isotopic values of brown shrimp and grass shrimp were similar between areas, habitat types, and species, and (4) brown shrimp distributions were unaffected by river inputs. Freshwater from the structure clearly influenced the estuarine environment within the inflow area. Releases from the Caernarvon structure freshened the inflow area as intended and increased SAV and daytime dissolved oxygen concentrations. The response by macrofauna to these increased freshwater flows and habitat changes involved mostly changes in density and biomass rather than shifts in species composition. Although we detected no strong effect of the freshwater diversion on brown shrimp abundance or size in the inflow area, results of the sulfur stable isotope analysis indicated that brown shrimp collected in the inflow area had been growing in higher salinity waters, possibly following downstream displacement by the diversion. Species that would benefit most from continued freshwater diversions are likely to be those species that both use SAV as nursery habitat and thrive in a low-salinity environment. Nutrients carried by water from the structure were incorporated into the estuarine food web, and these nutrient inputs, together with an increase in SAV habitat, may enhance overall secondary productivity in the inflow area.

Simulation of Airfoil Icing with a Novel Morphogenetic Model

In this paper, we describe a new modeling approach to tackle the challenging problem of in-flight icing prediction. In this approach, termed morphogenetic modeling, we predict the structural details of aircraft ice accretion by emulating the behavior of individual fluid elements. A two-dimensional morphogenetic model is used here to predict the ice accretion shape forming on a National Advisory Committee For Aeronautics 0012 airfoil under various atmospheric conditions. The influence of the surface heat transfer formulation on the ice accretion shape is examined. We complement the numerical simulation with an analytical model for airfoil icing that is based on a simple form of the mass and heat conservation equations. This analytical investigation allows us to identify a significant new dimensionless ratio, the runback factor, defined as the ratio of the impinging water mass flux to the freezing mass flux at the stagnation line. An increasing runback factor leads to a quantifiable downstream displacement of the accretion mass. We also use the analytical model to verify the morphogenetic model during the early stages of icing, and find that there is reasonable agreement between the two models in terms of ice accretion shape. A comparison with experimental data and other models shows that even simple morphogenetic modeling is competitive with existing models. Further improvements will take advantage of the model's unique ability to simulate discontinuous ice accretions in complex geometries, leading to a considerable advancement in the simulation of in-flight icing.

Geochemical signature as paleoenvironmental markers in Holocene sediments of the Tinto River estuary (Southwestern Spain)

High absolute concentrations of metals associated to sulfide deposits located at the river source (Fe, Cu, Zn, Pb, Cd, and As) have been determined from the study of major and trace element concentrations in sediment samples obtained in two vibracores in the Tinto River estuary. In most cases, the enrichment factor (EF) for these metals is over 2. Cu, Zn and As have much higher EF, over 10 in surficial samples. Fe oxy-hydroxide precipitation in high-chlorinity zones and its scavenging effect on metals is the main control process on the chemical composition of sediment. Vertical evolution of the sedimentary and geochemical characteristics of the cores show that the estuary has undergone a process of progressive shallowness and a relative increase in acidic river water volume with respect to seawater. This effect has originated a downstream displacement of acid mixing processes affecting the estuary, which is reflected in higher metal concentrations in sediments of the outer zones of the system.

Effects of Holding Environment and Exercise Conditioning on Swimming Performance of Southwestern Native Fishes

AbstractRare native fish are often reared in ponds or tanks at hatcheries for later stocking into streams with depleted populations. Fish reared in standing water may experience increased stress, downstream displacement, or high predation mortality when released into lotic environments. We compared the swimming performance of captive fish held in standing water, captive fish exercised in flowing water, and wild fish captured from a stream to evaluate the effects of exercise conditioning and holding environment on swimming performance. The swimming performance of flannelmouth sucker Catostomus latipinnis, bonytail Gila elegans, razorback sucker Xyrauchen texanus, and spikedace Meda fulgida held in standing water increased by 10, 15, 26, and 40%, respectively, after exercise conditioning in flowing water (10–100 cm/s) for as little as 10 d. Exercising fish reared in standing water may improve swimming performance and increase survival.

The effect of freshwater flow on siltation in the Humber Estuary, north east UK

This paper investigates the impact of variations in freshwater discharge from the Rivers Trent and Ouse on a number of factors related to siltation in the Humber Estuary. These factors include suspended sediment concentrations, bed sediment composition, bed levels and channel mobility. Seasonal increases in freshwater discharge can reduce siltation rates and lower bed levels in both the Inner and Outer parts of the estuary. Such seasonal variations in freshwater flow also lead to the downstream displacement of sandy bed sediments as well as the turbidity maximum. More extreme flood events can lead to discrete shifts in the channel configuration within the Inner Estuary. It is suggested that in the Humber there are critical values of freshwater discharge, with the exceedance of upper threshold conditions leading to reductions in bed levels and the switching of channel positions, and the reduction of flows below lower threshold conditions leading to pronounced increases in sedimentation.

Evolution of the fluvial system of the Prypiat, Desna and Dnieper during the Late Middle - Late Pleistocene [ Evolution des systèmes fluviatiles des rivières Prypiat, Desna et Dniepr au cours du Pléistocène moyen récent et du Pléistocène supérieur.

A hypothesis that envisages Late Pleistocene braided rivers with enlarged meanders, unlike the present rivers of the European plains, anses from a tentative model for the development of the Prypiat-Dnieper-Desna alluvial plain in the late Middle - Late Pleistocene. This plain (local relief - 30-60 m) consists of a staircase of six river terraces and corresponding alluvial suites that have been forming since the Dnieper (Saale) Glaciation. Each terrace occupies a definite altitudinal step in this staircase and has a unique position within the plain as well as characteristic relief features related to terrace boundary shapes, longitudinal and cross profiles, primary fluvial and superimposed aeohan, cryogenic and erosional landforms. The alluvial suites have a cut-and-fill framework and noticeable differences in faciès composition expressed by sedimentary structures and other hthological features. Data inferred from the relief and from geological structures indicate complex fluvial system development with six main stages of varying duration and significance. These comprised one accumulative phase and five erosion-accumulation cycles, associated with weak pulsating tectonic uplift, expressed as 7-20 m of incision. The direct influence of the decay of the Dnieper ice sheet, alternations of glacial -interglacial runoff and stream load, significant fluctuations of precipitation, displacement of vegetation zones as well as permafrost thaw, all caused changes in the fluvial erosion-sedimentation regime. Glacially-fed suspended load streams, sandy meandering and braided streams, as well as sandy bedload braided and low sinuosity streams succeeded each other following the Dnieper Glaciation. These changes were accompanied by remarkable transformations of the river network general westward displacement, downstream displacement during the latest stages, abandonment of tributaries and inner fan formation. Proof of the existence of discontinuous permafrost or deep seasonal frost penetration has been obtained for the first time in the Late Pleistocene fluvial deposits of the area. Relying on the evidence of cryogenic and aeohan activity, the final phases of accumulation of the 1st Terrace Suite presumably correlate with the maximum of the Last Glaciation and the formation of the Peripheral Terrace Suite with permafrost degradation at the end of the Late Pleistocene.

The relation between particle path length distributions and channel morphology in gravel-bed streams: a synthesis

The path length (downstream displacement over a given time period) of individual bed particles in gravel-bed rivers is central to morphological methods for measuring bed load transport rate and is also fundamental to understanding the bed load transport process and the development of channel morphology. Previous studies of particle movement using tracers report predominantly strongly positively skewed frequency distributions of path length with modes close to the point of entrainment. However, gravel-bed rivers often have regularly spaced erosion (scour pools) and deposition (channel bars) sites that are several channel widths apart and it is reasonable to expect that particle path length would reflect this morphological scale, at least during flows large enough to create and modify the morphology. Here, we synthesize and re-analyze results from published bed load tracing experiments in gravel-bed rivers to identify the variety of possible path length distributions for differing channel morphology, channel dimensions, bed particle size, and particle mobility (i.e. flow magnitude) and to look for occurrences of path length coinciding with the length scale of the morphology. The results show that path length distributions may be positively skewed, symmetrical, and uni-, bi-, or multi-modal and may include modes that coincide with known or expected pool–bar spacing. Primary path length modes equivalent to possible pool–bar spacing are more probable at higher non-dimensional bed shear stress, from which it is inferred that both particle mobility and channel morphology exert an influence on particle path lengths and that particle movement is unlikely to be stochastic except at relatively low particle mobility. Existing data are inadequate for more than a preliminary analysis of this problem consequently there is a need for new data collected explicitly and systematically to confirm these preliminary results, isolate the effect of the several variables that influence the characteristics of path length frequency distributions and identify the conditions under which path length coincides with the length scale of the dominant morphology.

A parasite spatially structures its host population

Acanthocephalan parasites are capable of altering the behavior of their intermediate amphipod hosts by inducing hyperactivity and increasing downstream drift. We used a laboratory and field study to investigate how Echinogammarus stammerii amphipods infected by the acanthocephalan Pomphorhychus laevis compensated for downstream displacement and responded to a fish extract predator cue. Experiments conducted in laboratory streams showed infected amphipods drifted more and compensated for drift less than non‐infected individuals. When fish extract was added to the channels, non‐infected individuals decreased their activity while infected individuals did not. A field study examined the up‐ and down‐stream movements of a population of E. stammerii infected with P. laevis in the River Brenta, Italy. Drift by E. stammerii was positively influenced by acanthocephalan infection. Infected amphipods made up 40% of the drift but accounted for just 5% of the E. stammerii population. Infected individuals, however, were under‐represented among upstream‐moving amphipods collected from the main river channel and a near‐shore migration column, comprising only 3% of sampled individuals. These data suggest that infected individuals may be effectively displaced downstream, creating a mechanism for spatial separation of the non‐infected and infected sub‐populations.

Crayfish as geomorphic agents and ecosystem engineers: effect of a biomass gradient on baseflow and flood‐induced transport of gravel and sand in experimental streams

SUMMARY 1. Using experimental streams, we studied the impact of the crayfish Orconectes limosus on (i) the transport of gravel and sand at baseflow; (ii) the sediment surface (bedform, particle consolidation, proportion of sand, algal and gravel cover); and (iii) the critical shear stress (τC) causing incipient gravel and sand motion during simulated floods. We examined (i) and (ii) in experimental outdoor flumes that replicated riffle‐pool sequences and (iii) in a larger laboratory flume, in which we exposed sediments retrieved from the outdoor flumes to a progressively increasing discharge.2. Habitat changes induced by crayfish, such as bedform alterations in riffles (downstream displacement of riffle heads) and the increase of gravel on sand dunes in pools, had major impacts on the spatial and temporal patterns of the baseflow transport of gravel and sand.3. In addition to their impact on bedform in riffles and on gravel cover in pools, crayfish prevented the physical consolidation of particles in riffles and reduced the algal cover and the proportion of sand in the surface layer in both riffles and pools. These crayfish impacts on sediment surface variables had complex, interacting effects on the mobilisation of gravel and sand during subsequent flood simulations. For sand, crayfish progressively decreased the τC (i.e. the sum of bedform drag and skin friction) by about 50% along the entire biomass gradient in pools, whereas the presence of crayfish abruptly decreased the τC by about 75% in riffles. For gravel, the discharge causing motion in riffles produced a shear stress (in terms of skin friction) on an even bedform that was about 75% lower in all flumes with crayfish compared with the flumes without crayfish. Crayfish had no impact on τC for gravel in pools.4. Scaling‐up these experimental results to real streams suggests that crayfish could affect the patch dynamics of major sediment transport events and habitat suitability for other organisms that, at larger spatial scales, could increase the overall spatio‐temporal habitat diversity and thus the overall structural and functional biodiversity of lotic communities.

Experimental investigation of trailing-edge devices at transonic speeds

AbstractThe influence of trailing-edge devices such as Gurney flaps and divergent trailing edges of different height on the aerodynamic performance of an aerofoil at transonic speeds has been investigated experimentally. The investigation has been carried out in the Transonic Wind Tunnel Göttingen (TWG) using the two-dimensional aerofoil model VC-Opt at freestream Mach numbers of M ε [0.755, 0.775, 0.790] and a Reynolds number of Re = 5.0 x 106.The results have shown that the trailing-edge devices increase the circulation of the aerofoil leading to a lift enhancement and pitching-moment decrease as well as an increase in minimum drag compared to the baseline configuration. The maximum lift-to-drag ratio is considerably improved and the onset of trailing-edge flow separation is shifted to higher lift. Besides the increased rear-loading, a downstream displacement of the shock provides the main lift enhancement in transonic flow.The simple Gurney flap provides the largest additional circulation of all geometries tested. The smoother turning of the flow due to the additional ramp of the divergent trailing edge leads to a smaller increase of circulation. Slightly less lift but considerably less viscous (pressure) drag is generated enhancing the maximum lift-to-drag ratio compared to the Gurney flap. The negative affect of the Gurney flap on the pitching moment is also reduced.For the high divergent trailing edges, different ramp slopes have a significant influence on the aerodynamic performance whereas at low device heights the influence is considerably diminished.The results show that the divergent trailing edge proves to be the better trailing-edge device at transonic speeds. The application as an element for an adaptive wing is generally possible.

Long-distance migration of South African deep-water rock lobster Palinurus gilchristi

Long-term movement patterns of deep-water rock lobster Palinurus gilchristi were investigated off the south coast of South Africa using tag recapture data. Over a 12 yr period (1988 to 1999), 30 043 lobsters were tagged at 5 sites. From west to east, these were Cape Agulhas, West and East Agulhas Bank, Mossel Bay to Port Elizabeth, and Port Alfred. The overall recapture rate was 7.51%, with individuals remaining at large for up to 10.3 yr. Overall, 547 (25.8%) tagged lobsters of both sexes moved >20 km within or between sites. We hypothesized that counter-current migration of juveniles would occur to redress downstream dispersal of phyllosoma larvae by the westerly flowing Agulhas Current. The vast majority of migrants (72.6%) did prove to be immature (carapace length 50 km, either southeastwards offshore to the West and East Agulhas Bank sites (106 lobsters, covering a mean distance of 154 km), or eastwards alongshore to the Mossel Bay to Port Elizabeth site near the center of the range (33 lobsters, 461 km). The fastest 5% of migrants moved at 0.43 to 0.78 km d -1 . In general, Cape Aguihas migrants reached West Agulhas Bank within 1 yr, East Agulhas Bank within 2, and Mossel Bay to Port Elizabeth within 3 yr. The West Agulhas Bank population comprised mostly juveniles and small mature individuals, and 38% migrated, all moving eastwards, either to East Agulhas Bank (66 lobsters, 46 km), or to Mossel Bay to Port Elizabeth (73 lobsters, 426 km). The easterly migration of P. gilchristi was evident but diminished at East Agulhas Bank, virtually absent at Mossel Bay to Port Elizabeth and non-existent at Port Alfred (at the eastern extreme). There was no evidence suggesting a westwards or return migration at any of the 5 sites. It is concluded that Cape Agulhas is an important settlement area for post-larvae originating between Port Elizabeth and West Agulhas Bank, and that juveniles migrate eastwards to redress the downstream displacement. The Port Alfred population is non-migratory and receives no immigrants from elsewhere, adding to other evidence that it is a separate stock, and it is inferred that phyllosoma larvae from this area use a different larval dispersal and return mechanism.

Effects of Flooding on Recruitment of Roundtail Chub, Gila robusta, in a Southwestern River

Effects of Flooding on Recruitment of Roundtail Chub, Gila robusta, in a Southwestern River

Swimming Performance of the Topeka Shiner (Notropis topeka) an Endangered Midwestern Minnow

Abstract The Topeka shiner (Notropis topeka) is imperiled by extensive changes in stream hydrology. Responses of shiners to changes or variation in stream hydraulics, however, have not been quantified, hampering conservation efforts. We quantified swimming endurance and behavior for Topeka shiners in a laboratory swim tunnel. Sustained swimming (>200 min) was observed at water velocities of 30–40 cm/s. Prolonged and burst swimming (approximately 10 min to less than 0.1 min) was observed at water velocities of 40–75 cm/s and endurance was negatively correlated with water velocity. Larger individuals (4.4–5.5 cm standard length) exhibited greater sustained swimming ability than smaller individuals (3.0–4.2 cm standard length). Oral grasping of wire mesh within the swim tunnel was frequently employed at moderate water velocities (35–50 cm/s); this behavior may limit downstream displacement of shiners during freshets. Topeka shiners are capable of swimming speeds faster than water velocities which they typica...

Analysis of a Taylor–Poiseuille vortex flow reactor — II: reactor modeling and performance assessment using glucose-fructose isomerization as test reaction

In a previous work, Giordano, Giordano, Prazeres and Cooney (1998, Chemical Engineering Science, 53(20), 3635–3652) reported experimental evidence of a significant by-pass stream around toroidal vortices that occur in a Taylor–Couette–Poiseuille apparatus, for low axial Reynolds numbers ( Re ax from 0.172 to 1.067, corresponding to mean residence times between 30 min and 5 h). With the exception of rotational Reynolds numbers ( Re θ ) close to the critical value, downstream displacement of vortices was slower than the fluid's superficial velocity. One specific geometry was focused (radius ratio η=0.677 and aspect ratio Γ=18.30). This work presents a lumped-parameter model of this system that uses empirical values of the vortex drift velocity, V d (the ratio between the speed of the vortex core and the fluid's superficial velocity). The model has two mass transfer parameters, estimated after injections of a tracer. It is validated against the data of an enzymatic test reaction, fructose–glucose isomerization, chosen in view of the small changes of viscosity that occur along the VFR. Thus, no detectable alterations of flow patterns are caused by the progress of the reaction. The temperature is 40°C, and the catalyst is glucose isomerase in soluble form. Mean residence times are approximately 2000 s, for a reactor volume of 1.5×10 −4 m 3. Two different models presented in the literature (the classical axial dispersion approach and a progressive vortices model, with V d =1.0) are compared with the empirical data and with the response of the model proposed here. Re θ ranges from 130 to 3700 (1.6< Re θ / Re θ, c <46). The reactor's performance is close to a plug-flow response for low rotations, but the by-pass flow, coupled with greater inter-vortex mixing, have a detrimental effect at higher rotations. A careful assessment of these features is essential, therefore, when the VFR is used in the manufacture of high-valued products.

The influence of environmental factors on movements of lowland-river fish in the Yorkshire Ouse system

The influence of several environmental parameters on movements of fish within the Yorkshire Ouse system, north-east England, is considered. Automated monitoring of fish with passive integrated transponding (PIT) tags, at the entrance to a fish pass on the lower Derwent, was used to examine factors influencing upstream migration of fish between May and August 1998. Overall, 85% of records were from the cyprinids, chub Leuciscus cephalus, dace Leuciscus leuciscus and roach Rutilus rutilus, which are the dominant species in the lower Derwent. Daily numbers of PIT-tagged fish entering the pass in summer were significantly and positively correlated with daylength, but not significantly correlated with mean daily temperature or mean daily flow. There were significant variations in the diel pattern of PIT records between species. Adult chub mostly approached the fish pass at night, while other species entered over a wider range of times, and stocked juvenile cyprinids entered mostly during daytime. Analyses of movements of radio-tracked barbel from the lower Nidd showed that downstream displacements associated with high flow in summer were often followed by homing to the original residence area, but that in autumn displacements were significantly more frequent and homing was significantly less frequent, resulting in a tendency for barbel to move downstream. It is concluded that an appreciation of the effects of environmental parameters on movements by different fish species is important in understanding the causation of spatial variations in distribution of fish in lowland rivers.

Microhabitat Use by Age-0 Brown Trout and Grayling: Seasonal Responses to Streambed Restoration under Different Flows

We examined microhabitat selection by age-0 brown trout Salmo trutta and grayling Thymallus thymallus in experimental flumes with channelized (monotonic configuration and low amount of stones) or restored (highly heterogenous substrate) streambed structures at low and high flows and at different times of year. Both brown trout and grayling used higher water velocities in summer high flows. Both species also occupied higher-velocity microhabitats in channelized than in restored flumes, but for grayling this difference was significant only in summer. Overall, brown trout used lower water velocities than grayling. In winter and in high-flow treatments, brown trout were more susceptible to downstream displacement in channelized compared with restored flumes. For grayling, the effect of flume type occurred only in winter when fish were located further downstream in channelized flumes. Both species were more aggregated in winter. Brown trout also exhibited a different spatial pattern among the flume types, being more aggregated in channelized flumes. The flume type did not affect the spatial pattern of grayling, but they were more contiguously distributed in high flows. Because both brown trout and grayling used lower velocities in winter, the potential for interspecific competition may increase in winter. The spatiotemporal differences in fish habitat preferences suggest that assessment of the physical carrying capacity of a stream in relation to species-specific habitat requirements is a prerequisite for effective management of brown trout and grayling populations. Preference curves indicated that the habitat requirements were narrowest in winter and that the crucial habitat factor was the availability of flow refuges; thus, the curves helped identify the timing of, and the key factor underlying, a potential habitat bottleneck in northern boreal streams.

Assessment of Streambed Stability in Steep, Bouldery Streams: Development of a New Analytical Technique

Substrate stability in streams is fundamental to benthic communities, many of which decline in biomass and taxon richness with decreased stability. Although of obvious importance in describing a stream's disturbance regime, substrate movement is often difficult to quantify accurately. Traditional techniques rely either on subjective scoring systems (e.g., Pfankuch stability score), or require repeated site visits to follow downstream displacement of painted stones. Newbury's Instability Index measures substrate movement based on calculations of a stream's tractive force at bankfull discharge, but the assumptions on which this method is based limit its use to lowland streams. We have developed a new method to assess substrate movement suitable for shallow, steep mountain streams with a high bed roughness, based on an extensive review of existing hydraulic literature. We calculated the % of the streambed in motion at bankfull discharge (P BF ) in 48 New Zealand headwater streams using our method, t...

Movement of rainbow trout (Oncorhynchus mykiss) during the spawning migration in the Tongariro River, New Zealand

The movements of 74 rainbow trout (Oncorhynchus mykiss) were monitored every 2–3 days using radio‐telemetry during the spawning migration up the Tongariro River, New Zealand, between June and November 1995. Contrary to the views of anglers, movements of individual fish were highly variable and upstream movement could not be predicted from environmental conditions and fish related variables (length, sex, and reproductive status). There was no significant difference in individual movement between male and female fish. When individual movements were in an upstream direction, maiden trout moved faster, but less often, than fish that had spawned in previous years. The flow in the lower river explained 9% of the variance in individual downstream movements. Net upstream movement occurred throughout most of the study period with above mean activity when the river was in spate, especially early in the migration season when fish responded to flow changes as low as 7%. Above mean upstream daily movement occurred 8 times on rising and 9 times on falling barometric pressure. Large floods caused downstream displacements, but fish tended to move up stream again on the flow recession. The most noticeable downstream movement occurred following the eruption of Mount Ruapehu.

Rheotactic response of fry from beach-spawning populations of sockeye salmon: evolution after selection is relaxed

Rheotactic response was quantified for newly emerged sockeye salmon (Oncorhynchus nerka) fry from a beach population (Pleasure Point) and from an adjacent inlet population (Cedar River). The Pleasure Point population was small in number and had been established by straying from the much larger Cedar River population no more than 13 generations previously. When tested in laboratory raceways, fry from the Cedar River population were displaced downstream in the dark but not in the light. Such behavior is typical of inlet populations and presumably reflects selection for rapid movement to rearing lakes with minimal losses to predation. Fry from the Pleasure Point population showed greater downstream displacement than the Cedar River fry. Behavioral divergence of the Pleasure Point population could not be explained by selection, because water movement was absent from the beach environment. Genetic drift appeared to be a more plausible divergence mechanism. We suggest that the rheotactic response of beach populations should reflect the founding genotypes, gene flow from other populations, and random genetic drift. The results of previously published studies on the rheotactic response of beach fry in two other lake systems qualitatively support our hypothesis.