A comprehensive hypothesis on the migration of European glass eels (Anguilla anguilla)

The European eel (Anguilla anguilla) is a catadromous fish that spawns in the Sargasso Sea. As larvae, eels cross the Atlantic Ocean and reach the continental slope of Europe, where they metamorphose into post‐larval glass eels. These reach the continent, where some enter fresh water, some remain in marine waters, and others move between fresh and marine waters. After 5–25 years, as adult silver eels, they migrate back from fresh water to the Sargasso Sea to spawn and die. The glass eel stage is a critical step during which the eels cross the continental shelf and recruit to estuaries, where they facultatively transition to fresh water. Extensive research has been conducted to understand the behavioural mechanisms and environmental cues that aid and guide glass eels' migration. Glass eels follow odours and salinity gradients, they avoid light, and they change orientation and depth according to the tides. Recent work revealed that European glass eels also use Earth's magnetic field and lunar cues to orient. However, while we understand many aspects of their orientation behaviour, a unifying theory describing how glass eels migrate from the continental slope to fresh water is lacking. The goal of this review is to develop a comprehensive hypothesis on the migration of European glass eels, integrating previous knowledge on their orientation behaviour with recent findings on magnetic and celestial orientation. This review follows the journey of a hypothetical glass eel, describing the nature and the role of orientation cues involved at each step. I propose that, although glass eels have the sensory capacity to use multiple cues at any given time, their migration is based on a hierarchical succession of orientation mechanisms dictated by the physical properties of the environments that they occupy: (i) lunar and magnetic cues in pelagic water; (ii) chemical and magnetic cues in coastal areas; and (iii) odours, salinity, water current and magnetic cues in estuaries.     

Magnetic Compass Orientation in the European Eel

European eel migrate from freshwater or coastal habitats throughout Europe to their spawning grounds in the Sargasso Sea. However, their route (∼ 6000 km) and orientation mechanisms are unknown. Several attempts have been made to prove the existence of magnetoreception in Anguilla sp., but none of these studies have demonstrated magnetic compass orientation in earth-strength magnetic field intensities. We tested eels in four altered magnetic field conditions where magnetic North was set at geographic North, South, East, or West. Eels oriented in a manner that was related to the tank in which they were housed before the test. At lower temperature (under 12°C), their orientation relative to magnetic North corresponded to the direction of their displacement from the holding tank. At higher temperatures (12–17°C), eels showed bimodal orientation along an axis perpendicular to the axis of their displacement. These temperature-related shifts in orientation may be linked to the changes in behavior that occur between the warm season (during which eels are foraging) and the colder fall and winter (during which eels undertake their migrations). These observations support the conclusion that 1. eels have a magnetic compass, and 2. they use this sense to orient in a direction that they have registered moments before they are displaced. The adaptive advantage of having a magnetic compass and learning the direction in which they have been displaced becomes clear when set in the context of the eel’s seaward migration. For example, if their migration is halted or blocked, as it is the case when environmental conditions become unfavorable or when they encounter a barrier, eels would be able to resume their movements along their old bearing when conditions become favorable again or when they pass by the barrier.     

Shifted magnetic fields lead to deflected and axial orientation of migrating robins,Erithacus rubecula,at sunset

The migratory orientation of the robin was tested in shifted magnetic fields during the twilight period after sunset, under clear skies and under simulated total overcast. The horizontal direction of the geomagnetic field was shifted 90° to the right or left in relation to the local magnetic field, without changing either the intensity of the field or its angle of inclination. Experiments were conducted during both spring and autumn, with robins captured as passage migrants at the Falsterbo and Ottenby bird observatories in southern Sweden as test subjects. Generally, the orientation of robins was affected by magnetic shifts compared to controls tested in the natural geomagnetic field. Autumn birds from the two capture sites differed in their responses, probably because of different migratory dispositions and body conditions. The robins most often changed their orientation to maintain their typical axis of migration relative to the shifted magnetic fields. However, preferred directions in relation to the shifted magnetic fields were frequently reverse from normal, or axial rather than unimodal. These results disagree with suggested mechanisms for orientation by visual sunset cues and with the proposed basis of magnetic orientation. They do, however, demonstrate that the geomagnetic field is involved in the sunset orientation of robins, probably in combination with additional visual or non-visual cues that contribute to establish magnetic polarity.     

Orientation responses of American eels, Anguilla rostrata, to varying magnetic fields

1. The locations of freshwater yellow eels in an eight-chambered octagonal behavior tank were videotaped during six-day intervals while the animals were being subjected to normal and experimental magnetic fields. 2. The earth's magnetic field (0.5 g) was utilized for two control periods at the start and completion of each run for each animal. 3. During each run, the sequence of applied magnetic fields was +1.0, 0.0, -0.5 and -1.0 g, each being applied for a period of 24 hr. 4. Under the influence of the earth's magnetic field, the eels showed a preference for a northeast direction (27.01%). During the second control period (i.e. after being subjected to variations in the magnetic field), the animals showed a dual preference for north and northwest directions (23.02% and 25.9%, respectively). 5. In a 0.0 g field, the eels preferred the north chamber (24.43%) and the vestibule of the behavior tank (19.46%); a preference for north was also obtained with a field of +1.0 g (25.95%). 6. The preferred direction with the -0.5 and -1.0 g fields was southeast (20.93 and 26.71%, respectively).     

Influence of geomagnetism and salinity on the directional choice of eels

Summary 1. A modified arrangement has been used for the photographic recording of the directional choice of silver eels (Anguilla anguilla andA. rostrata) in a circular tank and under the magnetic action of a Helmholtz coil.2. The movement of the eels in the experimental tank was very restricted as obvious by photographs taken every 6 seconds.3. Nine male specimens from the Elbe River near Hamburg, 11 male specimens from the Elbe estuary and 3 to 5 female specimes from Rhode Island (USA) were examined for 17 to 22 hours, either in sea water or in fresh water, and yielded 225 to 946 photographs for each examination.4. In sea water, eels of all three samples changed their preferred direction from north or south to east, when the normal horizontal north (X) component of the geomagnetic field was compensated to zero. The east component (Y) remained (<–0.01 Gauss) during all observations. In fresh water, eels also preferred more easterly directions in the compensated field, but to a lesser degree.5. Reversal of the horizontal North component (X) of the magnetic earth field to the South caused no consistent change in eels examined in sea water compared with the controls. In fresh water a slight but consistent deviation to the right was observed.6. Sea water and fresh water controls includingA. rostrata showed a significant directional difference; sea water specimens chose North or South; fresh water specimens selected East to Southeast.7. Perhaps because of the very small sample ofA. rostrata, differences in this species and compared withA. anguilla were mostly not significant. But in all cases the same tendencies as in the European eels have been observed.8. The question of whether selection of swimming course is affected by the earth magnetic field itself or by magnetohydrodynamically produced electricity is discussed. Mechanisms of orientation under the influence of geomagnetism seem to be different from those observed in birds.9. An ecological example from the Elbe estuary is given which demonstrates the combined action of geomagnetism and salinity on migratory orientation of silver eels. It is evident that the salinity necessary to release northward swimming is comparatively low; this was also obvious from the results of earlier laboratory investigations (Tesch &Lelek 1973b).

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Einfluß von Erdmagnetismus und Salzgehalt auf die Richtungswahl von Aalen

Kurzfassung 25 Blankaale vonAnguilla anguilla undA. rostrata wurden in Süß- und Meerwasser sowie unter veränderten Bedingungen des erdmagnetischen Feldes hinsichtlich ihrer Richtungseinstellung untersucht. Hierzu dienten ein Rundbecken, ein Helmholtz-Spulen-Paar sowie eine photographische Registriereinrichtung. Die Aale bewegten sich in der Versuchsanlage wenig. Sie bevorzugten im Seewasser und unter natürlichen Magnetfeldbedingungen, ähnlich wie bei früheren Untersuchungen, eine nördliche oder südliche Richtungseinstellung. Kompensierung der Nordkomponente (0°) des Erdfeldes rief im Meerwasser bei Aalen aller drei Herkunftsorte (Elbe, Hamburg und Brunsbüttel, Küste von Rhode Island, USA) eine Bevorzugung östlicher Richtungen hervor. Umkehrung der magnetischen Nordkomponente auf geographisch Süd verursachte keine Veränderung gegenüber dem Verhalten im natürlichen Feld. In allen neun Tests unter verschiedenen Magnetfeldbedingungen im Süßwasser (beide Arten im natürlichen Erdfeld, bei kompensierter Nordkomponente und bei auf geographisch Süd verlagerter Nordkomponente) lag die bevorzugte Richtung zwischen 93° und 184° (SO). In vier dieser Fälle stellten sich die Aale auch in entgegengesetzter Richtung ein. Es wird diskutiert, wodurch Unterschiede in der Richtungseinstellung zwischen Feld- und Laborversuchen zustande kommen konnten und wie weit magnetische oder elektrische Reize die Richtungswahl bedingen. Ein ökologisches Beispiel aus dem Gebiet der Elbmündung macht die vermutlich kombinierte Wirkung von Erdmagnetismus und Salzgehalt auf die Aalorientierung offenbar: das in Feldversuchen ermittelte Schwimmen in nördlicher Richtung wird erst beim Eintritt der Aale in Meerwasser geringer Salinität aktiviert.

     

Spontaneous preferences for magnetic compass direction in the American red-spotted newt, <Emphasis Type="Italic">Notophthalmus viridescens</Emphasis> (Salamandridae,Urodela)

In addition to other sensory modalities, migratory vertebrates are able to use the earths’ magnetic field for orientation and navigation. The magnetic cue may also serve as a reference for other orientation mechanisms. In this study, significant evidence is shown that, even in darkness, newts (Notophthalmus viridescens, Salamandridae) spontaneously align according to the natural or to the deviated earth’s magnetic field lines, thereby demonstrating a magnetic compass sensitivity. All newts preferred compass directions close to east or west or chose the E/W axially and hence sought to maintain a specific angle or axis relative to the magnetic field vector. Such an active alignment is considered an essential precondition for magnetic orientation. When the horizontal magnetic vector was experimentally compensated, animals became disoriented. We infer that the animals have either learned the preferred magnetic direction/axis individually or that these choices are innate and could even be seasonally different as in migrating birds. It is still an unanswered question as to how and where the physical and physiological mechanisms of magnetic transduction and reception take place. The visual system and other light-dependent (radical pairs) mechanisms alone are often claimed to be in function, but this must now be reconsidered given the results from animals when deprived of light. The results may therefore point to putative receptor mechanisms involving magnetite elements in specialized magneto-receptors.     

Innate preference for magnetic compass direction in the Alpine newt, <Emphasis Type="Italic">Triturus alpestris</Emphasis> (Salamandridae,Urodela)?

Magnetic compass orientation was first discovered for migrating/homing birds in which all individuals of a population or species prefer a predictable magnetic direction during a particular migratory situation. If all other sensory cues are absent, the Earth’s magnetic field may serve as a reference for other orientation mechanisms. It will be demonstrated that alpine newts (Triturus alpestris, Salamandridae) spontaneously align according to the natural or the deviated magnetic field lines of the Earth. They are able to do this in the dark and by apparently seeking to maintain a specific angle with respect to the magnetic field vector. When the horizontal component of the magnetic vector was eliminated, animals became disoriented, and orientation became random. We infer that the animals observed had learned to prefer a particular magnetic direction following environmental/geographical cues. Alternatively, the magnetic directional alignments are innate as, e.g. in migrating birds, but these may be modified/altered according to season, age, hormonal status, and environmental factors such as “landmarks”, light-, sound-, or olfactory cues. Numerous observations of the aligning showed that the preference for a certain magnetic compass direction/axis was not only individual but also specific for the population-subgroups tested. Specimens roughly preferred magnetic directions close to east or west. However, the larvae were able to learn to align to obviously attractive hiding spots (tubes) that were provided in a direction that deviated with respect to the first magnetic preference. The new conditioned alignments were, again, referred to magnetically by the animals and remained stable, even if the hiding tubes were absent. Animals preferred that direction until, eventually, a new directional cue became attractive.     

Factors affecting recruitment of glass eels into the Grey River, New Zealand

The arrival pattern of glass eels of the shortfin eel Anguilla australis and longfin eel Anguilla dieffenbachii was studied over a two successive migration seasons in the Grey River, South Island, New Zealand. Fishing was carried out on selected nights during September to November, for 3 h per night during 2000 and 2001. The number of shortfin glass eels exceeded longfins in both years; earliest glass eels (September) were larger than later glass eels (November), and longfins larger than shortfins, but there were significant differences in size between years for both species. Environmental variables affecting recruitment differed between years, but common variables for both years were sampling date, time after high tide (incoming tide preferred), time after sunset and moon phase; the influence of moon phase appeared to be expressed both through tides (spring tides preferred) and moonlight. During the year of higher discharge variability, both increasing discharge and increased turbidity were also important.     

Orientation of anosmatic pigeons

Summary Test releases performed at five symmetrically arranged sites around the loft, at a distance of 78–99 km from it, showed that 1) anosmatic birds transported without alteration of the earth's magnetic field were completely random-oriented, 2) anosmatic birds transported in a container inside which the intensity of the magnetic field was strongly reduced were unable to orientate homewards and mostly departed according to a preferred compass direction, 3) control birds, which could smell, and were transported without alteration of the magnetic field, were homeward oriented and performed better in homing than both experimental groups. The conclusion is that anosmatic birds are unable to detect home direction at unfamiliar sites and that magnetic stimuli perceived during the outward journey are unable to substitute olfactory cues.Abbreviation PCD preferred compass direction Supported by a grant from the Consiglio Nazionale delle Ricerche     

The role of skylight polarization in the orientation of a day-migrating bird species

To assess the role of skylight polarization in the orientation system of a day-migrating bird, Yellow-faced Honeyeaters (Lichenostomus chrysops, Meliphagidae) were tested in funnel cages for their directional preferences. In control tests in the natural local geomagnetic field under the clear natural sky, they preferred their normal migratory course. Manipulations of the e-vector by depolarizing the skylight or rotating the axis of polarization failed to affect the orientation as long as the natural geomagnetic field was present. When deprived of magnetic information, the birds continued in their normal migratory direction as long as they had access to information from the natural sky, or when either the sun or polarized light was available. However, when sun was hidden by clouds, depolarizers caused disorientation. — These findings indicate that polarized skylight can be used for orientation when no other known cues are available. However in the hierarchy of cues of this species, the polarization pattern clearly ranks lower than information from the geomagnetic field.     

Manipulations of polarized skylight calibrate magnetic orientation in a migratory bird

1. Young migratory birds enter the world with two representations of the migratory direction, one coded with respect to the magnetic field, the other with respect to celestial rotation. The preferred magnetic direction of migratory orientation is malleable early in life: it may be calibrated by celestial rotation, observed either in daytime or at night.
2. Previous experiments showed that early experience with skylight polarization was necessary for calilbration to occur in daytime. In this study, we performed a direct manipulation of patterns of polarized skylight at dawn and dusk.
3. Hand-raised Savannah sparrows (Passerculus sandwichensis) were allowed to observe the clear sky for 1 h prior to local sunrise and for one h following local sunset. They never saw the Sun nor stars. The birds observed the sky through bands of polarizing material (HNP'B) aligned with the e-vector axis in one of three orientations with respect of the azimuth of sunrise and sunset: group 1) 90°; group 2) 45° CW; group 3) 45° CCW.
4. Tested indoors in covered cages in both shifted and unshifted magnetic fields, the autumn migratory orientation of the three groups differed significantly. Group 1 oriented magnetic N-S, group 2 oriented magnetic NW-SE, and group 3 oriented magnetic NNE-SSW. These observed orientation directions are very close to those predicted by the manipulations of polarized skylight.
5. These results indicated that a fairly simplified, static polarized light pattern viewed a limited number of times only in dawn and dusk snapshots is sufficient to produce calibration of the preferred magnetic migratory orientation direction.

     

Estuarine habitat preferences of <Emphasis Type="Italic">Anguilla australis</Emphasis> and <Emphasis Type="Italic">A. reinhardtii</Emphasis> glass eels as inferred from laboratory experiments

We tested the habitat preferences of Anguilla australis (shortfin) and A. reinhardtii (longfin) glass eels using circular tanks in an aquarium, containing four types of estuarine habitat (sand, mud, rocks/cobbles and seagrass). Shortfin eels either showed a tendency to occur in heterogeneous habitats, or in rocks/cobbles. Longfin glass eels showed a significant preference for rocks/cobbles in both experiments. Tests on shortfin and longfin glass eels in tanks with only rocks/cobbles available showed that eels were not clumped, indicating that individuals select habitat for re-settlement independently. Therefore, we assumed that the uneven distribution of glass eels observed in the habitat type experiments were the result of habitat preference. Given a choice of habitats in tank experiments, shortfin and longfin glass eels preferred habitats containing structure, and in particular, rocks/cobbles.     

Magnetic Orientation in the Savannah Sparrow

Although magnetic compass orientation has been reported in a number of invertebrate and vertebrate taxa, including about a dozen migratory bird species, magnetic orientation capabilities in animals remain somewhat controversial. We have hand-raised a large number of Savannah sparrows (Passerculus sandwichensis) to study the ontogeny of orientation behavior. Young birds with a variety of early experience with visual and magnetic orientation cues have been tested for magnetic orientation during their first autumn migration. Here we present data from 80 hand-raised sparrows, each tested several times in both normal and shifted magnetic fields. Birds reared indoors with no experience with visual orientation cues showed axial north-south orientation that shifted by almost exactly the magnitude of 90° clockwise and counterclockwise shifts in the direction of magnetic north. Other groups of birds with varying early experience with visual orientation cues showed different preferred orientation directions, but all groups shifted orientation direction in response to shifts in the magnetic field. The data thus demonstrate a robust magnetic orientation ability in this species.     

Thermal preferenda and behavior of Atlantic eels (genus Anguilla) in relation to their spawning migration

Synopsis The final preferred temperatures (FPTs) of adult premigratory and migratory life-history phases of American eels, Anguilla rostrata, were determined by chronic tests in a horizontal thermal gradient. Mean FPTs were between 17 and 20°C and were not significantly different between life-history phases, acclimation temperatures, illumination regimes, photoperiods or sexual maturation states. Thermal behavior of eels was highly variable, both among individuals of the various test groups and among repeated tests of single individuals. Light inhibited behavioral thermoregulation by promoting shelter-seeking. The following inferences are drawn from the laboratory findings and observations of migrating A. rostrata and A. anguilla (European eels) in the North Atlantic: (1) decreasing temperatures may initiate downstream migration of silver eels, (2) eels may select temperatures close to their FPT in thermally stratified environments, but will tolerate higher and lower temperatures depending on illumination or other physical constraints, (3) the oceanic phase of the migration to the Sargasso Sea may take place at relatively shallow depths in the open ocean, probably within the upper 1000 meters. The strong eurythermality observed in eels may facilitate their occupation of and migration through thermally diverse and unpredictable habitats.     

Anguilla japonica is already magnetosensitive at the glass eel phase

Magnetosensitivity of the Japanese eel Anguilla japonica at the glass eel phase (newly metamorphosed juveniles) was examined by conditioning and electrocardiography. The glass eels were conditioned to an imposed magnetic field of 192 473 nT parallel to the fish body placed along the earth's west‐east axis. After 10 to 40 conditioning runs, all the glass eels exhibited a significant conditioned response ( i.e . slowing of the heart beat) to a 192 473 nT magnetic field and even to a 12 663 nT magnetic field that combined with the geomagnetic field (32 524 nT) at the laboratory and produced a resultant magnetic field of 21° easterly. These results indicate that glass eels have high magnetosensitivity and probably acquire geomagnetic information early in life. It is hypothesized that silver‐phase adult eels find their way back to the oceanic spawning ground by reversing the geomagnetic direction that had been detected and 'memorized' during the glass eel phase when migrating from the open ocean towards the continental shelf and coastal waters.     

Microelectronic detection of activity level and magnetic orientation of yellow European eel, <Emphasis Type="Italic">Anguilla anguilla</Emphasis> L., in a pond

Synopsis We studied the circadian and monthly activity, the distribution patterns, and orientation to the earths magnetic field, of yellow (non-migratory) female eels in a freshwater pond by means of microchips injected into their muscles. Detectors for microchips mounted in tubes were placed in the pond to detect if eels oriented themselves with respect to earths magnetic field. Based on the frequency of tube visits (search for shelter), the data indicated that the presence of eel in the tubes decreases gradually during the study period. We saw more activity during the night in the first months. There was a seasonal component in the orientation mechanism, with a significantly lower preference component in the summer compared to the fall. A preference for tubes oriented in a south-southwest direction (the direction of the Sargasso Sea) in fall suggests an orientation to the earths magnetic field.     

Experiments on the effects of a continuous 16.7 Hz magnetic field on melatonin secretion, core body temperature, and heart rates in humans.

The present study investigated the hypothesis that a strong extremely low frequency magnetic field partially suppresses the synthesis of melatonin and subsequently elevates the core body temperature. Seven healthy young men (16-22 years) took part in a control and in an exposure session. Three men experienced first the control and then the exposure session, four men experienced the sessions in reverse order. Control sessions were performed as constant routines, where the participants spent 24 hour periods continuously in bed while air temperature was 18 degrees C, illumination less than 30 lux, and the sound pressure level 50 dBA. The exposure sessions differed from that protocol only between 6 pm and 2 am when a strong extremely low frequency magnetic field was continuously applied (16.7 Hz, 0.2 mT). Assuming that the participants were unable to perceive the field consciously, they were blind against the actual condition. Salivary melatonin levels were determined hourly; body core temperatures and heart rates were registered continuously throughout. Neither of these parameters revealed alterations that can be related to the influence of the magnetic field. The present results, taken together with other investigations using that particular field, lead to the hypothesis that the effects most likely, occur, only after repetitive exposures to intermittent fields.     

Three-dimensional migratory behaviour of European silver eels (Anguilla anguilla) approaching a hydropower plant

The global population of European eel (Anguilla anguilla) is rapidly declining, and migration barriers in rivers are believed to be one of several key causes. While progress has been made in the development of bypass solutions, they are often constructed based on a limited knowledge of swimming behaviour. A bypass close to the stream bed is often recommended at fish passage facilities to accommodate downstream eel migration. The results of this recommendation are poorly studied, and the few studies that exist show varying bypass efficiencies. The current study used acoustic telemetry with depth sensors to explore the three-dimensional migratory behaviour of downstream-migrating silver eels. The eels were tracked as they approached a hydropower plant with a state-of-the-art angled bar rack and full-depth bypass. Downstream and upstream swimming differed in preferred vertical and lateral positions. During periods of local downstream movement, the density of observations was largest in the upper middle section, away from the river boundaries and in higher velocities. Conversely, when moving upstream, eels tended to avoid the upper layers of the middle part of the river, swimming closer to the riverbed and using the bank areas to a greater extent. Downstream-moving fish swam higher in the water column during night and in turbid conditions (high discharge). When approaching the impassable bar rack and the full-depth bypass, the eels searched most intensely but not exclusively along the bottom third of the rack, often exploring at new depths after changing direction. The impediment passage efficiency was 100% when both bypass solutions were considered. The study provides knowledge of the swimming behaviour of silver eels, which is relevant for the design of bypass solutions for eels at migration barriers.     

Observations on the Nocturnal Activity and Feeding Behavior of Anguilla japonica Glass Eels Under Laboratory Conditions

Glass eels of the temperate anguillid species, Anguilla japonica, clearly showed a nocturnal activity rhythm under laboratory conditions. Light–dark cycle was a determinant factor affecting their photonegative behavior, nocturnal locomotor activity, and feeding behavior. Under natural light conditions, glass eels remained in shelters with little daytime feeding, but came out to forage during darkness. They moved and foraged actively in the following dark, and then their activity gradually declined possibly because of food satiation. They finally buried in the sand or stayed in tubes immediately after the lights came on. Under constant light, glass eels often came out of the shelters to forage in the lights but spent little time moving outside the shelters (e.g. swimming or crawling on the sand). Glass eels took shelter to avoid light and preferred tubes to sand for shelter possibly because tubes were much easier for them to take refuge in than sand. Feeding and locomotor activities of the glass eels were nocturnal and well synchronized. They appeared to depend on olfaction rather than vision to detect and capture prey in darkness. Feeding was the driving force for glass eels to come out of sand under constant light. However, in the dark, some glass eels swam or crept actively on sand even when they were fully fed. The lunar cycles of activity rhythms of glass eels that have been observed in some estuarine areas were not detected under these laboratory conditions.     

ILLUMINATION OF UPPER AND MIDDLE VISUAL FIELDS PRODUCES EQUIVALENT SUPPRESSION OF MELATONIN IN OLDER VOLUNTEERS

Bright light treatment has become an important method of treating depression and circadian rhythm sleep disorders. The efficacy of bright light treatment may be dependent upon the position of the light-source, as it determines the relative illumination in each portion of the visual field. This study compared illumination of upper and middle visual fields to determine whether melatonin suppression is different or equivalent. Thirteen older volunteers received three illumination conditions in counterbalanced orders: 1000 lux in the upper visual field, 1000 lux in the middle visual field, or dim diffuse illumination <5 lux. A four-choice reaction time task was performed during tests to ensure eye direction and illumination of the intended portion of the visual field. Illumination in the upper and middle visual fields significantly suppressed melatonin compared to <5 lux (p<0.001). Melatonin suppression was not significantly different with upper or middle field illumination. These results indicate that bright light treatments placed above the eye level might be as effective as those requiring patients to look directly at the light source. Clinical comparative testing would be valuable. In addition, this study demonstrates that significant suppression of melatonin may be achieved through the use of bright light in healthy older volunteers.