THE BEHAVIOUR OF MINNOWS (PHOXINUS PHOXINUS L.) IN A LIGHT … · In the gradient tank, normal, well-fed minnows were almost constantly active. Fish most usually turned at the end

VOLUME 33, No. 2 JUNE 1956

THE BEHAVIOUR OF MINNOWS (PHOXINUSPHOXINUS L.) IN A LIGHT GRADIENT

BY P. M. J. WOODHEAD

Zoological Laboratory, University of Cambridge, andFisheries Laboratory, Lotoestoft

(Received 16 September 1955)

INTRODUCTIONIt is well established that light can be a factor influencing the behaviour and thedistribution of fish. Three centuries ago Izaak Walton wrote: ' For you are to note,that the great old Trout is both subtil and fearful, and lies close all day, and doesnot usually stir out of his hold', and ' In the night, the best Trouts come out oftheir holes... draw your bait over the top of the water to and fro' and if there be agood Trout in the hole, he will take it, especially if the night be dark,—for then he isbold, and lies near the top of the water'. In recent years there has accumulated aconsiderable number of field observations of the diurnal movements of fish sug-gesting correlations with changes in light intensity. The modern development ofecho-sounding techniques for fish detection has demonstrated considerable verticalmigrations of fish from upper waters at night into deeper waters during the day.Such vertical migrations occur both in the sea (Krefft & Schiiler, 1951) and in freshwaters (Scharfe, 1951) and have been related to definite light intensities (measuredin arbitrary units) for certain marine pelagic fish (Richardson, 1952). A horizontalmigration into shallow, inshore feeding grounds as light intensities decreased atsunset has been described for yellow perch, Perca flavescens Mitchill. (Hasler &Bardach, 1949; Scott, 1955), and for rock bass, AmbloplitesrupestrisRaf. and suckers,Catastomus commersonU Lac. (Spoor & Schloemer, 1938) in North American lakes;with increasing light intensities after sunrise, the fish moved offshore again intodeeper waters.

Relatively little work has been carried out analysing the causal relationshipsbetween such movements and changes in illumination. The present experiments,which form part of a laboratory study of the behaviour of fish in relation to variouslight stimuli, were carried out at the suggestion of Dr F. R. Harden Jones, who verykindly allowed me to use a light gradient tank which he had constructed.

MATERIAL AND METHODSThe fish used in the experiments were adult minnows, Phoxinus phoxinus L., from5-0 to 6-6 cm. long, caught in the River Cam. They were kept in glass accumulatorjars filled with water which was constantly aerated and changed at regular intervals.The fish were fed daily on Tubifex and, under these conditions, remained in a healthycondition throughout the experiments.

17 Exp. BioL 33, 2

258 P. M. J. WOODHEAD

All experiments were carried out in a blacked-out room at the ZoologicalLaboratory, Cambridge. The apparatus used consisted of a Perspex trough 360 cm.long and 15 cm. wide, filled with water to a depth of 7-5 cm. Sheets of frosted glasswere laid end to end on a frame 15 cm. above the water surface, and over these wereplaced sheets of thin black cardboard which were bent in such a way as to form alight-proof tunnel extending the length of the trough. An electric light, enclosedin a metal box, was placed at each end of the tunnel and mounted so that the troughwas illuminated by diffuse light passing down through the frosted glass. The lightsat either end of the tank could be switched on separately or both together. Anattempt was made to eliminate some of the directional properties from the gradientby including vertical black paper screens between the frosted glass and the water-level inside the tank. The screens, being continued to bench level outside the tankalso served to subdivide the tank into twenty units, or compartments, for purposes ofobservation.

The intensity of the light falling vertically on to the water surface was measuredat a number of points in the tank with a photomultiplier described by Jones (1956),which was sensitive to io~s metre-candles (m.c). The distribution of light intensityin the gradient tank is shown in Fig. 1.

Observations showed that for a period varying from 4 to 12 hr. after being placedin the gradient tank the minnows swam up and down the trough at abnormally highspeeds, often turning rapidly. After this time the fish appeared to become adaptedto the tank, swimming more steadily and in a less confused manner.

The experimental procedure adopted was to place a single fish in the gradient tankone evening and leave it overnight with the light shining at one end of the tank. Onthe following morning the observer entered the dark room as quietly as possible andsat in a position about 3 ft. away from the centre of the tank. After the observer hadbeen in the room for about 15 min., the successive positions of the fish in the tankat 10 sec. intervals were recorded in pencil on a small pad of paper, the observerremaining quiet, and moving as little as possible in order not to disturb the fish. The10 sec. time intervals were determined by use of a Palmer time-clock connected tomuffled earphones which produced a just audible click; this timer was left runningthroughout the time that the fish was in the experimental tank and had no apparenteffect on the behaviour of the fish.

During each experimental period the position of the fish in the tank was recordedfor up to an hour, the lights were then reversed, so that the light now shone at thepreviously dark end of the gradient, and the fish was left for 2 hr. before a furtherperiod of observations was made.

In all experiments, except those with hungry fish, a small number of Tubifex werepresent in the tank. These were placed in all parts of the tank in order that resultsmight not be influenced by the distribution of the worms. Well-fed fish generallyappeared to ignore the Tubifex, and only on a few occasions were such fish seen tosnap up a worm. No water circulation was run during the day when experimentswere in progress, but the water in the tank was completely renewed each evening.Using this procedure it was possible to obtain uniform and repeatable results. The

Behaviour of minnows in a light gradient 259

Frost ed

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glass

- U -

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i i r iTank ~ - ~

screeni

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Mil l

y 100

ght

inte

nsi

ty (

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i

0001

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-

-

100W.

_ o '

W

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Right

— — ,

-

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10

0-1

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0O01

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Right

Fig. 1. Diagram of the light gradient tank and the distribution of the vertical light intensity in thetank using different light sources: (a) 200 W., (6) 100W., (c) 15 W. In each graph the distribu-tion for illumination from the left-hand end of the tank is shown as - • - and from the right-handend as - o - .

temperatures at each end and in the middle of the tank were usually taken aftereach period of observation. On some occasions slight temperature differences ofabout i° C. were observed, but the behaviour of the fish did not show any con-sistent relationship to such temperature gradients.

The results presented below are based upon 208 experiments involving between60,000 and 70,000 recordings.

17-2

26O P. M. J. WOODHEAD

RESULTS

The behaviour of normal fish

In the gradient tank, normal, well-fed minnows were almost constantly active. Fishmost usually turned at the end of the trough in the dark part of the gradient, but onapproaching the light end they turned back to the dark at a definite point beforereaching the lightest parts of the tank. All fish exhibited this behaviour, and it wasconsidered that the minnows were limited in their movements by light above a certainintensity. The fish generally swam quite freely between this limit and the darkerend of the tank and their behaviour could be described as 'normal and unexcited'.On approaching the limiting intensity their speed of swimming often decreased, andthey usually turned to swim back to the dark. Some fish moved very slowly whenalmost at their limit, pausing motionless in mid-water or sinking slowly to thebottom, to remain still, for periods varying from a few seconds to hah0 an hour. Allfish in the gradient orientated head towards the lighter end of the tank (positivephototaxis) when inactive in mid-water or sinking to the bottom, and only orientatedaway from the light when swimming actively into the darker part of the tank. Nofish was observed pausing, more than momentarily, orientated away from the light.Although fish also turned below their limit both when approaching and retreatingfrom the light, these lower turning points varied in position. No lower light limitwas observed, although this was investigated to the limit to which fish could beseen. The movements of two typical minnows in the tank are illustrated in Fig. 2 a, b.

When a fish passed the limiting light intensity its behaviour changed. It swamjerkily, darting forward, pausing, then darting forward again. The swimming move-ments seemed exaggerated and the fish appeared 'excited', perhaps darting forwardto the lightest part of the tank, to turn and swim, with great rapidity, back below thelimit. The time spent by any fish above its limit was usually small (from o to 5 %of the total time).

On reversal of the lights fish moved to the opposite end of the tank, their move-ments now being limited at a position where they had previously moved freely whenthe newly lit end of the trough had been dark. In a few experiments both lightswere switched on, restricting the movements of the fish to the centre of the tank.Immediately after the reversal of lights fish exhibited a ' shock-reaction' initiallygoing into the darker parts of the tank then later venturing farther and farther intothe light During this period the fish appeared rather excited, moving rapidly, andturning often, but after about 15 min. they had usually begun to assume their normalbehaviour and were swimming freely below their limit. Selection of the limitvaried with individuals but often tended to improve with time, as is shown inFig. 2c.

The minnows appeared to maintain their position at or below their limit by acomparison of light intensities. From time to time a fish has been observed to havea well-established limit at one point in the tank and to stay motionless in mid-waterat this point. Respiratory movements (the 'jetting' of water from the opercula)


would slowly move the fish to a position perhaps 6 cm. farther up the tank into thelight. The fish would then turn and swim back to the dark, or, occasionally, main-taining its position facing the light, it would propel itself gently backwards withits pectoral fins until it reached its limit, whereupon the reversed pectoral beatsceased. This latter reaction was totally different to the normal swimming movements

R.20 r20 R.

20 R,

12 18 2-4114Time (mlrO

120 126

Fig. 2. The tracks to and fro of three typical well-fed minnows in the tank: (a) ioo W. right light;(6) ico W. right light, (c) the tracks of a fish immediately after light reversal, showing ' shockreaction', followed by the movements of the fish 90 min. later.

of minnows seen in this apparatus, and was a most convincing demonstration of thepresence of a light limit. Such behaviour must depend upon an actual comparisonof intensities by the fish.

As the light intensity in the gradient decreased roughly in inverse proportion tothe square of the distance from the source, it was possible that the minnows mighthave been responding to a given rate of change of light intensity rather than to anyparticular intensity. This possibility was investigated by varying the strength of thesource, using different electric light bulbs in different experiments. In this way theposition of any intensity could be moved up or down the gradient, although the rate ofchange of light intensity remained the same (Fig. 1). The results of these experiments


showed that the fish were responding to light intensity alone; as the limitingintensity was moved along the tank, the length of tank in which the fish movedfreely and unexcitedly increased or decreased accordingly. In certain experimentsthe limiting effect of light in this apparatus was brought out particularly clearlywhen individual fish were 'held' in the darkest part of the tank by using a 500 W.,or, in one case, a 200 W. light source; in these experiments the movements of min-nows were restricted to the darkest one or two compartments for up to 45 min.,when observations ceased. The percentage distributions in the tank when differentlight sources were used are shown for two fish in Fig. 3.

Compartments Compartments

Fig. 3. The distribution of typical well-fed minnows in the gradient tank shown as the percentage ofthe time spent in each compartment. Positions of a given limiting intensity using differentstrength light sources are indicated by ( 4-): (a) four distributions of a fish limited at about0-030 m.c; (b) four distributions of a fish limited at about 0*14 m.c

Each minnow had its own limiting intensity, which might differ from those ofother fish. A different limit was also demonstrated for the same fish on differentoccasions; however, individual fish held the same limit for at least 48 hr. duringexperiments. The distribution of the values of limiting light intensities observedfor normal, well-fed minnows is shown in Fig. 4. Although these values are mainlyderived from different fish, some values of different limits observed in the sameindividual on different occasions have also been included. Each limit included inthis diagram has been derived from the results of from two to six experiments, inwhich the same limit was maintained by an individual fish.

During the course of earlier experiments all fish observed in the gradient tankwere kept in the dark room for some days or even weeks before being used in experi-ments, in order to eliminate the vestiges of possible diurnal activity rhythms, whichit was thought might occur. Later experiments were carried out with fish taken


directly from aquaria which had been exposed to normal diurnal changes inillumination. No difference was observed in the behaviour of these fish, the lightlimits in the gradient being the same for both types of fish.

0001 00017 0003 0006 001 0017 003 006Log light intensity (m.c)

01 017 03 06

Fig. 4. The frequency distribution of the light limits of minnows in the gradient tank.

The effect of htmger on behaviour

Jones (unpublished) carried out a number of preliminary experiments on thebehaviour of fish in the gradient. From his results he suggested that whereas well-fed fish were mainly found in the darker parts of the tank, hungry fish spent aconsiderable time at high light intensities. Further experiments were performed toinvestigate the effect of hunger on the light responses of minnows.

The fish used in this series of observations had not been fed for about 10 dayspreceding the experiments. The fish were observed by the standard procedure,except that no Tubifex were present in the tank. All experiments with hungry fishwere carried out using 100 W. lamps as light sources. After the behaviour of ahungry fish had been observed for at least one day, a small ball of Tubifex was placedin a lighter part of the tank. The food was left in the tank for about 20 min., althoughthe fish ate most of the worms in a very short time; after this period any wormsremaining were siphoned out of the tank. Observations were resumed 2 hr. later.

Jones's results were fully confirmed. Hungry fish spent much of their time in thehighest light intensities in the tank, making repeated excursions into that region.Indeed, on two occasions a hungry fish remained in the lightest compartment of thetank for up to half an hour (when observations ceased); such behaviour was neverobserved at any time during the course of the experiments with fed fish. After


feeding the minnows retired to the darker parts of the tank for about 24 hr., afterwhich they showed an increasing tendency to return to the hght with time.

Although the lights were regularly reversed during these experiments, by referringthe percentage distribution of the fish to the lighted end of the tank, the sum of thedistributions observed before feeding can be compared with the sum observedduring the first 24 hr. after feeding. The summed results of a number of experimentswith two typical hungry minnows are shown in Fig. 5. The time spent in the fourlightest compartments (where the greatest change in light intensity occurs) is highin hungry fish, but falls to a very low value after feeding.

100YV. left

1 0 0 W - 'eft 100 W. left

Compartments Compartments

w e>)Fig. 5. The distributions of two typical hungry fish (a) and (6), in the gradient tank, shown as the

percentage of the time spent in each compartment. The distribution of the fish before feeding isshown in the upper diagrams, and their distribution after feeding in the lower diagrams. Note themarked change in distribution in the four lightest compartments.

The behaviour of minnows prior to spawning

In mid-April, immediately prior to their breeding season, shoals of minnowswere seen on sunny days swimming at very high hght intensities in shallow streamsaround Cambridge. These observations were in contrast to the experimental resultsobtained with minnows in the light gradient, and it was decided to carry out furtherexperiments with prespawning fish in this apparatus. The fish used in these experi-ments had been kept in aquaria in the Cambridge Zoological Laboratory for atleast 7 months prior to the experiments, and belonged to the same stock from whichfish were taken for earlier work. These fish had begun to assume nuptial coloration,particularly noticeable being the milk-white spots at the bases of the paired fins andon the opercula, described by Frost (1943) as characteristic of the spawning livery.

Unlike the minnows examined in the late autumn and winter, these prespawningfish swam rapidly up and down, turning often and entering all regions of thegradient. Although they generally showed a tendency to spend a greater time in thedarker parts of the tank, their excursions into the lightest parts were frequent andlittle evidence of any fixed hght limit was shown even when strong light sourceswere used. These fish examined in April were much more active than the previousfish examined in the winter, as can be seen by comparing the movements of the twotypical prespawning fish shown in Fig. 6 with those of the fish shown in Fig. 2.

The method of recording the movements of minnows in the gradient tank, andlater plotting these results against time, allows calculation of the speeds of swimming


I I I I I I I I I I M I I I I M

Compartments


of fish in the apparatus. In this way the activity of these minnows just prior tospawning, as expressed by mean swimming speeds in single experiments, may becompared with the activity of minnows in December. The mean swimming speedsobserved in twenty-four experiments on prespawning minnows made between nand 22 April 1954 have been compared with similar results for 24 hr. experimentsmade in December 1953. The December experiments were taken 'as they came',but the two series are strictly comparable since equal numbers of experimentsusing the same light sources and illuminated from the same end of the tank havebeen used. The mean swimming speed for the December fish was 2-57 cm./sec.,

151

2 4 6 8 10 12Mean swimming speed (cm./sec)

2 4 6 8 10 12Mean swimming speed (cm./sec)

Fig. 7. The frequency distribution of the mean swimming speeds of fish observed in December (a)and in April (6) in the gradient tank.

whereas the mean swimming speed for the fish examined in mid-April was 675 cm./sec., an increase of 159 %. The distribution of these results for the December andApril series are shown in Fig. 7. The temperature of the water was 17-190 C.during the December experiments and 18-200 C. during the mid-April experiments,and it seems improbable that the difference in activity observed at these differenttimes of year can be attributed to the small difference in temperature.

No attempt was made to distinguish male from female fish in these experimentsand possible differences in the activity of the two sexes have been ignored.

DISCUSSIONAt the time of completion of these experiments Trincker (1954) published diagramsof the movements of a shoal of goldfish in an apparatus which would appear to actas a type of light gradient. Although his work was carried out for different purposes,the movements of the shoal in the control experiments strongly resemble themovements of minnows in the light gradient. Like single minnows, the goldfish


shoal was constantly active but did not swim into the lightest parts of the apparatus,apparently being limited in their movements by light above a certain intensity.Trincker also conditioned the goldfish to feed in the lightest parts of the tank. Theconditioned fish then modified their normal behaviour pattern, spending severalhours at high light intensities. Hungry minnows in the gradient similarly failed torespond to light intensities which they avoided when well-fed. It is not possible todetermine from the present experiments whether this reaction was innate, or due toa previously learnt association between light and food.

Hoar, MacKinnon & Redlich (1952) observed the behaviour of small groups ofPacific salmon fry and smolt in a light gradient tank. Their results were presentedsimply as the percentage distribution of the fish in the lighter and darker halves ofthe tank. It is apparent that different species, and different stages within the samespecies of salmon, differed in the percentage of the time spent in the lightest half ofthe gradient, but the method of presentation does not indicate whether the salmon,like the minnows, had a definite light limit. Similar light gradient experimentswere carried out by Kawamoto & Nagata (1952), although in their apparatus thelight was not diffuse. No evidence of a limit was shown for either of the speciesstudied, but, in view of the observations on the initial behaviour of minnows afterbeing placed in the gradient, it seems probable that the results of Kawamoto &Nagata are based largely on the observation of shock reactions in the fish.

Scharfe (1953) has reviewed a number of field experiments on the reactions offish to electric lights. Fish, singly, or in shoals, moved away from a light shone onthem; there was often an initial shock-reaction, but a final position was taken up at adefinite distance from the light; this distance varied with species and with physio-logical state within a species. Varying the intensity of the light proportionatelyincreased or decreased the distance of the fish from the source. The results of theseexperiments suggest that the fish were limited by light above a certain intensity but,also, that they had a positive phototaxis orientating their movements towards thelight source. In all these field experiments the lights used would act as point sources;nevertheless, the results seem to agree well with the results of the light gradientexperiments where diffuse, although directional, illumination was used.

In experiments with a number of minnows in a 'split-tank', half of which hadbeen covered to exclude the light, Jones (1956) found that the fish remained in thedarkened half of the tank until the light intensity outside was reduced to between0-17 and 0-08 m.c. Below this intensity minnows crossed freely into either half ofthe tank. It seems that the minnows in the split-tank were limited in their movementsby light above about o-i m.c., a value which agrees well with that observed in thelight gradient.

Discrimination of the light limit in the gradient tank was very fine. A change ofone-tenth of a log unit at 0-002 m.c. or 0-006 m.c. was readily and repeatedly detectedby individual minnows. These results are perhaps surprising, in view of the carefulwork of Bull (1928, 1932) with marine fish. In Labrus bergylta, Bull (1928) foundthat discrimination could only be made with difficulty between 7500 and 80 m.c.;in later experiments (Bull, 1932) with BletmiusphoUs he found that the fish could not


be conditioned to discriminate between 20,000 m.c. and intensities down to 36 m.c,although discrimination was made between higher light and 20 m.c. He concludedthat intensity discrimination in fish was extremely poor.

The apparent discrepancy between the present results and those of Bull probablylies in the lower order of the light intensities used in the gradient tank. The visualrange of fish extends well below the light limit described here, and, in view of Bull'swork, it is reasonable to conclude that the limit to the movements of the fish was alsothe limit (threshold) to some physiological mechanism, rather than the response toone intensity selected from the middle of a range which could all be perceived.

Scharrer (1928) conditioned blind minnows to respond to light. The fish onlyshowed a response when the head was illuminated, and removal of the pineal andassociated parts of the mid-brain, the pineal-complex, abolished the response.Scharrer obtained consistent reactions from fish at intensities down to 0-5 m.c,whilst the lowest intensities at which he observed responses in particularly well con-ditioned individuals were 0-0226 and 0-017 m-c- If these results are comparedwith Fig. 3, showing the distribution of limits in the gradient, it is apparent that thethreshold for pineal stimulation and the limiting light intensity are of the same order.These results suggest that the pineal might be the receptor controlling the limit ofminnows in the gradient.

Certain evidence derived from experiments on other fish lends support to thissuggestion. Hoar (1955) has investigated the role of the pineal organ in the photo-tactic responses of the sockeye salmon smolt Onchorhynchus nerka. Unoperatedsmolt were described as 'negatively phototactic'; this was expressed as the per-centage of fish found in the exposed half of a circular tank, half of which had beenartificially darkened. Blind smolt had essentially the same percentage distributionas the controls, whereas the distribution of bund fish in which the pineal had alsobeen destroyed approached random. In similar ' split-tank' experiments Breder &Rasquin (1947) related the sign of the ' phototaxis' to the degree of exposure of thepineal region in the blind Mexican characins belonging to the genus Anoptickthys.Later (Breder & Rasquin, 1950) they extended the relationship to a number ofnormal, eyed teleosts. Kuhn & Kahling (1954) have made a more detailed studyon one of the blind characins, Anoptichthys jordani Hubbs & Innes. They con-ditioned the blind fish to respond to a light stimulus and found that both young andolder fish had the same threshold for stimulation, despite marked differences in thestate of degeneration of the optic cysts. Whilst they point out that the sensoryreceptors for this reaction are still not known, it is of interest that the threshold forstimulation is about 0-12 m.c, a value similar to that found by Scharrer (1928)for stimulation of the pineal in blind minnows.

The possibility that the light limit in the gradient tank is the threshold for thechange from rod to cone vision in the eye, rather than the pineal threshold, cannotbe ignored. From the results of experiments on visual acuity in the minnow, sup-ported by observations on retinal histology, Brunner (1935) concluded that thechange-over from rod to cone vision occurred between 0-008 and 0-002 m.c. Thisvalue is lower than the majority of the limits observed in the gradient, and a log unit


lower than the mean of these limits. However, in the sunfish, Lepomis, Wolf &Zerrahn-Wolf (1935) found the threshold for cone vision, as determined by flickerfusion studies, to be 0-4 m.c.; a value as much greater than the gradient limits asBrunner's result was below them. Clearly, further work is required to determine thetrue nature of the receptor for the light limit in the gradient, but so far it has notbeen possible to carry out the critical experiments with blind fish.

The increased activity prior to spawning observed in minnows in the lightgradient, agrees well with field observations. The experiments were made about3 weeks before the peak spawning dates reported by Frost (1943) for minnows inWindermere. Frost observed considerable activity in the fish at this time; theymigrate from lakes to running waters, from deeper rivers into shallow side streams,and may leap clear of the water in attempting to pass small barriers.

The experiments described here were mainly carried out in the late autumn andthe winter. Frost (1943) reports that in Lake Windermere minnows are not seenin open water at this time, but are found under stones. However, in the summer,she found shoals of non-breeding minnows swimming in surface waters only a fewfeet deep. Unfortunately, it was necessary to terminate the gradient experimentsin the early spring, but it may be well to note the possibility that a fit between thebehaviour of individual fish under the very specialized conditions in the gradienttank and the observed behaviour of minnows in the field, might not have beenobtained had experiments been carried out in the summer months.

SUMMARY1. Minnows in a light gradient tank were almost constantly active, but were

restricted in their movements by Hght above a certain intensity.2. The value of the limiting intensity differed for individual fish but lay between

0-2 and o-oo2 m.c. These values are of the same order as the threshold for lightstimulation of the pineal complex in blind minnows, described by Scharrer. It issuggested that the pineal might be the receptor for the hght limit, and evidencesupporting this suggestion is discussed.

3. Hungry minnows were not limited by light, and spent a considerable per-centage of the observation time in the lightest parts of the tank. After feeding theyassumed normal limited behaviour.

4. In mid-April, prior to spawning, minnows examined in the gradient wereextremely active and showed httle sign of a hght limit. The mean swimming speed offish in mid-April was 159 % faster than that of fish observed in December.

I wish to express my thanks to Prof. Sir James Gray for allowing me the privilegeof working in his laboratory at Cambridge. My grateful thanks are due to Dr F. R.Harden Jones for allowing me to use the hght gradient, and for the use of his photo-multiplier, and especially for his constant advice and encouragement throughoutthis work. This work was carried out during the tenure of a Development Commis-sion Fisheries Training Grant, which is gratefully acknowledged.


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THE BEHAVIOUR OF MINNOWS (PHOXINUS PHOXINUS L.) IN A LIGHT … · In the gradient tank, normal, well-fed minnows were almost constantly active. Fish most usually turned at the end