Natural Stream Water Research Articles

Laboratory simulation of flocculation processes in a flooded tailings impoundment at the Kristineberg Zn–Cu mine, northern Sweden

A laboratory mixing experiment was performed to simulate the flocculation processes that can be expected when natural stream water mixes with the saline water in flooded tailings impoundments. Mixing plots where dissolved (<0.22 μm) Ca, Mg, Na, K, S and Si were plotted vs. a conservative mixing index revealed a conservative mixing behaviour for these elements. Similar plots for dissolved Fe, Mn, Al and total organic C (TOC) showed that dissolved Fe and Al flocculated within 24 h after the mixing of the waters. Dissolved Mn was removed from solution 13–98 days after mixing, presumably due to the oxidation of Mn 2+. Removal of TOC could not be detected in the mixing bottles. However, the flocculants that settled in the mixing bottles (1.7–3.4 mg flocs L −1 of stream water) contained 18 wt% C and 14–19 wt% acid-leacheable Fe. Organic C and Fe oxyhydroxides thus appear to form substantial fractions of the flocculants. If stream water is diverted through tailings ponds as part of a remediation programme, flocculation processes are likely to contribute to natural sedimentation in the pond. However, the relative contribution from flocculated particulate matter may be significant only when the stream-water transport of suspended matter into the impoundment is low (<10–15 mg L −1). Trace metal uptake in the flocculants that settled in the mixing bottles resulted in removal of Cd (0.024–0.028 μg L −1), Co (0.15–0.17 μg L −1), Cu (1.8–3.5 μg L −1) and Zn (15–29 μg L −1) from the dissolved phase. Relative to the dissolved trace metal concentrations in the tailings pond water used in the experiment (Cd=0.435–0.438 μg L −1; Co=0.738–0.763 μg L −1; Cu=3.16–5.05 μg L −1; Zn=26.6–32.6 μg L −1), the trace metal uptake exceeded 50% of these concentrations only for Cu and Zn.

A multi-component migratory pheromone in the sea lamprey

Stream finding by the anadromous sea lamprey is mediated by a pheromone released by stream-resident larvae which functions as an instinctively recognized indicator of habitat suitability. This cue is comprised of multiple components including the bile acid, petromyzonol sulfate, and at least one other as yet unidentified compound with a molecular weight of 704 daltons. These compounds are detected at picomolar concentrations and are released in large quantities; laboratory experiments demonstrate that each stream-resident larva activates at least 400L of water an hour. The actions of this pheromone are synergized by other compounds found in natural stream waters. Field studies demonstrate that the migratory pheromone plays a key role determining adult lamprey distribution in the Great Lakes. The cue does not appear to be species-specific or specialized, but rather a composite of biologically-relevant compounds that migratory adults have evolved the ability to recognize.

Potencial poluidor de rejeitos carboníferos: I - Caracterização química da água de lixiviação

O aumento da produção carbonífera no Rio Grande do Sul para o atendimento da crescente demanda por energia tem proporcionado o acúmulo desordenado de grandes volumes de rejeitos nas áreas de mineração. Esses rejeitos afetam a qualidade da água, do ar e do solo, bem como a vegetação e a fauna, podendo representar elevado comprometimento ambiental para a região. Assim, realizou-se o presente estudo com o objetivo de avaliar a composição química da água de lixiviação proveniente de colunas de PVC em experimento envolvendo diferentes espessuras de solo aplicadas na superfície do rejeito carbonífero, solo em posições de subsuperfície e solo misturado ao rejeito. A água de lixiviação foi coletada em 1993, durante o cultivo de milho e trevo-branco, em 1995, antes e após o cultivo de aveia-preta, e em 1996, após o cultivo de milho. Os dados demonstram que o baixo pH da água de lixiviação manteve solúvel parte dos elementos provenientes da dissolução do rejeito carbonífero, refletindo-se em elevada condutividade elétrica. Os elementos S, Fe, Mn, Zn, Ni e Pb foram os que ocorreram em maiores concentrações na água de lixiviação, com potencial poluidor dos recursos hídricos.

Laboratory assessment of the role of a larval pheromone and natural stream odor in spawning stream localization by migratory sea lamprey (Petromyzon marinus)

This study used large laboratory mazes and natural stream waters to test the role of olfactory cues, including a pheromone released by larvae, in spawning stream localization by migratory sea lamprey (Petromyzon marinus). We found that migratory lamprey strongly prefer stream water over lake water and that this response is dependent upon a functional olfactory system. Responses persisted among migratory lamprey even after stream water was diluted a thousand times but were not seen among non-migratory lamprey. Experiments using waters from five streams demonstrated that a larval pheromone is a key determinant of stream attractiveness: water from streams with larval populations were consistently more attractive than those without, and adding larval odor to the latter reversed this relationship. Larval odor was attractive at low, realistic concentrations, especially when presented together with natural stream water, suggesting that streams contain other odors that synergize the actions of the pheromone. Some, but not all, of the activity of the larval pheromone could be explained by two bile acids released by larvae (petromyzonol sulfate and allocholic acid). Together, these results strongly suggest that migratory lamprey locate streams using a larval pheromone. This cue could be useful in lamprey control.

Site investigation on heavy metals contaminated ground in Estarreja — Portugal

Owing to its five decades in the chemical industry, Estarreja is one of the most important industrial areas in Portugal. Intensive industrial activity along with both direct discharge of the effluents into natural water streams and uncontrolled waste disposal on the ground has, throughout the years, had strong impact on health and welfare. Recently an association between industry and local authorities was created — ERASE. The main goal for this association is to find, in co-operation with the Portuguese Environmental Ministry, a cost-effective solution to deal with the soil/sediments contamination and solid waste problem. The ERASE association planned to build a landfill for both solid waste and contaminated soil/sediments disposal. In order to determine more accurately the volume of material to be disposed of in the landfill, a site investigation was carried out during September/November 1998. The site investigation consisted mainly of systematic soil sampling at shallow depths, both within the industrial area and along the natural water streams. The site investigation results revealed high concentrations of toxic pollutants, mainly heavy metals (namely As, Hg, Pb and Zn), in the soil of the industrial area. Much higher concentrations were found in the sediments of the water streams, several kilometres away from the industrial complex (the pollution source area). In most cases, concentration increases with depth, reaching groundwater. Therefore the site investigation programme carried out could not determine the full extent of the contamination. Consequently, further studies were strongly recommended, which should include a wider and deeper investigation area and groundwater sampling.

The hepatic stress protein (hsp70) response to interacting abiotic parameters in fish exposed to various levels of pollution

Two indigenous fish species, brown trout (Salmo trutta f. fario) and stone loach(Barbatula barbatula) were exposed tocomplex stressors (mixtures of environmentalpollutants) in laboratory and semi-fieldexperiments (aquaria connected to stream water)and in field studies. As a biomarker of effect,the level of the 70 kD heat shock protein(hsp70) was quantified in the liver of troutand loach. Laboratory experiments withdifferent pollutant mixtures did not mimic thehsp70-inducing or inhibiting potential of fieldconditions, whereas effects of long-termexposure in the bypass systems showed asignificant correlation with effects recordedin feral fish. Laboratory as well as semi-fieldstudies revealed the stress response to followan optimum curve, resulting in a maximum hsp70level under stress but rather low hsp70 levelswhen stressors (chemicals, high temperature)become too severe. Consequently, the hsp70level in the liver of both species was highlyseason-dependent with two peaks in late springand fall, and rather low hsp70 levels insummer, particularly in fish exposed to waterand sediment of the complexly polluted stream.In winter, the low hsp70 level of lab controlswas elevated by exposure to natural streamwater only, but elevation did not occur undercontrol conditions independent of apre-exposure to polluted streamwater two months earlier. Despite the highvariability of the hsp70 level within one yearand among five subsequent years, patternanalysis indicated the prevailing importance ofwater temperature on stress protein response.Temperature alone, however, could not explainthe regularly observed low summer levels inhsp70. Non-linear regression analysis on labcontrols revealed an optimum temperature(Topt) for the highest hsp70 level forboth fish species. In both investigated streams, thechemical influence led to a decrease in thehsp70 level only when Topt was surpassedby the ambient temperature at the same time.Otherwise, the chemical impact resulted in anelevated hsp70 level relative to the control.The study demonstrated the suitability of hsp70stress protein levels to integrate the responsedynamics of several different stressors and,therefore, to effectively function as abiomarker for the integrated effect of allenvironmental stressors acting on an organism(not only of chemical pollution). Rathercomplex kinetics of hsp70 elevation anddecrease should be taken into consideration.

Apparent solubilities of schwertmannite and ferrihydrite in natural stream waters polluted by mine drainage

The apparent solubilities of schwertmannite and ferrihydrite were estimated from the H+, OH−, Fe3+, and SO42− activities of the natural stream waters in Korea and mine drainage in Ohio, USA. Both chemical composition of the stream waters and the mineralogy of the precipitates were determined for samples from two streams polluted by coal mine drainage. This study combines these new results with previous data from Ohio, USA to redetermine solubilities. The activities of the dissolved species necessary for the solubility determinations were calculated from the chemical compositions of the waters with the WATEQ4F computer code.Laboratory analyses of precipitates indicated that the main minerals present in Imgok and Osheep creek were schwertmannite and ferrihydrite, respectively. The schwertmannite from Imgok creek had a variable chemical formula of Fe8O8(OH)8−2x(SO4)x· nH2O, where 1.74 ≤ x ≤ 1.86 and 8.17 ≤ n ≤ 8.62. The chemical formula of ferrihydrite was Fe2O3· 1.6H2O. With known mineralogy of the precipitates from each stream, the activities of H+, OH−, Fe3+, and SO42− in the waters were plotted on logarithmic activity-activity diagrams to determine apparent solubilities of schwertmannite and ferrihydrite. The best estimate for the logarithm of the solubility product of schwertmannite, logKs, was 10.5 ± 2.5 around 15°C. This value of logKs constrains the logarithm of the solubility product of ferrihydrite, logKf, to be 4.3 ± 0.5 to maintain the stability boundary with schwertmannite observed in natural waters.

On-line monitoring of phosphate in natural water and effluent streams using sequential injection analysis

Automation of the molybdenum blue method by sequential injection (SIA) for the on-line monitoring of phosphate in natural waters is presented. Although sequential injection analysis runs at one fifth the speed of conventional FIA, it presents many advantages such as simplicity of the manifold, robustness and computer compatibility. A reduction in reagent and sample consumption is also observed. Flow detector fouling does not occur in SIA manifolds as the detector is in contact with water between analysis. The proposed SIA analyser is able to monitor phosphate in the range 0–70 mg l−1 with a standard deviation of 0.9%. The detection limit is 0.5 mg l−1 PO 4 3− .

Determination of manganese in natural water and effluent streams using a solid-phase lead(IV) dioxide reactor in a flow-injection system

The determination of manganese(II) in natural water and effluent streams, using a solid-phase reactor incorporated into a flow-injection system, was investigated. Mn 2+-ions in samples injected into a carrier stream, were oxidised by solid lead(IV) dioxide suspended on silica gel beads to form MnO 4 −-ions which were detected spectrophotometrically at 526 nm. The linear range of the system is between 1 to 5 mg l −1 with a detection limit of 0.56 mg l −1. The proposed system is suitable for the determination of manganese(II) in natural water and effluent streams with a relative standard deviation of better than 1.8%.

Alternative Predator Avoidance Syndromes of Stream‐Dwelling Mayfly Larvae

Experiments were conducted to compare the patterns, mechanisms, and costs of predator avoidance behavior among larvae of five species of mayflies that co—occur with the predatory stoneflies, Megarcys signata and Kogotus modestus in western Colorado streams. Mayfly drift dispersal behavior, use of high vs. low food (periphyton or detritus) patches, microhabitat use, positioning, and activity periodicity were observed in the presence and absence of predators in circular flow—through chambers using natural stream water. Also, distances from predators at which prey initiated escape responses were compared among prey and predator species. Costs of predator avoidance behavior were assessed by measuring short—term (24 h) feeding rates of mayflies in the presence or absence of predatory stoneflies whose mouthparts were immobilized (glued) to prevent feeding. The intensity and associated costs of predator avoidance behavior of mayfly species were consistent with their relative rates of predation by stoneflies. Megarcys consumes overwintering generation Baetis bicaudatus &gt; Epeorus longimanus &gt; Cinygmula = Ephemerella; Kogotus consumes summer generation Baetis &gt; Epeorus deceptivus = Cinygmula; Megarcys eats more mayflies than Kogotus. While Megarcys induced drift by Baetis, Epeorus, and Cinygmula, this disruptive predator avoidance behavior only reduced food intake by Baetis and Epeorus. The morphologically defended mayfly species, Ephemerella, neither showed escape behavior from Megarcys, nor any cost of its antipredatory posturing behavior. Only Baetis responded by drifting from Kogotus. No mayfly species shifted microhabitats or spent less time on high—food patches in the presence of foraging stoneflies. However, predators enhanced the nocturnal periodicity of Baetis drift, which was negligible in the absence of stoneflies as long as food was abundant. Lack of food also caused some microhabitat and periodicity shifts and increased the magnitude of both day and night drift of Baetis. Thus, Baetis took more risks of predation by visual, drift—feeding fish not only in the presence of predatory stoneflies, but also when food was low or they were hungry. All other mayflies were generally nocturnal in their use of rock surfaces, as long as food was abundant. Finally, the distances at which different mayfly species initiated acute escape responses were also consistent with relative rates of predation. This study demonstrates alternative predator avoidance syndromes by mayfly species ranging from an initial investment in constitutive morphological defenses (e.g., Ephemerella) to induced, energetically costly predator avoidance behaviors (e.g., Baetis). Although the costs of Ephemerella's constitutive defense are unknown, experiments show that prey dispersal is the mechanism underlying fecundity costs of induced responses by Baetis to predators, rather than microhabitat shifts to less favorable resources or temporal changes in foraging activity. A conceptual model suggests that contrasting resource acquisition modes may account for the evolution and maintenance of alternative predator avoidance syndromes along a continuum from Baetis (high mobility) to heptageniids (intermediate mobility) to Ephemerella (low mobility). Prey dispersal (swimming) to avoid capture results in reduction of otherwise high fecundity by Baetis, which trades off morphological defense for enhanced ability to acquire resources. Thus, improved foraging efficiency is the selection pressure maintaining the highly mobile life style in Baetis, which increases resource acquisition and fecundity, offsetting the high mortality costs associated with this behavior.

Occurrence of non-monomeric species of aluminium in undersaturated soil and surface waters: consequences for the determination of mineral saturation indices

The occurrence of a polymeric and/or colloidal Al pool was demonstrated on an experimental basis in various natural acid soil solutions and stream waters, even in the pH range 3.3–4. A number of these waters were found to be undersaturated with respect to various relevant mineral phases (amorphous Al(OH) 3 and AIPO 4, gibbsite, boehmite, variscite, imogolite, kaolinite, halloysite, jurbanite, basaluminite and alunite). Non-monomeric Al species should not occur in undersaturated aqueous systems in equilibrium; therefore the polymeric/colloidal Al pool observed in these undersaturated waters cannot be in equilibrium with monomeric Al. As a consequence, the traditional determination of saturation indices (S!) by model equilibrium calculation using total concentrations of every element is considered to be unreliable, and any valid procedure should involve the determination of monomeric species (or a part of them) by analytical speciation.

Morphological and morphometric changes in the gills of mosquitofish ( Gambusia holbrooki) after exposure to mercury (II)

Physiological measurements suggest that mercury (Hg) affects ion regulation in aquatic organisms. This implies that Hg should cause morphological changes in gills, the major ionoregulatory organ of fish. Previous studies have shown severe Hg-induced gill damage at acutely lethal Hg concentrations, but in soft fresh waters, where ionoregulatory disturbances should be most pronounced, there have been no quantitative studies to date of the effects of dissolved Hg at sublethal concentrations. We exposed mosquitofish ( Gambusia holbrooki) to nominal concentrations of 75, 150 and 300 nM Hg (II) in natural stream water of low ionic strength (Ca = 4.8 μM) in a static-renewal experiment. Mercury concentrations dropped rapidly after Hg additions, and most dissolved Hg was probably complexed with humic substances. Gills were sampled after 7 and 14 days exposure. Examination with scanning electron microscopy (SEM), indicated that Hg caused progressive loss of secondary lamellae and loss of cell surface microridges. Effects became more severe at higher Hg concentrations. Using light microscopy and morphometric techniques, the percent of gill filament occupied by chloride cells (volume density) was significantly greater in all Hg treatments than in controls, and thickness of the primary lamellar epithelium increased with Hg exposure. Thickening of the primary lamellar epithelium filled some interlamellar spaces, obliterating secondary lamellae. However, width of the exposed portions of secondary lamellae was not affected by Hg, suggesting that blood-to-water diffusion distances did not increase in response to Hg exposure. Autometallographic staining for Hg showed that the metal was distributed throughout the gill epithelium, and not concentrated inside chloride cells.

Microhabitat and Activity Periodicity of Predatory Stoneflies and Their Mayfly Prey in a Western Colorado Stream

Experiments were conducted to determine whether overlap between microhabitat preferences and activity periodicities of four mayfly species and their stonefly predators could explain species-specific differences in predator-prey encounter frequencies. Preferences for rock type (slate or granite), flow microhabitat (high or low), rock surface (top, bottom, upstream or downstream sides), and periodicity of drift and the use of rock tops were measured in a stream-side system of flow-through circular Plexiglas chambers receiving natural stream water and light levels. These parameters were compared among the predatory stoneflies, Megarcys signata or Kogotus modestus, and four species of mayflies that vary in their encounter rates with the stoneflies. Based on predator-prey encounter rates previously observed in similar chambers, we expected greater overlap between Megarcys and Ephemerella infrequens and the overwintering generation of the bivoltine mayfly, Baetis bicaudatus than with Cinygmula sp. Likewise, we expected Kogotus microhabitat use to overlap more strongly with that of summer generation Baetis than with later instars of Cinygmula and Epeorus deceptivus. Results ran counter to our predictions, indicating that microhabitats of the prey species with high predator encounter rates did not overlap more strongly with the stoneflies than did mayflies with low predator encounter rates. Most mayflies and stoneflies preferred the bottom surfaces of granite rocks, and showed few flow preferences. Most were nocturnal in their use of top rock surfaces, in drift and feeding activity periodicity. Therefore, nocturnal activity periodicities of both mayflies and stoneflies confirm that mayflies have not evolved feeding periodicity to avoid encounters with foraging stonefly predators. We conclude therefore, that neither temporal nor spatial microhabitat overlap is a reasonable explanation of differential encounter rates between predatory stoneflies and their mayfly prey. Alternative explanations for differential encounter rates are that more abundant or more mobile mayflies have higher encounter rates with predators, and effective pre-contact predator avoidance responses of other mayflies reduce their encounter rates with stoneflies.

Supply and Transfer of Oxygen in Natural Stream Water Body by Reaeration

Supply and Transfer of Oxygen in Natural Stream Water Body by Reaeration

Relationship between total dissolved organic carbon and SO 4−2 in soil and waters

Soluble organic matter in forest soils appears to play a decisive role in the mobility of SO 4 −2. In this laboratory study, a forest Haplorthod soil was leached to investigate the relationship between total dissolved organic carbon (TOC) and SO 4 −2. The leaching solutions were distilled water (DW), sulfuric acid (S), sulphuric acid and nitric acid SN, (SN1, 1:1 M and SN2, 1:2 M). The pH of the acid solutions was 3.7. Pour chemical amendments were used, control (C), lime (L), calcium phosphate (P) and lime and calcium phosphate (LP). After leaching, the soil columns (48 columns) were sampled and analyzed. Evaluation of the leachate data consistently showed that the TOC was negatively or positively correlated with the mobility of SO 4 −2 when the EC was >100 or <100 μS/cm, respectively. However, results of the extracted soil solution showed that at any given depth the TOC was positively correlated with SO 4 −2 (EC < 100 μS/cm). The positive relationship between TOC and SO 4 −2 can be attributed to competition between TOC and SO 4 −2 on the exchangeable positive charge sites. In contrast, the negative relationship between the TOC and SO 4 −2 was due to increasing salt content that caused the TOC to precipitate. Sulfate, then, becomes the dominant soluble anion and is present at a higher concentration than TOC. A negative correlation between TOC and SO 4 −2 in waters is often found in data from natural stream and lake waters. It is apparent that the salt effect and competition mechanism between TOC and SO 4 −2 provide an explanation for the increasing TOC levels in natural water, especially in areas known to be affected by with high sulfate deposition.

Direct determination of chromium(III) and chromium(VI) with ion chromatography using direct current plasma emission as element-selective detector.

A method is described that utilizes direct current plasma atomic emission spectrometry as an element-selective method of detection for ion chromatographic determination of chromium(III) and chromium(VI) species. The eluting chromium-containing species are detected on the basis of the atomic emission of chromium without any species conversion. Both anion and cation separator columns give similar results when used with varying sample matrices. By employing an on-column preconcentration procedure, the detectable concentrations of the chromium species are reduced to less than 1.0 ppb. This method is applied to the determination of chromium species in human serum, natural water, and industrial process stream samples.

Influence of pH, Aluminum, and Organic Matter on Stream Invertebrates

The interactive effects of low pH, aluminum, and organic matter on five species of invertebrates collected from low-alkalinity (160–300 μeq/L) Michigan streams were measured using five 28-d experiments during 1982 and 1983. Such interactive effects had not been tested previously for these species. Each experiment consisted of a control channel (pH 7.0), a pH modified (4.0 or 5.0), and pH plus Al (250 or 500 μg Al/L) modified channel. Three experiments were conducted using natural stream water (dissolved organic carbon of 42–47 mg C/L), while two were conducted using simulated stream water with no dissolved organic carbon added. The invertebrates included Asellus intermedius Forbes, Pycnopsyche guttifer (Walker), Lepidostoma liba (Ross), Nemoura sp., and Physella heterostropha (Say). Survival of all species was significantly decreased at pH 4 compared with controls but not at pH 5 except for Asellus (15–20% mortality). Adding 500 μg Al/L caused significant additional mortality under some conditions for all species except Physella. Removal of organic matter from the water caused complete mortality at pH 4 for Nemoura and Asellus when 500 μg Al/L was added and decreased survival for Pycnopsyche to less than 20%. Addition of citrate as an organic ligand reduced susceptibility to Al in the low-organic streamwater treatment.

Photodegradation of the Lampricide 3-Trifluoromethyl-4-Nitrophenol (TFM) 1. Pathway of the Direct Photolysis in Solution

Solutions of TFM (3-trifluoromethyl-4-nitrophenol) in distilled and in two Lake Ontario tributary stream waters were exposed to spring and fall natural sunlight in Pyrex glass containers. TFM degraded in the natural stream waters with a half-life of about 3 days. The principle identified photoproduct is shown to be gentisic acid (2,5-dihydroxybenzoic acid). A second major photoproduct is believed to be a condensation product of TFM and gentisic acid (molecular weight 371). Structures based on mass spectra are proposed for four other photoproducts. A photolysis pathway is proposed in which the primary degrdation route leads to polymeric, humic acid-like material.

The release of arsenic from contaminated sediments and muds

Abstract Stream muds and sediments, contaminated with arsenic from an industrial waste water, have been studied, under laboratory conditions, for release of this contaminant to overlying waters. The arsenic content of the muds was determined initially whereupon simulated reservoirs were prepared from these muds and natural stream water. The systems studied were: anaerobic conditions at 25o, 20o, 15°, 10°, and 4°C, aerobic conditions at 20°and 10°C with dissolved oxygen (DO) levels of 100%, 50%, 25%, and 10% saturation. Distribution coefficients, the dimensionless ratio of AS /AS ,, were determined. In both the water mud aerobic and anaerboic systems, lower temperatures produced smaller distribution coefficients (those in the 4–10oC range were about 40% of the coefficients at the higher 20–25°C range). Very little difference was found in the distribution coefficients at a given temperature between reservoirs having either 10%, 25%, 50%, or 100% of saturation of dissolved oxygen. Anaerobic conditions led to...

Das Verhalten von Wanderaalen,Anguilla rostrata, gegenüber Strömung, Salzgehalt und Lunarperiode

1. Behaviour of migratory silver eels and immature yellow eels,Anguilla rostrata, was studied in a choice-chamber apparatus. 2. Silver eels display a negative rheotaxis in both well water and natural stream water. 3. Silver eels reversed the sign of their response, to a positive rheotaxis, when salt water was introduced into the tank. 4. Rheotaxis and the salinity response are proposed as an effective orientation mechanism in the seaward migration of the silver eel. Responses are anticipatory in nature and are thought to be only part of a sequential arrangement of orientation behaviours. 5. Yellow eels showed no rheotactic response in August, but displayed a negative rheotaxis two months later into the fall. This seasonal difference in rheotactic responses of the yellow eels suggests periodic movement away from a home range in what might constitute a “dry run” for the eventual offshore migration. 6. Yellow and silver eels in heterogenous samples showed similar rheotactic responses in fresh water but were segregated by response in salt water. 7. The non-orientation of the yellow eels in the freshwater-saltwater choice suggests that it is the response to salinity which contains the yellow eel in its feeding habitat and later guides the silver eel away from it. 8. Perception of saltwater by silver eels is olfactory. 9. Locomotor activity of silver eels peaked to coincide with periods of naturally occurring darkness, between sunset and moonrise. The relationship between eel migration and lunar period is discussed in this respect. 10. Exogenous control over locomotor activity is indicated by experiments which induced activity during daylight hours. The presence of an endogenous factor is also suggested.